Damp stains on a chimney breast and adjacent ceiling – It may be salts.

I’m finding that the number of enquiries relating to damp stains on chimneys is on the up.


The common complaint is that the roofer has either fixed the stack or the flashing, but the damp still comes in.  Usually the roofer has been back a couple of times or three and still the problems persist.

Damp chimney in Harrogate due to hygroscopic salt contamination

Above – stains of the face of the front bedroom chimney breast

Damp ceiling in Harrogate, Yorkshire, caused by salts from a chimney

Above – Stains on the rear bedroom chimney breast and ceiling.

This is a short example of the problem I investigated this week in Harrogate, West Yorkshire. Salt contamination from concentrated flue gas salts which persist has caused this issue.

Typical symptoms are damp patches, which may or may not be salty looking.  They often come and go with damp weather, though it doesn’t necessarily have to rain.  A muggy day will be sufficient for the stains to become more clearly defined. Often the damp stain will feel greasy to the touch (like your skin does when you’ve been swimming in the sea).

Check for leaks

Of course, the first thing to do is check for leaks and fix them.  Leaky flashings, pointing and open chimney pots are the most common causes.

But when these are clearly in good condition, or have been repaired, hygroscopic salts are the most common cause.

On the featured survey I was able to get in the loft and check for leaks myself.   In the following image there are signs of water ingress. With salts on the timber.  However, the trimming joist is securely fixed in intimate contact with the chimney stack, shouldn’t it be wet?

It is air dry at 11% WMC. Some say that salts make a conductivity moisture meter over-read..that’s true.  But it’s also true that salts which are dry do not do this.  So clearly, with visible salts and a low reading from my conductivity meter, I know that this wood is dry and the salts are merely a sign of drying out – they are efflorescent salts

Dry salts on a trimming rafter in a Harrogate roofa


Contrast this with the reading seen below, taken from the back of the ceiling plaster.  This reading is 45% WME – substantially higher yet visibly free of salts.


This is because the salts are hygroscopic and are causing the meter to over-read; because they are absorbing water vapour.

You may wonder where we get these two different salts? Well, the salts accumulated on the roof timbers are there due to years of tiny water ingress and condensation on the underside of the roof slates.  This runs down and wets the timber, leaving minerals they dissolved along the way.  Repeated wetting and evaporation has concentrated those seen here. These are from the slates, back-pointing, external chimney pointing and such – they are relatively free of flue gas derived ammonium nitrates and chlorides.

Further down the stack, the chimney is encased in a porous plaster and this, combined with high humidity in the room below means that once wet, drying is slow, there’s plenty of time for salts to migrate through to the plaster and stay there.


Wet hygroscopic salts on the ceiling plaster - Harrogate

 In the above image, note that the plaster lath ceiling goes right up against the chimney brickwork.


Our houses are quite humid these days, what with double glazing and all manner of steam producing gadgets in the kitchen.  Not to mention showers, baths and drying clothes. Combine this with a cold old chimney stack and any salt contamination can develop into a serious and disfiguring problem.


There are two main causes on this house*:

  1. Previous leaks, which allowed rainwater to soak through the chimney, dissolving salts and contaminating the plaster
  2. High humidity, resulting is vapour from the house penetrating the chimney, where the cooler temperature raises the Relative Humidity to near dew point.  Some of the salts derived from flue gasses can become deliquescent at 85%RH or so (I have Graham Coleman to thank for this science bit), so in effect they become liquid and are drawn through to the plaster via capillarity – salts move in solution; no other way.

The end result is that the plaster is now hygroscopic and will always be so.

The only solution is re-plastering. Disposing of the contaminated material to waste.

Remedial work is straightforward but some diligence is needed. Don’t forget; diligence is the rarest commodity in the construction industry, so you must find a contractor who will take sufficient care to follow the rules…which are:


  • Don’t use a lightweight gypsum based plaster to repair the damage.
  • Either use a cement based water resistant system or, dry line the wall on an independent lathing system or over a vertical DPM. If you must use a lime based plaster then either accept that salt may work through again later or, render onto laths, leaving a small gap behind, so the lime is isolated.
  • Care is needed with the ceiling if that is effected.  When ready to replace the ceiling use some vertical membrane or butyl tape to separate the plasterboard from the chimney.  The chimney is saturated with salts and any gypsum products which touch the area affected will draw more salts.
  • If you are using a hard wet plaster method, a layer of latex sealer or a cementitious ‘tanking’ slurry will help too. Why not ad some SBR to the cement mix to reduce permeability?  I like Safeguards Bond-aid plus or SikaBond SBR.

I hope the above is useful for surveyors, contractors or homeowners.


Dry Rot.

*As Chartered Surveyor Martin Conners rightly points out below, failure to ventilate redundant flues is also an important factor. This doesn’t apply in this case study, but if the damp patches are on unventilated redundant flues and not immediately adjacent to the external stack/flashings, condensation in the flue may be the cause.  If that’s the case then ventilate the flue – though by then the damage is done; ventilation will not remove the salts from the plaster, brickwork or stone.


  1. martin connors says

    Hi Brian

    An interesting article. Do you not also agree it is also wise to vent un used flues at low level internally and cap and vent pots externally.

    • Dry Rot says

      Hi Martin,

      Thank you for looking in. Yes of course you are correct. redundant flues in old chimneys are a natural moisture sink; absorbing water from various sources and holding it, becoming wetter and wetter. ventilating the flue is generally good practice because at least the moisture in the flue is being diluted when the wind blows.

      There are always other opinions though; a client told me that a ‘mould remediation expert’ claimed that the air in the redundant flue was contaminating the home environment with mould spores and the chimney needed sealing up because it was making his tenant seriously ill! You couldn’t make it up.

      best regards


      One of my damp diagnosis group chaps has raised this too. My post is on a particular house ventilation didn’t apply and I tend to do these ‘on the fly’. As I wrote it, the post developed into a more general advice piece. I’ve added another short paragraph now, reflecting your input – thanks for the feedback, your comments are valuable and I’m lucky to have fellow experienced surveyors to in effect ‘peer review’ my stuff. Keep it up!

      • martin connors says

        Hi Brain
        Only just read your reply, thanks for your comments

  2. Don Martin says

    Hi Brian
    Nice article and very helpful.
    This is a real problem in our industry and very common. Ilooked at a chimney breast in a terraced property just this week that the owner has had “Loads of builders in to sort” and not one had diagnosed the problem correctly. I dont suppose i should be that perturbed as they aren`t experts.
    This one had been drylined but using dot & dab adhesive (perhaps gypsum based) and the chimney and party wall adjacent to it looked like it had got measles where the adhesive patches had absorbed the salts.
    I always enjoiy your articles! Keep up the good input!

    • Dry Rot says

      Hi Don,

      Thanks for looking in. Dot and Dab – great in new build and great (for us), in refurb work. Of course it’s awful for use on damp walls. It’s down to training and so many plasterers and builders use the same techneques and materials for re-furb works as for new-build. My blog is about changing that in some small way.

      I’m glad you found the article helpful and thanks for commenting.



    • Andrew Tolley says

      Hi Bryan

      Just looked in on this site for the first time – great, quality posts and helpful discussion on the subject of hygroscopic plaster. The majority of these confirm my own experience over the last 15 years but in respect of the dot and dab method of fixing plasterboard, would this system of fixing not be possible in conjunction with foil backed or foam backed plasterboard?

      I tend to specify replacement of the salt contaminated plaster with renovating plaster on an SBR slurry which seems to do the job reasonably well, particularly when only part of the plaster is removed and has to be patch repaired.

      Would appreciate your/your readers comments on this approach.

      Best regards


      • Dry Rot says

        Hi Andy,

        Thank you for looking in.I am glad you’ve found the posts useful.

        You are right that patching can be done with render or a good cement based renovating plaster. An SBR gauging solution and pre-wash is a good idea to kill the suction a little and discourage inter-mixing between the tap water in the plaster and the hygroscopic salts in the wall.
        Most insulated plaster board is ‘closed cell’ so you can usually get away with it. However, if I was set on a dot n dab method, I would prefer a membrane such as Safeguard’s excellent Oldroyd XP. It’s a physical barrier and will also create an air space between the dabs and the wall so if the house is a little humid there should be less of a ‘cold’ spot’ at the dab… hence less chance of any mould issues.

        Keep up the good work Andy, it sounds like you are doing the best for your clients.


        • Hiya, I have had builders in to remove an old chimney in a room and plaster the walls to make the room a good bedroom again. Ever since the SBR,hardball and skim have been applied it really does stink of Ammonia. Is this likely to disappear over time as the plaster dries or do I need to now do further work to fix this problem?
          I also have a young family so I am concerned that the fumes will cause harm?
          Any advice would be very much appreciated.

          • Dry Rot says

            Hi Time,

            This should pass as the walls dry. Not really a health problem provided you keep the room well ventilated.

  3. Hi Bryan,

    Do you recommend any kind of treatment or replacement of the salt affected timbers? If the salts are hygroscopic, then as you say, they will continue to attract moisture and I am assuming that this will eventually lead to timber defects.



    • Dry Rot says

      Hi Dan,

      Thanks for looking in.

      That’s a very good point, but remember that the salts on the roof timbers are mainly efflorecent salts rather than hygroscopic ones. They do cause a conductivity meter to over-read, but they will not pull in moisture and cause decay; they are a product of drying out.

      The salts on and in the chimney itself are the hygroscopic ones and yes, they will pull in a vast amount of moisture.

      best regards


  4. Bryan,

    As ever Informative, interesting good stuff.



    • Dry Rot says

      Thanks Mark, I am glad you found it useful.

      best wishes for Christmas and the new year.


  5. Great post Bryan, this is such a common problem. Lots of people have spent a small fortune on roof and chimney repairs to find the problem is still there…..

  6. Hi All

    I had my chimney stack removed (2 years ago) from my bungalow (letting in water and gas fire no longer working) amd damp seen on the chimney breast.
    The roofer cut the terrocotto pot and pushed rockwool insulation in the top of the chimney. The gas fire had cardboard replaced at the bottom of the chimney to prevent daraughts). I still have damp on the top of the chimney and on the ceiling.
    Is this because there is still a requirement for airflow or can the damp be coming from the original wet in chimney coming through the walls?

    Many thanks

    • Dry Rot says

      Hi Chris,

      If the stack is no longer exiting through the roof then we know that there cannot be rain water involved. Two years is more than ample time for any ‘free’ water to dry out.This means that the remaining moisture is probably water vapour pulled into the wall by the residual hygroscopic salts. If the plaster at the wall and ceiling was damp before it probably will remain damp, because the plaster is hygroscopic now.

      Just replace it with knew material. ideally a cement based renovating plaster – dry-lining on laths or a membrane. Do not use modern gypsum based materials without protecting them from the salty walls. Lime plaster could be an option but it may let the salts migrate through and then you are back to square 1.

      Although salts do suck in lots of water vapour, the problem tends to be much worse in a property which is too humid to start with. If you had a nice open chimney and now it is gone, it’s reasonable to assume that the ventilation in the house will have reduced. Make sure that the kitchen and bathroom have great extraction – it really is important these days, particularly if the house is fully double glazed.
      This post on why many modern houses are mouldy tells you more about the need for good extraction – mould and humidity

      Thanks for looking in Chris.


  7. Pauline Walker says

    I have damp at the top of a bedroom chimney breast and the surrounding ceiling. No salt that I can see, just the paper coming loose then drying out leaving stain marks. I have had the roof, lead and chimney checked but still get the damp but only from time to time. There is a gas fire at the bottom of the chimney breast in the lounge below which is only used occasionally. And the chimney is supposedly lined. The fire and flue were installed about 5 years ago. Anything you can suggest please.

    • Dry Rot says

      Hi Pauline,

      I can’t diagnose this over the internet I’m afraid. However, it sounds like salt contamination so a plastering job as suggested in the above post is the likely treatment.

      It wont harm to get a professional to look at it. Try the Property Care Association web site for a contractor or independent surveyor.

      best wishes


  8. Daniel says

    Hi Bryan,

    I have heard conflicting advice over venting of redundant chimney flues. I have always understood that you should ventilate the flue in the room, where the old appliance was and also at the top, though being careful to ensure that rainwater is prevented from entering.

    However, I have recently been told that you should still cap and vent the flue on the stack, but that it is better to seal everything up on the inside, as this prevents warm moist air from inside entering the flue, which is actually more likely to result in condensation, as it cools on its way up and out of the property.

    Any thoughts on this?

    • Dry Rot says

      Hi Daniel,

      The BRE guide says always vent. However, I agree that sometimes this is not a good idea, particularly if the stack is on an external wall, where the inside will be very cold in winter. In my view you need to treat each on its merits. I think that ‘top’ ventilation is a good idea always, but the internal vents into the house should be avoided where the stack is on an external wall, because as you say, the warm damp air will be drawn up and will inevitably condense in the flue, perhaps mobilising the salts, which can then pass through into internal plaster..

      best wishes


  9. Hourglass says

    Hi Bryan,
    For the first time I have heard a real explanation for the damp patches on our newly plastered walls. The patches are either side of an old flue and up onto the ceiling, in the top floor of our three story house, external wall. Well, the walls were plastered two years ago but haven’t painted as trying to figure out what to do. i have rebuilt the chimney, replaced the lead flashing, removed and replaced the tiles, re-rendered the exterior, all to no avail. I have been told it is condensation as there are tiny black specs on the plaster, so we leave all the windows open, but that doesn’t help. There is a shower room in the next room , clearly that is contributing to the problem, and the new extraction system must not be coping. The fact that we are within 200m of the sea and the house is 130 years old with solid exterior wall probably doesn’t help much either.. Bryan, would Vandex Refurbishment Plaster do the job? if so, should we use anything under it? We don’t really have to opportunity to use drylining due position of doorway (cut into the flue – not by us). I understand that Vandex refurbishment plaster is lime based. I am concerned about the requirement to allow sold walls to ‘breathe’.

    • Dry Rot says

      Hi Ther,

      Sorry for the delay in replying to you – I have been on Holiday 🙂

      You can re-plaster with Vandex or any cement based renovating plaster. Another way is to dry-line the wall with plasterboard on laths (not dot and dab). Let me know how you get on.


  10. Katie Jackson says

    Hi Bryan

    I was so pleased to find this discussion thread. We have just had a new bathroom fitted, fully re plastered and new plasterboard ceiling. The plasterer has advised that he waterproofed rendered the window wall and the wall below the bath, but only skimmed the wall above the bath as the original render/plaster was still there.
    The part of the wall that was not waterproofed joins onto the outside ground floor to above roof level stack. The lead flashing was missing from the chimney as well as lack of rear render so we have had a roofer back three times to find out why our wall is getting wet. All repairs have been complete but still on a damp day the wall is getting wet. Our shower has now been installed and when we first used it the walls damp increased quite a lot. After reading all the previous comments I guess this is salt contamination?? I will try and attach a photo to this comment if I can? I have seen that you can buy a hygroscopic salt solution that you can paint on good plaster saving the time of taking it all off and starting again, do you think this would work?
    My other concerns are that the plasterer is back to work on the kitchen. Our kitchen is below the bathroom, so I now have concerns that we will have the same problem. Many thanks for any advice that you can give me. Katie

    • Dry Rot says

      Hi Katie,

      I have been on holiday – hence the delay.

      Don’t be taken in by magic paint on chemicals they rarely work.

      One way of checking if the problem is salts is close the door and boil a kettle in the room. After a few minutes the wall will look a lot ‘damper’ if salts are to blame. If they are then re-plastering is th eonly option But use cement based render or dry line the wall on laths – not dot and dab. Also you could apply a membrane to protect the linings.

      best of luck and thanks for the comment.


  11. Richard says

    Hi Bryan,

    An interesting article….

    I’ve spent the past 2 years battling a damp problem on the wall of a bedroom in line with an external chimney stack. I’ve had the flashing replaced and t
    he stack full repointed and silicon washed in an attempt to give it some level of waterproofing. It’s a single skin stack.

    Heavy rain over the weekend and water came in again, to the level of wet wallpaper causing it to bubble off the wall. Had the roofer back today, he stripped back tiles and the lead and all appears 100% dry. His only thought is that the old (original 1940s) plaster is acting as a sponge to the moisture and I should re plaster the wall with a waterproof layer.

    Feels like it would be treating the symptom and not the problem but then stumbled on your article. Any advice?

    Many thanks

  12. Katie jackson says

    Thank you very much for your reply Bryan. Katie

  13. michael brotherton says

    hi bryan im a small building firm and have a problem on a chimney that is on an outside wall , there is a large stain in the room at gutter back level now first thing i checked was the lead but it had been replaced. so all i did to that was hack out the mortar and pointed with lead mate . no change but the customer had the wall plastered straight after the lead had been changed . there was 2 flues one had a mushroom cowel and its illegal for the gas fire. so i put a gc2 in and the next flue was totally capped off no vents at all so i put an air grate in this and reflaunched. then i ground out all the pointing on all four sides and repointed .there is a vent in the room where the damp is where the old fire was . im at my wits end and so is the customer as ive kind of inherited this job.i dont want the customer to spend any more until im certain of what it is , please can you advise?

    • Dry Rot says

      Hi Michael,

      One way of checking for salts is just boil a kettle in the room. If the damp patch grows or looks more intense then it is likely to be hygroscopic. In this case re plastering using a cement based renovating plaster – dry line on laths or a membrane will sort it.
      Have fun and thanks for looking in


  14. michael brotherton says

    thanks for that bryan going to try the kettle thing ill post here how it it goes.

  15. Anna Davidson says

    We had a leak through our chimney last winter resulting in water running downn the walls. The chimney was subsequently removed to below the roof level and tiled over around 6 months ago which seemed to solve the problem. However upon replastering it appears that we still have damp plaster where the top of the chimney breast meets the ceiling. The roofer can see no obvious external problem and we can’t see any signs of damp in the roof space above. The affected wall has been stripped back to the brickwork and the small damp area is still exposed, the rest of the room has been replastered. Any idea what the problem could be and what I should ask the plasterer to do to finish off the job? The chimney breast is on an external wall, vented in the affected room and open in the room below. Many thanks.

  16. Amanda Bristow says

    What is your view on the use of a breathable membrane? We have repaired our leaking chimney but are left with salt marks on the chimney breast in two rooms. In one where the stains are small and intermittant it has been suggested to us that we cover the wall with a breathable membrane & replaster, rather than removing all the plaster over a wide area. Is this a good option?

  17. I have lived in my victorian mid terrace for 15 years and the lounge chimney breast was already removed when I bought the house. I noticed damp in the last year or so and have stripped off the lining paper to find damp in the middle of the wall where the chimney breast would have been. The neighbour had a course if damp injections so could it have pushed it through to my side? Also the damp starts at waist height upwards, below that the wall is not damp but is different plaster, perhaps wall was previously tanked? I am wanting to get room skimmed but need to treat this damp first with something?

    • Dry Rot says

      Hi Becky,

      When a chimney is removed the interior of the flue is exposed and plastered over. This often contains the salts I mention in my article. They are hygroscopic and will tend to absorb water vapour from the air and remain damp. This is especially in a house with double glazing and missing or bricked-up fireplaces.

      The new plaster at the bottom section implies that the house has had some damp proofing done before. This material is probably a cement based renovating plaster or render. That is why it is not damp; it is suitable for application to a salt contaminated surface.

      Installation of a DPC into a wall does not force damp elsewhere – that is a myth and goes against the laws of thermodynamics… sorry Becky I am a nerd 😉

      I would either; extend the sand/cement render up the wall making sure that the work is extended at least 500mm past the last evidence of any staining or, dry-line the wall with plasterboard on laths or over a vertical damp proof membrane – Either will work. It is a cosmetic issue.

      In saying this, I am not on site and am blind to things you may not mention, but which an experienced surveyor would see and be aware of. I recommend you get a surveyor to check it out for you.

      Best wishes

      Dry Rot.

  18. Hi
    I bought a 1920’s house and in the upstairs bedroom the fireplace had been removed. A fire continued to be used in the room immediately below. However a damp stain is seeping through in the bedroom. It appears on the wall adjacent to where the bedroom fire had been.

    We’ve checked the chimney stack and roof above and it is bone dry even after heavy rain.
    A vent hole does appear to have been created where the bedroom fire had been, but I suspect that this had been covered by the skirting board.

    If the damp is a result of a previous leak and hygroscopic salt contamination is the only option to re-plaster or will using the fire and ensuring ventilation dry out the wall?

    • Dry Rot says

      Hi Nelli,

      Thank you for looking in – I hope you find my blog useful.

      It sounds like a re plastering job I’m afraid. Once these salts are in the plaster then the plaster itself is hygroscopic. It will absorb and retain water vapour very readily. Any re plastering should be done carefully. Do not apply gypsum plaster or lime directly to the brickwork. If you want to use these products then they must be isolated from the wall with either a lathing system or a vertical damp proof membrane. An alternative is a dense cement render coat, which will not be permeable so will not allow salt contaminated water to pass through and contaminate your finish plaster. Don’t worry about any ‘breathing’ nonsense, you will only be using the material on a fraction of the house surface area, so it will have zero effect on the internal environment.

      The disused flue should be vented of course.

      Please note that I cannot survey the problem by word press 🙂 This is general advice and ideally you should get confirmation, perhaps from a local PCA member.

      Best wishes


  19. Lizzie says

    Hi Bryan,

    Really interesting article. I bought a Victorian semi 3 years ago and a oily mark appeared on the landing wall where an old chimney breast used to be. The top part of the chimney is still there but is held up by an RSJ in the loft. Over time the patches have increased. I have had the lead flashings attended to, chimney repointed and the chimney ventilated and a hat placed on top but the marks came back and get worse when the weather gets cold. I have been into the loft and noticed that the beams have a layer of mould type substance and there looks like there are tiny droplets on the RSJ but I can’t see any leaks.
    I wondered what you would suggest. I saw in one article you suggested replastering the wall was a way to fix the problem.
    Any advise would be most helpful.

    Thank you!

    • Dry Rot says

      Hi Lizzie,

      I’m glad you found the article interesting.

      From what you’ve written it seems that salt contamination is the cause.This is especially the case if there is no direct connection from the lower level damp patches to the remaining chimney – when you think about it, it can’t be continuing water ingress.

      Replastering using either a dense cement render coat or a lathing system to isolate the plaster from the wall is appropriate. Applying a latex/SBR solution first will kill the suction and reduce contamination of the new plaster.

      From what you say about the loft, there may be some condensation up there. Make sure the roof void is ventilated at the eves and ridge if possible.

      I cannot survey via the interweb so maybe a local PCA contractor or independent surveyor member will be a good idea.

      Best regards



  20. Hi Brian,
    I have been through all the chimney repair processes even treating the 110 year old brickwork with a quality breathable sealant. I am now pretty much convinced that all is in good order externally.
    I have had the damp ceiling patch for about a year now and it does feel sticky to the touch as you describe. additionally there is a damp patch on the ground floor side chimney breast. This appeared a year ago and does get damper after heavy rain as does the bedroom stain.
    Where do you suggest I go from here bearing in mind your comments regarding diligent builders?

    There are two flues in the stack. One has always been ventilated with the original pot with a ventilated top and the other only recently (July this year).

    • Dry Rot says

      Hi Malcolm,

      Thanks for getting in touch. If the damp patch is greasy then it is contaminated with hygroscopic salts and will never dry out. Follow the advice on replastering and it should be fine. Always extend the replastering at least 300mm past any staining. The ceiling will require gypsum plaster on plasterboard (probably), so make sure the the new work does not touch the chimney masonry. You can use a strip of butyle tape for this and just trim it after so it’s insulating the edge of the ceiling from the wall, but not visible.

      The wall can be either rendered with sand/cement at 3:1 with a water-proofer in, or dry lined on laths. Another way is plastered over a vertical DPM such as Oldroyd XP.

      best wishes


      Please consider getting a surveyor to check it – there may be evidence that changes the above advice but which only a site visit would reveal. The PCA find a member page is a good starting point for a local PCA member.

  21. Andrew Moss says

    Hi Bryan,
    Thanks for the very useful article. I think I have a damp patch on internal chimney breast due to salt contamination.

    But my question relates to a damp patch on the external brick work on the outside chimney stack. The chimney stack rises 4 feet from a flat roof and is 3½ feet wide. The external bricks are damp (40% reading) in the bottom right hand quarter of the chimney stack which is on the north side. There are no visible signs of water ingress and the chimney is top ventilated with half moon ridge tile acting as cap.

    Presumably salt contamination can occur on a an external chimney brick wall – is there a specific test to verify it? Also should I do anything about it?

    Thanks very much . . . . Andrew

    • Dry Rot says

      Hello Andrew,

      Thank you for the compliment, I am glad the article is useful for you.

      The external brickwork or stone of a chimney will often exhibit similar salt contamination. Taking electronic moisture meter readings from it can be very misleading because the bricks contains salts and so the meter will over-read. That is why it takes experience to use the meters correctly. Also, the external face gets rained of of course and this rain takes along time to dry out, which further confuses the issue.

      There’s no need to worry about it though. It is merely a cosmetic issue. If you see salts appear in very dry weather, just brush them off and they will reduce somewhat, over time.

      best regards


  22. Don Higgs says

    Hi Brian,

    Many thanks for the superb level of information provided on your site.

    I wonder if you can clarify something for me. I have a semi with a redundant chimney stack on the “end” wall which had been taken down to below roof level and left open (not capped off).

    When I bought the house damp patches were evident on the chimney breast in the relative bedrooms and on the sitting room chimney brest – at about 1.2m AFFL (plus a little on adjacent ceiling areas of bedroom). On checking further in the loft void I noticed that the roofing felt above the open chimney stack was very wet and the chimney stack itself in the loft area appeared to be damp.

    For reference, there is an air vent at the bottom of the chimney stack into the sitting room

    I believe that the problem is principally condensation with salts etc possibly contributing to the damp patches on walls in the bedroom and sitting room. In order to minimise the condensation problem I intend to install air vents into the loft void – both low down and near the top – to try to disperse the warm air rising up the chimney, however, as a secondary measure I am wondering whether I should actually cap the chimney stack off in the loft void, possibly leaving a small vent hole to allow free flow of air up the chimney itself.

    Can you please advise whether you would leave the chimney open or would you cap it and, if the latter would you cement – say – slates with insulation blocks on top. Also can you provide a guideline as to the spacings etc of vented roof tiles.

    Many thanks. Don

    • Dry Rot says

      Hi Don,

      I wouldn’t cap it completely – leave a small vent.

      Consider dry lining the chimney on Laths so the the plaster is not touching the surface – it’s a cosmetic issue.

      Thanks for the kind comments

      Dry Rot.

      • Brian J says

        Really interesting article and discussion. We have a 1902 house which we have just renovated, putting internal insulation on the solid walls, installing central heating, double glazing etc and repointing/reflashing all the chimneys,etc. We are aware of the need to remove water vapour from cooking etc and the house is dry and cosy. One of the fireplaces on the ground floor on the external wall originally had the cooking range which was replaced by a gas AGA approx.35 years ago, ducted up through the chimney on the roof. We have removed the AGA and ducting, and professionally vented this chimney at the top with an open vent inside the fireplace at the bottom. On cold days, if you look up the inside of the chimney with a torch, you can see water starting to trickle down from a point about 2/3 of the way up – it appears not to be coming in from outside, but starts all round the chimney at about the same height, presumably as the air temperature reaches the point where the water vapour condenses onto the chimney walls (brick/stone). These “trickles” eventually reach the bottom and emerge into the fireplace. If this is condensation, and the open vents do not generate sufficient air flow in winter to warm the chimney so that condensation doesn’t occur, I have seen apparently experienced authors recommending that the chimney is swept, sealed at the bottom, filled with Vermiculite and sealed at the top. Does anyone have any experience of doing this, or have any opinions about this please?

        • Dry Rot says

          Hi Brian,

          That’s a fine piece of detective work with the torch.

          There are a couple of possibilities. The first is as you say, the warm mist air from the house gradually cools on the way up the chimney, condensing and causing the problem. Another is that the RH, being very high results in the salts in the soot, absorbing so much water vapour that they become deliquescent and add to the problem.

          Maybe a bit of both.

          I’ve often thought that the common advice – that is to vent disused chimneys, can backfire if the house is quite humid. This is possibly a good example of just that sort of situation.

          As for remedial action – I don’t think such drastic action as filling the thing up is wise or economically sensible.

          why not have the chimney swept and reduce the size of the internal vent? Maybe close it completely for a bit and monitor the result…. if it works, make that situation permanent. Finding this out will cost nothing.

          Please do let me know how you get on with it Brian.

          Dry Rot.

          • Brian J says

            Thanks for your comments. We will certainly get it swept though it looks reasonably clean – although we did remove several buckets of residual soot from the bottom during renovation. Our builder installed a small vent at the bottom (along with the vented mushroom at the top) which we removed and temporarily sealed when we first spotted the problem, following your line of thinking about the warm air exactly. That didn’t appear to help and may have made it worse, but part of the problem is that I suspect it may take months for the results of any particular action to become clear, and the variation in outside temperature whilst we experiment probably has more influence than anything else! I’ll report progress (or lack of it!) as we proceed. One of the surprises is that even though the top is vented, there is no downdraught even in cold windy weather, so I will do a basic smoke test with the bottom vent open to see if we have any updraught. Thanks for your help.

  23. Afzaal says

    Hi Brian,
    We have wet patches upstairs which are most probably due to hygroscopic salts from the chimney. My course of action is to strip away the contaminated area and re-plaster the area with a good sand:cement mixture. I would like to know if there is any thing I need to do to exposed bricks. If yes, please guide through step by step.
    All the help is greatly appreciated….Afzaal

    • Dry Rot says

      Hi Afzaal,

      Thanks for getting in touch.

      If you are sure that the salts are to blame and there are no leaks, a good sharp sand with ordinary Portland cement at 3:1 will be impervious to any salt contaminated water. If the thickness (cover), is thin, then you can apply acetic acid to solidify salts near the surface and even apply a cement slurry with a little SBR added before the render goes on. That too will reduce suction and cross contamination between new tap water in the render and the salt contaminated moisture in the flue.

      Of course you can dry-line on a lathing system too if you don’t want the cold render there.

      Good luck with it and let me know how you get on.

      Dry Rot.

  24. Afzaal says

    Cheers Bryan

  25. Mark Alston says

    Really good thread here, which encourages me to follow your suggestions, but there are a few technical terms you’ve used which I hope you can clarify for me…

    ◾When you refer to a “cement based water resistant system”, do you mean basically a waterproof render applied to the stone of the chimney breast? Presumably then finished with a normal skim coat?
    If so, is the render a ready-mix product or can you mix it yourself (what would be the ratios?).

    ◾On the ceiling, is it possible to replace a patch of affected plasterboard, or are you saying the whole lot has to be replaced? – My ceiling has just got a few small areas affected, so do you have a method for such small patches?

    ◾Also, you mention using a “hard wet plaster method” – is that different to the first point I asked about above?

    Finally – what is “SBR” short for?

    Many thanks in advance for any further help


    • Dry Rot says

      Hi Mark,

      1 – yep waterproof render – skim as normal

      2 – Mix it yourself – three parts sharp washed river sand – one part ordinary Portland cement plus water. Adding a waterproofing additive such as Safeguard’s excellent Renderguard gold works great. If you don’t fancy mixing it Safeguard also do a nice pre-mixed render too http://www.safeguardeurope.com/products/product_menu.php.

      3 – patch the ceiling but do the wall first and then the plasterboard will not be touching the contaminated bricks or stones – only the new uncontaminated render 🙂

      4 – Styrene-butadiene rubber (SBR) Safeguard sell that too. http://www.everbuild.co.uk/building-products/PVA-SBR-Bonding-Agents DINT USE PVA!

      Hard wet plaster is the same thing.

      Thanks for the kind comments good luck with your repair. Do please remember that I haven’t seen the house so no guarantee buddy 😉

      Dry Rot

  26. Michael says

    Hi Brian

    I’m looking for some advice, I recently bought my own house within the past year and had set my heart on reinstating a open fire but due to the flue only being 6 inches I had to install a stove instead.

    A while ago I noticed a damp patch in our bedroom on the ceiling adjacent to the chimney breast, when we inspected it we found the flashing around the chimney was damaged.

    Now that has been repaired but it must of been like that for quiet a while as the wallpaper was coming away from the wall because of the dampness

    Before installing my stove I had the chimney cleaned and was told the stove would needa few days to be broken in as we would get a smell from it.

    now the stove has been in a month but everytime we light it we get chest well i suffer dry mouth, nose effects me and feel strange now it gives off all the signs of Co2 but it s not as ive 3 Co2 sensors(ground floor, upstairs and attic) and 2 smoke alarms

    Now im wondering is this a side effect of a chimney drying out or from spores, has anybody ever come across this, any help would be much appreciated.

    I also have all the salt deposits on my chimney in the attic too and the chimney has a liner aswell.

    Thanks in advance


    • Dry Rot says

      Hi Michael,

      I’m not an expert on flues so I’d suggest a gas fitter to to be safe. I doubt that spores are responsible – if you were that allergic to mould your symptoms would not come on when the open fire is running – rather the opposite.

      Good look with it and do please let me know if you get to the bottom of it.



  27. Michelle Purcell says

    Hi Brian ..I have just read ur article .my loft is covered in the salt your talking about ..I have had the council out .to many time ..
    I have bad damp in both bedrooms .which is no good for me as I have c.o.p.d .and all the are going to do is take the vent out and cover it ..
    Don’t they have to treat the loft or anything ..3years I have been trying to get this fixed ..
    Thanks shell x

    • Dry Rot says

      Hi Michelle,

      If the bad damp looks black or mouldy then treatment in the loft is not the answer. If you have respiratory problems then great air quality is what you need. Make sure that the bathroom and kitchen have excellent extraction via fans. ask the council for a proper damp surveyor to come and look for you. Make sure you keep as warm as you can.

      Good luck with it.


  28. Hi Bryan, thanks for all the advice given above. This problem is exactly what we have in our 1929 stone build house. Have tanked and rendered as per your advice, and about to use Thistle Dri-Coat as under coat. Is this the correct product to use and what make of finishing plaster should I use after this? Just not too sure as to what constitutes lightweight gypsum based plaster. Was also going to use a salt neutraliser before the Dri-Coat, but local builder said it may discolour the plaster? Would appreciate your input.



    • Dry Rot says

      Hi Terry,

      As long as the masonry has a good coat of a nice dense render the dri-coat should be fine. It isn’t good straight onto the walls of a salty chimney though… here’s a quote from the application instructions… “Buildings such as old farmhouses, stables and barns not originally built with a damp-proof course, or buildings that have been exposed to storage of chemicals, are particularly at risk from this problem. Thistle Dri-Coat should not be used in these situations unless a proper survey shows that the risk from salts is minimal. An independent wall lining may be a better solution. C

        chimney breasts are another area where salt deposits may be heavy

      Skim as per normal plaster.
      Good luck with it terry and thanks for your comment


      • Hi Bryan, thanks for your reply, sorry to ask again but could you just clarify that you dont think we need to use a Salt Neutraliser and that as we have tanked the wall completely and given it a good thick coat of render then a coat of Dri Coat applied as per normal plaster will be ok?

  29. Crystal says

    Hi Bryan,

    We viewed a mid-terrace house at the weekend which had a loft conversion bedroom. We noticed that on both walls where the chimney stacks are, there were discoloured patches, they looked a bit raised and we think salty with a thin brown outline. The patches also happened to be situated around the ends of the timber beam running across the roof of the room. After reading your excellent forum I am wondering if these patches could be due to hygroscopic salts. It turns out that the bigger patch (we felt them and they weren’t wet particularly just unsightly) which was about a metre across coincides with the chimney stack that belongs to the neighbours, so while we could make changes to our chimney stack to cap, seal or vent it, we couldnt make changes to the neighbours without permission etc. I’m also worried that the timber beam might be contaminated with hygroscopic salts too and vulnerable to wet rot. Do you think we should stay away from purchasing this property or are these problems easily fixed?

    Any advice would be greatly appreciated.


    • Dry Rot says

      Hi Zoe,

      Typical Salt Damp.

      Sounds like the builder completing the loft conversion applied gypsum based plaster to the salty brickwork. It will need to be removed completely. I’d recommend you then dry-line the walls on laths. Do not dot and dab as this will fail. It is cosmetic though not a reason to avoid the house. You could apply a sand/cemt render if you wish, or have an membrane applied before dry lining (Oldropyd XP or similar), that would be belt and braces though (it’s what I’d do).

      Still worthwhile getting a roofer up to check your stack and the neighbors.

      best of luck and thank you for looking in. If you need any more advice regarding damp problems get in touch.

      Dry Rot

  30. Hi
    Thanks so much for this email thread – very useful article.

    I bought our end terrace house 15 months ago. It is an old Victorian property, stone to the front and end terrace (rendered) and brick and render to the rear extension. Almost immediately we picked up problems with my chimney and the one belonging to our neighbour. Rain poured down both walls during the bad storms of winter 2013-14 and three was a huge damp patch on the wall next to the chimney breast in one bedroom. Emergency repairs were carried out to take away the neighbour’s chimney and to repair mine (I was having a wood burner fitted at the time so couldn’t take my chimney away too). I also replaced all boards, gutters and downpipes. The summer was ok, but as soon as the weather changed the damp patch reappeared. Our builder rendered the chimney, replaced all lead and ensured that the flue was checked and properly capped off where required. The job was completed about 10 days ago – it is good and the chimney is watertight. However, the damp patch keeps reappearing. Sometimes it is faint, sometimes it is sweaty to the touch. Could it just be “free” water in the stonework drying out, or would you suggest a salt problem?

    • Dry Rot says

      Hi Sue,

      It sounds like the original leak flushed salts into the plaster. ‘Free’ water in masonry gradually dries out, so it wouldn’t appear to re-wet. Also, if the surface is appearing to ‘sweat’ that too is likely to be high humidity and cold, combining to bring the salts to a deliquescent state. If the moisture feels a little greasy, like your skin after a swim in the sea then salts are the cause for sure.

      Provided the previous repairs are sound you should be able to re-render following the mixes I’ve previously highlighted or dry lining over an Oldroyd XP membrane or on laths.

      Good luck with it and thanks for getting in touch. Do make sure that you take sufficient render off to get at least 300mm past the last sign of damp or salting.


      Dry Rot.

  31. anthony says

    Hi Bryan,

    We had damp patches on bedroom chimney breast which we had covered with foil backed paper which the painter took over onto ceiling about 12 inches from top of chimney breast. The damp has gone beyond the foil paper on ceiling. When we looked in loft region the ceiling joist is damp which is touching the chimney stack although the chimney stack is quite dry. we cant find where the problem is coming from. We have an open fire so could the damp patches be salts. We are going to have the chimney repointed. About 10 years ago we had the chimney lined with cement due to smoke coming through mid feathers. Weve been told to remove plaster in bedroom on chimney stack completely and replace ceiling joist and treat woodworm. Then we were told not to remove plaster in bedroom. no need to as foil back paper covering damp areas and just to repoint chimney stack. I f we leave the chimney breast and not knock the plaster off will the foil back paper be sufficient and just repoint the chimney and replace joist., we also get problems with condensation on the walls by chimney stack and have black marks forming. please can you help. thanks

    • Dry Rot says

      Hi Anthony,

      Thank you for getting in touch.

      The foil may act as a good sealer, but the salts are hygroscopic so they can absorb water vapour through the reverse face. If enough is absorbed then the salts become mobile and hence can creep along an that sounds what has probably happened.

      I’d be tempted to just re plaster for a proper job.

      Good luck with it.

      Dry Rot.

  32. Hi Bryan/dry Rot.
    Great website and some really good information..collected from a bunch of questions posted by others.
    I have one more question. How far from a chimney breast can the hygroscopic salts show up?
    I recently bought a 1920s house which for sure had a coal fire in the front room. We did as others and investigated for leaks..we found 1. Broken downpipe shoe below the ground level, 2. Plastic roof tiles 3. Chimney needing fixing and flashing replaced and 4. Water leaking from the connection of the steel mains water pipe to the newer copper connection – quite common due to galvanic reaction.
    So thought we had fixed the problem of the damp patches on the chimney breast and about 1 metre either side of the chimney breast…but the patches came back even after a thorough demudifiying and heating of the house. Now I’ve read a bit and from your site I am comfortable it’s due to salts. The thing that mistifies me is that there are also the same patches under the bay window. From the chimney breast to the corner or the room is about 1m, then about 50cm of wall leads to a small bay window..the damp patches are at the other end of the bay window about 4 feet away from the start of the bay window. There are patches all the way along but not continuous….

    Is it possible for salts to have migrated that far?

    • Dry Rot says

      Hi Anjai,

      Thanks for looking in. I’m glad you have found the post useful is solving your damp problems. Salts can’t walk and only move through solution. I’ve seen salts a meter or so either side of a chimney, but as for the opposite end of a bay window, well I have my doubts about that. Salts can be present for several reasons and although this post is about chimneys, rising damp can also bring these into walls in other locations. Have a look outside where the damp is and see how high the path/ground is in relation to the DPC and if there are any salts on the external brickwork.

      best regards


      • Hi Bryan
        Thanks for the response.
        I think you have confirmed something for me.
        The corner (which I referred in the previous post) was the place where the underground (clay?) downpipe shoe was cracked. During testing we poured half a bucket of water into it (before we saw it was cracked) expecting to see water flow into the drain (manhole cover was removed) and not a drop came out that way. We realised then the shoe was cracked and all the water flowed directly to the foundations in that corner.
        When we lifted the floorboards we saw the underground timbers were fully soaked. I think whats happened is that water has soaked its way along the joists (which run under the floor under the bay window) its most likely its that water that the plaster (under the bay window) has ‘sucked’ up and resulted in salt movement.
        Does that sound at all plausible? I am not sure if the plaster touches the floor (I havent removed the skirting board as yet) but am guessing the bricks soak up the water first and then present it to the plaster.
        There is a 4″ concrete ‘plinth’ (house is 1920’s build) applied to the brickwork outside under the bay window but this seems in good shape and recently painted over with a bitumen paint. The front garden is paved over (maybe creating a splash from rain)? Ive checked the double glazed window and its sealed properly and the drip groove in the window sill is fine.

        Just want to at least have some sensible justification before I pull down all the internal plaster work ;-))

        Thanks again for this brilliant post…feel for the chap who pulled down his entire chimney stack!

  33. Ian Brown says

    Very interesting article but my salts problem couldn’t be further away from the chimney. The problem is in the hallway, three metres from the front door on a party wall. A “damp patch” has appeared just above the skirting both sides of the hallway, with several small patches elsewhere, slightly higher. There appears to be a yellow substance oozing out of the wall and the top of the skirting is actually wet.

    The house is one of a terrace of four, built about 1850 with no foundations and no DPC.

    Any advice would be gratefully received.

    Ian Brown

    • Dry Rot says

      Hi Ian,

      Thank you for visiting the blog. Mmmm, your issue sounds fascinating and wish I could see it. Rising damp never causes water to ooze out of a wall. Sometimes very heavy salt contamnation can cause the surface to bead with water and even run, due to deliquescent salts turning to liquid during periods of very high humidity. However this is rare and they are not normally yellow. If there are metals there, such as angle beads these can rust and then the water could be reddish…

      I’d recommend you visit the Property Care Association web site and find either an independent surveyor member or a contractor to take a look for you. I’m afraid that this is one of those occasions when I could mislead you, without being on site.

      good luck with it and do please let me know how it transpires..


  34. Carole Goodwin says

    We moved into a bungalow 6 months ago. Just had new soffits and fascia and some felt renewed. Three weeks later a damp stain has appeared on the chimney breast and the corresponding external bricks are quite wet. The wall needs some repointing. Which is responsible for the damp stain? We are not young, not wealthy and do not know who to trust.

    • Dry Rot says

      Hi Carole,

      Why not take some images and send them to me? I can’t promise I’ll have an answer but I’m willing to try. A few shots of the damp patch and external ones of the brickwork. Do you know if the walls are cavity or solid construction? Is the chimney used? How many flues in it? I have sent you an email to reply back to 🙂

      best regards

      Dry Rot

  35. Sarah Tennant says

    Hi Bryan, thanks for the useful article. We have damp pale yellow-cream patches appearing at the base of our chimney breast in the living room. Frustrating as we just redecorated. We had our damp course checked – twice – and they said it was fine. We had a leak previously in the property which led to a drainage survey – again all fine – and the culprit turned out to be a crack in the concrete in the outside drain (fixed and all allowed to dry out). Now that we waited to redecorate and have damp stains in the plaster around the chimney I am at a loss which expert to call next? The damp course man didnt pick up a problem around the chimney and neither did the surveyor, although my parents think our chimney may need repointing and some mortar fixed. Who do I call to confirm dampness around the chimney and who would know how to fix it? Thanks.

  36. I have what is almost certainly hygroscopic salts permeating the plaster in an upstairs bedroom chimney breast. The chimney was originally used for burning coal and wood but has not been in use for many years. I am sure I will have to get the wall re-plastered. However, I would like advice on whether the chimney which was tightly capped some 4 years ago when re-built, should actually have ventilation. I am sure this problem has arisen, or at best got worse, since the chimney was re-built.

  37. David Rawlins says

    Hi Bryan,
    Thanks for referring to your article from LinkedIn. I agree that lime plasters are just as susceptible to salt contamination as gypsum plasters, although a St Astier sulphate resistant plaster will last longer. Traditionally, chimneys were lined with cow dung & straw which was then finished with lime plaster. Interestingly this does work! Instead, in these circumstances I am inclined to advise dry lining with breathable boarding such as wood wool fibre, if the wall is confirmed dry, or to use Calsitherm or Cork, before providing a finishing coat of lime plaster.

    • Dry Rot says

      Thank you David,

      Always good to have a traditional weapon in the armory!


  38. Thomas McGovern says

    Hi Brian,

    After a full re-roof (old roof had it’s day) it seems like the wall in my son’s room became wet (in the shape of the old fireplace). the property is over 110 year old and we are on the second floor (so I thought no raising dump possible). Roofer says it is condensation caused by the stonework with touches the external walls etc
    Does this sound right? He advises we should bring wall back to brick, chip the stone back 5 cms and then use plaster board (not touching the stone) and re-plaster on top and also installing a vent. I am reluctant to doing anything as I thought it was strange this issue only happened when the new roof was installed. Can this be condensation?

    I appreciate any help/pointers on this one.



    • Dry Rot says

      Hi Thomas,
      The new roof shouldn’t effect the wall at all. I can look at some images if you post them on a dropbox and send me a link…


  39. Lee Broderick says

    Hi Brian

    I’ve had a chimney that has clearly leaked for several years leaving the original plasterwork with Hygroscopic Salts in it. The chimney has now been repointed, had new vented pots placed on it, new lead work all round and the brickwork sealed with masonry cream. I’ve recently removed the plaster back to the brickwork and am about to embark on the repair. The system I was intending to use was to insulate with Celotex direct against the brickwork, making sure all joins are foil taped and then board with moisture resistant plasterboard. I would then apply a finish coat of plaster.

    Do you think this will be sufficient? I note in your original post that you mentioned a vertical DPM if dry lining.

    Many thanks


    • Dry Rot says

      Hi Lee,

      From memory I Celotex has an aluminium sheet vapour control layer so a DPM shouldn’t be needed. The thing to remember is that the wall isn’t really that wet (provided you’ve fixed the leak), so all you need to do is make sure that the lightweight plaster and board products are not in direct contact with the wall. Direct contact could allow the salts to migrate into the plaster and boards but the foil backing of the insulation should stop this. Bear in mind that the use of insulation will mean that the wall is colder, so the taping of joints is crucial. However, a gas tight vcl is a fantasy so some warm moist vapour from your house will always be available for these salts to greedily suck up. Shouldn’t cause any problems if you are not diligent.

      How are you fixing the insulation to the wall?


  40. Hi
    We have damp in two bedrooms on the chimney. From reading above it sounds like the salt issue where water is being absorbed all the time. We counteract this by using a dehumidifier.
    However we have been advised to baton, insulate and replaster the walls to get rid of the problem. One bed still has fireplace (capped) and downstairs coal fire still in use although on an infrequent basis. The wet can travel half way down the wall at times, problem has increased since we double glazed this room.
    Please could you advise if better to remove old plaster and start again or will the baton, insulate and replaster method work (with out the removal of the old plaster). Or should we remove old plaster then baton, insulate and replaster.
    Thanks , any help appreciated

    • Dry Rot says

      Hi Tracy,

      I would tend to hack off the old plaster first, merely to avoid any chance of mould growing on the wallpaper behind the new boards. Also this would avoid the loss of space that over boarding brings. Not really crucial though.

      If the double glazing has made thins worse you need to check you have good extraction in the kitchen and bathroom – that is crucial.

      thanks for looking in.


  41. kelvin says

    Hi ive vented the chimney breast in my daughters room, and my chimney has been taken down to inside of the roof space, but the chimney breast is still wet, its not mouldy but soaking wet it even runs down the wall, i dont know how to fix this, apart from taking the chimney breast completly out all together, any help would be very help full.

    Thanks in advanced

    • Dry Rot says

      Hi kelvin,

      I know this sounds daft but you could try tasting the water on the face of the breast. if it tastes salty then its the salts expressing as deliquescent due to very high humidity in that room. If there are lots of salts then they hungrily pull in lots of water vapour. The best way is to re plaster on boards or a membrane (see advice above), or reduce the humidity (do both if possible).

      Good luck with this and do let me know how you get on.

  42. Dickie says

    Hi Bryan,

    You are a credit to your profession and the PCA, thanks for taking the time to write these articles and respond to queries like you do.

    I just moved to a 1930’s detached place with 2 unused chimney flues on the left flank exterior wall (which are capped off and reduced down to roof level). The purchase survey highlighted some dampness on the chimney breast on the inside walls which we have investigated further and found to be salt damp. Roofer found the roof to be old but sound.

    The previous occupiers had apparently tried a number of fixes like fitting an external air brick towards the top of each flue and rendering the outside flue surfaces. Everything except do the fix that you recommend here!

    However, my question is not about the re-rendering of the interior walls (which we are in the process of getting done), it’s to do with ventilation. The front flue (lounge) ground floor fireplace has no vent in it, just boarded off, and the rear flue (dining room) fireplace has been completely removed and is now a flat wall, no ground floor ventilation either. Only the external air brick on each flue around 1m below the roof line.

    Do you recommend adding in ground floor vents to create better air flow through the flue, or would this encourage increased damp inside the flue during cold weather as the warm air rising causes moisture to settle on the flue interior?

    Thanks if you can offer any practical advice on how to play this one. We are in the process of engaging a local PCA accredited surveyor but would appreciate your perspective too. They highlighted the need for better ventilation at the time of survey but did not mention remedial ventilation actions in the report.

    All the best

    • Dry Rot says

      Hi Dickie,

      Thanks for getting in touch – I missed your comment in the moderation list – sorry if this is a bit late.

      Whether it is sensible to have a vent internally depends on the environment in the house. If you have good control of humidity, with extraction in the kitchen and bathroom then fine. If on the other hand you find your windows are streaming on cold mornings and occupancy is high then venting them internally may actually increase the amount of condensate forming inside.

      Where venting redundant flues is concerned it should be done as soon as the flue is made redundant. If it has already suffered so that you need to replaster because of salts, it’s a bit late. However, if there are still large areas of ‘at risk’ original plaster on the breasts then it’s still worth venting.

      Good luck with and forgive my omission back in November please.


  43. Richard Black says

    Hi Bryan,

    You are a credit to your profession and the PCA, thanks for taking the
    time to write these articles and respond to queries like you do.

    I just moved to a 1930’s detached place with 2 unused chimney flues on
    the left flank exterior wall (which are capped off and reduced down to
    roof level). The purchase survey highlighted some dampness on the
    chimney breast on the inside walls which we have investigated further
    and found to be salt damp. Roofer found the roof to be old but sound.

    The previous occupiers had apparently tried a number of fixes like
    fitting an external air brick towards the top of each flue and
    rendering the outside flue surfaces. Everything except do the fix that
    you recommend here!

    However, my question is not about the re-rendering of the interior
    walls (which we are in the process of getting done), it’s to do with
    ventilation. The front flue (lounge) ground floor fireplace has no
    vent in it, just boarded off, and the rear flue (dining room)
    fireplace has been completely removed and is now a flat wall, no
    ground floor ventilation either. Only the external air brick on each
    flue around 1m below the roof line.

    Do you recommend adding in ground floor vents to create better air
    flow through the flue, or would this encourage increased damp inside
    the flue during cold weather as the warm air rising causes moisture to
    settle on the flue interior?

    Thanks if you can offer any practical advice on how to play this one.
    We are in the process of engaging a local PCA accredited surveyor but
    would appreciate your perspective too. They highlighted the need for
    better ventilation at the time of survey but did not mention remedial
    ventilation actions in the report.

    All the best

    • Dry Rot says

      Hi Richard,

      Thank you – I’m glad you like the blog.

      BRE good building guides recommend venting top and bottom. However, whether that will cause a problem with moist air from the house condensing in the flues depends on the condition in the house in the first place. If you’ve read much of my stuff you’ll know that I like to see good extraction in all wet rooms, like kitchen, bathroom etc. This is especially the case where old open flues are no longer used. In effect ‘old’ houses, which have been made ‘modern’ by double glazing, draft exclusion and redundant fireplaces, tend to have higher vapour pressures in them. This air may be unsuitable for passing up through an unlined flue, whee it can condense and also be drawn into the salt contaminated masonry. So my advice is vent the flue, only if you know you have good control of water vapour in the house. If not then don’t bother or better still, install the correct extraction and then vent as per BRE advice.

      hope this helps and thank you for your kind comments.


  44. kelvin abbott says

    Hi ive tasted the wet on the walls, it doesnt taste of salt, it feels greasy to touch, someone said it could be cold/thermal bridging and to fill the chimney void with vermiculite.

    Thanks for your help

  45. kelvin abbott says

    Ok so ive just tasted again, yes it has a really salty taste.

  46. Al McNay says

    Hello Bryan, really pleased to have found this interesting and nuanced discussion of this topic. I had no knowledge of hygroscopic salt damp until I called on Peter Cox to re inspect some work I’d had done by them on our Victorian home; I’d assumed their chemical DPC had failed, but was informed that it was residual salts in the brickwork peeping through because I hadn’t followed the spec for re-plastering. This was chastising and I’ve been reminded of my naïveté by several ugly and permanent dark patches on the dining room chimney breast and exterior walls since.
    I’ve just stripped the walls ready to address the problem and would like some advice. My plan is to remove plaster back to brickwork in the affected areas and treat walls with salt neutraliser. I am then either going to render with mortar with a wAterproofing additive or plasterboard prior to plastering. I’ve read so much about allowing Victorian houses to breathe that I’m anxious about effectively ‘tanking’ large areas of wall with a cement render. Is there a preferable approach between cement plaster and plasterboard in this respect? And could you advise me about what you mean when you mention ‘independent lathing system for plasterboard’ – how do you isolate the board?

    Many thanks


    • Dry Rot says

      Hi Al,

      Thank you for getting in touch. Firstly I’d like to apologise for the delay in getting back to you. I was away at the time and have big backlog. |Anyway, I hope this helps you.

      Any wall you have with guarantees on should only be plastered to the contractors specification. I can’t interfere there because it would not be in your interests to stray from their advice – it’s what you have paid them for.

      However, where there is a possibility of salt contamination in very old brickwork there are options where re-plastering is concerned:

      1 – Use a lime based plaster like the original
      2 – Use a sand/cement mix
      3 – Use a dry-lining system

      Lime is more permeable and softer, so physically it is very sympathetic to an old soft substrate. This is good. It is also quite permeable. The permeability is great for a non-contaminated surface. However, if there are high levels of hygroscopic salts it will be at risk of contamination itself. This means that the new plaster may not give you a dry surface for decoration. It can also work out expensive because whilst the methods and materials are not expensive, you need experience and skill to use it correctly. It’s not hard but as with anything defined as ‘special’ there are some who will charge lots for it. I know some ‘Heritage’ types who make a small fortune over-charging gullible consumers for this stuff, so as with all things, shop around.

      Sand cement is denser (if the sand is the sharp well-graded type and the mix is no more than 3:1 for the initial base coat). This is hard stuff and so care is needed, especially on soft backgrounds, such as hand-made bricks or some stone. 18mm cover is enough to ensure that no hygroscopic salts will pass through to the face (they move in solution and it stops that). You can apply a weaker mix on top or a cement or lime based product if you’d like a more permeable and ‘warmer’ finish. The wall must be well raked to spread the tension forces during curing and avoid crazing and damage to the substrate. It should never need removing but if it is done wrong and does need replacing, it’s removal will damage soft substrates. Thus it is not the thing for historic fabric. Permeability is low. However, it’s often maligned as always ‘a bad thing’ due to lower permeability. It is a bad thing if cement is used where the moisture source is not controlled – for example on an external face of a wall, as external render or pointing. In these cases the wall is still getting very wet and cement will just trap water in and make things gradually worse. In the inside of a wall, you balance the benefit against the potential for problems. Contrary to advice from some parts – it will not cause condensation in houses. However, it will highlight excess humidity and condensation will happen on it in these cases – it acts like a canary in a mine in that regard. In a well heated and ventilated dwelling, where there may be a need to keep salts in a wall at bay – it’s a useful tool. All cement products can be destroyed if there is high sulphate levels in the wall – if in doubt (such as very old chimneys), use special sulphate resistant cement, which is available from all good merchants.

      Dry lining is a great way of providing a dry surface for decorations. What you are doing really is using a plasterboard (or tradition timber laths), as base for the new plaster. Which can be gypsum or lime or even a weaker mix cement if you wish. The plasterboard is vulnerable to hygroscopic salt contamination so it must be either protected by some form of metal or timber lathing system, or a vertical damp proof membrane (DPM). It sounds hard but really it is not. Laths can be screwed to a wall and the plasterboard merely nailed to it. Metal laths are a good bet too (see you merchant). If a wall is particularly wet and will take a long time to dry out a DPM is the best bet (we don’t want to risk the wooden laths getting fungal decay). Most DPM’s a a vapour check, so they will reduce permeability and evaporation. On an external wall the vast majority of moisture evaporates externally (wind and sunshine driven). There is very little evaporation to the internal face in buildings. However, for example, if you applied a 1m square DPM to a 1m square damp patch, you could easily end up with a ring of damp around the new work, as the damp – spread-out due to the reduced evaporation of the residual moisture. Always go well past for this reason and remember that the source of water should be controlled firs, so that drying happens eventually. Don’t forget that the dry-lining is a decorative measure – the cause of the original damp problem should be identified and dealt with if possible first.

      Advantages for a dry-lining method are very quick drying of the surface, because only a skim of this plaster is applied, rather than 20mm plus of backing coat. Never – ever apply plasterboard by the dot and dab method onto potentially salt contaminated masonry. Even if the wall is dry, always consider the possibility of cold bridging at the dab positions – these will attract condensate if the humidity is excessive. You can apply dot and dab on a DPM, but the ventilation and heating should still be good. Another advantage of dry lining is that internal insulation can be easily incorporated. Disadvantage include lost space (it’s thicker) and the associated need to re-position or replace sockets, joinery and such. In historic situations it has the advantage of being ‘reversible’ and our heritage friends like that. Removal of it will not break off the face of old bricks. Never ever put a vertical DPM over even the smallest but of built-in wood…. there lies the path to dry rot!

      I hope this helps Al

  47. Interesting reading, especially as my cottage has had a damp problem on the ceiling/upper wall by the chimney for most of the 10 years that I’ve owned it. I think the previous owner has redecorated to cover it up. My cottage is over a hundred years old and of solid brick construction. The stack is on the north gable and 2 of the 3 flues are not in use. There was only one pot when I bought the property but I had the other 2 pots reinstated and added mushroom type tops for ventilation. The flue to the front bedroom was already closed off and the flue to the rear is supported in the loft as the rest has been removed. The flue from the sitting room has been cast lined as I had hoped to install a stove. The lead soakers have been replaced, flashing redone and rechecked, flaunching done twice, north wall and stack repointed and also sealed on two occasions, slates checked etc. At one point we thought the problem had been sorted so I had the plaster hacked off and replastering done. No such luck, the ceiling still shows evidence of damp but the amount varies. The builder has put 3 vents in the gable wall in the loft to aid ventilation and also added one near the bottom of what remains of the third flue. Externally a damp area appears in the brickwork just beneath where the flues meet in the loft, directly under the stack. At worst it extends to about 3′ x 4′ approx. Internally repointing has been done to the flues in the loft at which time we also removed bricks higher up in order to take photos up the flues. It was January and it was definitely wet up there. We also opened up a small area where the front bedroom fireplace had been and as it was dry at that level it was then reclosed. On the bedroom ceiling the damp patch extends about a foot by 18 inches and about a foot or so down the wall. It tends to be smaller in summer but there’s always evidence and now some salts. I really need to get this fixed as I need to move area and the problem is just as bad as it was before the works were done. I did once see clear water coming out of a brick in the loft about a foot below where the flues join so one of the airbricks was put in that area in the hope that ventilation would help. Condensation in the flues seems to be a likely issue but, due to the dampness getting worse after periods of rain, ingression also seems possible, despite the builder not being able to identify how it is getting in. Reading the thread has been very interesting and i’ll definitely try the “taste test” if I see water coming out of the brick in the loft again. Do you think it’s likely to be just a combination of condensation and salts? Or is it possible that a combination of freezing weather conditions and water in the brickwork has caused deterioration and is allowing water into the bricks? Am I over thinking this? The damp area outside does slowly reduce in size during dry spells and sometimes it disappears in summer but it always reappears after a lengthy spell of heavy rain. The damp on the bedroom ceiling/walls varies. Can you offer any advice?

    • Dry Rot says

      Hi Jill,

      It was a good read and clearly this has been a long road.

      Assuming your builder/roofer is a good lad we can rule out a bad penetrating rainwater problem. |You may still get rainwater coming through a solid wall due to porosity, but this is quite rare, otherwise almost all solid wall houses would be damp and they are not.

      So we need to be less worried about the stack in the loft. I never look at mine and I don’t care if it looks salty or a bit damp. It’s cold up there and you are looking at bricks which are saturated only a few hundred millimetres higher than that – just above the roof. So of course the bricks thin the loft will be damp sometimes – that is natural.

      Choose a dry period and boil a kettle in the room where the problem is, with the doors and windows closed and the turn off the radiator in that room). After a few minutes (ten to 20) the damp patches may look a bit more obvious. If that happens then you know that the issue is being influenced by hygroscopic salts. If that is the case then dry-line or plaster using the methods I’ve described to others above.

      You can get all scientific with it using gravimetric methods, like I do in my damp diagnosis lab here in Yorkshire. The method is to take a sample of the damp plaster and weigh it carefully (down to .000). Then expose it to high humidity for a few days (in a vessel over a saturated chloride salt solution will give 75%RH), then re-weigh. Then oven dry it overnight. If the sample loses weight in the vessel then there is ‘free water’ present. In other words there’s a leak or something. In many cases it does not lose weight or, it may even gain weight. If this weight is then lost in the oven it is the ‘hygroscopic’ moisture content. This is what salt damp is.

      Have a good time and don’t forget that the ceiling will need cutting back and replacing too if it is contaminated. When replacing a ceiling, the simple use of a couple of layers of masking tape or butyl tape against the wall will stop the new ceiling from touching the stack and pulling more salt through (posh firms like mine use a special mesh membrane called Oldroyd XP or Drybase Flex for this, but as this is my consumer blog you can have the cheap, non-guaranteed short cut 🙂 )


  48. Louise says

    Hello Bryan

    I was really hoping you could advise me as you have given so much advice to others & I certainly need it.

    We have one particular wall in our house which seems to meet all your criteria for us having a salt problem however after removing the log burner in the room below, we have also noticed we have water dripping in the corner of the heath when it is raining. We have had various repairs done to the roof and chimney stack to try and avoid it but nothing works. The roofer and damp proofer are unsure of where to go. I’m not sure if this is a stupid question but could the salts cause water to drip down the inside of the chimney or would we have to have a leak (that as of yet has not been found) for this to happen? I really want to get to the bottom of this and I need someone who knows what they are talking about to advise me. Please help.


    • Dry Rot says

      Hi Louise,

      This is a really fascinating one – thank you for getting in touch.

      Has the chimney got a cowl or just an open pot? Open pots can let rain down of course, but most flues are not simply a straight up tunnel and have twists and turns, feathers and such, so it’s usual for the rain to get right to the bottom hearth. If the chimney was on a gable then in driving rain the water could get in at a lower level via poor pointing etc but I assume that’s covered here?

      I have seen a wall ‘weep’ due to a combination of very high humidity and high hygroscopic salt contamination. However, if the flue is clear for the log burner it should be too well ventilated for that. However if the log burner uses it’s own liner and is only occasionally used you could get condensate and ‘salt damp’ in the space between the metal liner and the inside parging of the chimney. This is more likely if the old chimney was never swept. In cases where a house is very humid, air from the house can exit up the chimney (when the fire is not in use so the chimney is cold), then the warm and wet air condenses inside the chimney – have you decent extraction in the kitchen, bathroom and any utility rooms?

      I wish it was local as it sounds really interesting…

      good luck with it Louise.


      • Louise says

        Thank you for your reply. We have only had the house a year but have never used the log burner because the chimney isn’t lined – I was a bit concerned about fumes leaking considering it is an older house. It was when we took the log burner out to look into getting the flue lined we noticed the water dripping down. As it only happens when it’s raining we assumed it was a rain water leak which is why we had the roofer do all the various repairs. Yes all pots have cowls and the chimney has been swept (although soot does still fall down – making it easier to spot the drips). We have some fans i.e. a drimaster heat fitted directly outside the room upstairs we have the salt problem with and a nuaire fan in one bathroom (both these run all the time). We are aware we need more i.e. in the kitchen and the other bathroom but have yet to do it. The drimaster seams to do a great job of getting rid of condensation i.e. we don’t get any on windows etc but suppose that’s not a great indicator of himidity in the house. We do need to check the pointing more closely, which is going to be one of our next tasks – it was ruled out to begin with but now we need to go back to the drawing board. Thank you for your help.


  49. Aiders says

    I’m so relieved to have found this article as I think at long last it may explain why I have 3 huge terrible damp stains in my 3 upstairs bedrooms – they are all on the ceiling and down the wall of external chimney breasts.

    I’ve just had a very old roof replaced with a brand new one and was horrified to find that I still had this water coming through. My poor roofer has been in a miserable state trying to find out what he could have done wrong.

    I have 4 external chimney stacks and one of these has never caused a problem and has no damp marks around the internal chimney breast. When I read this article I thought, if this this the problem I’ve got, then that stack is vented and the other 3 aren’t. I went outside to see if this one has an external vent and found that it has an enormous old vent about 2 foot above ground level. The other 3 have no external vents. There are no internal vents where the old first floor fireplaces have been blocked up either.

    So if I put in external vents on the 3 other stacks, it seems likely that this would help stop the problem. From what I’ve read, it seems external vents would be better than internal vents – would you agree?

    The other thing I’ve then got to do is remove the plaster and coving which is impregnated with salts. I actually licked the wall – and yes it tastes salty. It’s a shame to wreck the coving in these 3 bedrooms but I guess there’s no other option really.

    Thank you for this article!

    • Dry Rot says

      Thanks for this one – I can see your roofer pulling his hair out…. poor chap.

      Ventilating the old flues is a good move. However you may find that the damp persists, particularly when the humidity is high. I understand your reticence to replace the old coving. If you don’t mind the wait you could try improving the humidity in the house first. Make sure that the wet room (bathroom, kitchen, utility and such), are all served by good extraction or you could have a PIV installed http://www.preservationexpert.co.uk/positive-input-ventilation-as-a-condensation-control-case-study-in-york-2the-data/ . This may reduce the severity of the damp stains, merely by reducing the excess water vapour the salts are absorbing. If this works then a ‘stain-block’ paint may be the answer. In the light of all the interest the post has engendered I feel I should point out that the salt damp problem is common and also a ‘cosmetic’ issue. So of course you do not have to do anything if the damp stains can be reduced cosmetically….. No dry rot, rampant damage or health problems were ever caused by salt damp, as far as I’m aware. Chill!

      best wishes


  50. Jeremy Taylor says

    Chimney stacks newly re -painted with sandtex 365 , chimney cowls ,flashing and slates ok , flaunching not to bad, no previous damp problem but both breasts now showing bad damp in bedrooms below . . . . am suspecting sandtex 365 (a solvent based paint ) not microporus after all and moisture ingress can not escape . Can anyone help ?

    • Dry Rot says

      Hi Jeremy,

      So the paint is on the external of this chimney – I assume it is rendred, not painted onto brick or stone? I doubt whether simply applying this paint would make the internal face damp in anything like a short peiod of time. Takes a long time for this to change the dynamics enough to cause a problem. It may be that a combination of the weather and temperature has raised the rH generally, in which case you may be seeing the effects of long standing hygroscopic salts (as long as there is no surface disruption). If there is surface disruption then water is getting in – nothing to do with the paint. Try the boiling kettle test? It may inform you.

      best of luck with it and do please keep me informed Jeremy.


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