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.
Above – stains of the face of the front bedroom chimney breast
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
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.
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*:
- Previous leaks, which allowed rainwater to soak through the chimney, dissolving salts and contaminating the plaster
- 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.
*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.