Rising Damp for beginners (and homeowners).
So someone says your house has rising damp?
This isn’t a post about whether you have rising damp or not, that’s already been done before. What this post is about is a very basic explanation of what rising damp is and how it happens, what it can do and how it can be controlled, avoided or lived with.
Ever let a towel dangle over the edge of the bath and into the water? It can happen if you over-fill the bath and forget to remove it. If you have you will have seen how water is ‘wicked-up’ the material, sometimes several centimeters above the water line. It happens with a sponge dipped in water or if a piece of chalk is dipped in a puddle also.
This is a basic mechanism for rising damp and it affect all porous materials, other than those, which are water repellent.
Masonry is much heavier and denser than a towel so you’d think that it would be less prone to soaking up water. However, the opposite is the case, because water will rise higher in materials with smaller voids (called pores), than large ones, so in fact a brick wall may ‘wick-up’ water as rising damp a long way up – say a meter or so, whilst you would never get that effect with a towel.
All walls get wet at the base. This is due to rainfall and the subsequent surface water. Don’t be confused with talk of ‘water tables’ and such – these are not relevant to rising damp, which will happen in a house in valley and on top of a mountain. Water is always on or near the surface and the base of a wall is usually deeply embedded in the soil, often on a foundation. The ‘pore’ size of the masonry is usually smaller than that of the soil, so when it rains, whilst the soil may be well drained, the wall can wick-up quite a bit of water, which is retained for some time – this is all rising damp is.
We are not talking water in the sense of say a leaky gutter or condensation – the wall will rarely be wet to the touch (almost never). This is because the mechanical action of rising damp would need to be reversed for surface water to happen…. Why would a wall, constructed in fairly homogenous material, suck water in at the base and then push it out of the face higher up? Can’t happen.
The amount of water retained in the base of the wall can be enough to cause problems:
- Staining of decorations
- Rot in timber fixed or embedded in the wall near the base
- Damage to plaster, which becomes dry and friable or crumbly
- Damage to the external stones, pointing or bricks near the base, causing Flaking (spalling) and salting.
So to stop rising damp, all houses constructed these days and since about 1920 have physical damp proof courses. All these are, is a barrier in the bed-joint near the base of the wall, which do not have pores – they act to insulate the pores in the bricks or stones above them from contact with water in the ground.
These are made from impermeable materials like bitumen-impregnated textiles, poured bitumen or these days, usually plastic. Slate and courses of hard dense ‘blue’ bricks have been used also.
So rising damp is common and rare? How’s that?
Rising damp is very common because the rise of water through these materials is a physical fact. Given damp soil and a porous wall it will happen. However, some soils are so free draining that they do not retain rainwater for any length of time, so rising damp can be less common in some areas.
In saying that, rising damp, even if present may be slight and barely noticed (unless you use an electronic moisture meter to find it).
Another contradiction is that though it is common, it is also rare in houses, which already have a physical DPC, installed. So most of the time, when rising damp is present in a post war house, it is due to the DPC being bypassed (bridged), so that damp can rise past it, rather than through it. Nevertheless, survey reports do sometime refer to the existing damp course being ‘perished’ and damp proofing, in the form of a new DPC is recommended. In my experience most physical DPC’s are in good condition; despite the rising damp symptoms. Damp proofing may not be needed.
Where rising damp is present, but is not causing visible issues it may not be such a problem. However, there are cases where it will cause issues, such as damage to timber floor joists, skirting and such, An electronic moisture meter will detect very small amounts of moisture and there may even be a ‘rising damp profile’. Does this mean you should install a new DPC? Perhaps, but not certainly.
What a rising damp profile via a meter does confirm is that you need to consider treatment.
Treatment may be as innocuous as lowering a flowerbed that has been allowed to ‘bridge’ the original DPC. It may mean cutting back plaster in contact with a damp concrete floor.
It may also mean installing a new DPC, via chemical injection; the damp-proofing method. However, in cases where only an electronic moisture meter confirms there is a problem, we need to put that in perspective.
It’s rather like a doctor asking you in for a free health check. You feel fine but agree because you’re getting older and why not? He tells you that your blood pressure is a little high and perhaps you should get some exercise and lower your salt intake…. Fair enough. You rest easy and maybe take the advice, but don’t get too worried. That is like a surveyor visiting your house because it’s for sale and finding high moisture meter readings.
Then there are the other cases…. This time there are symptoms, which can’t be ignored. You feel tired all the time and when going up stairs you find yourself feeling dizzy. You call the doctor and after a consultation you are told you have high blood pressure and are prescribed some tablets… of course you take them; you’d be mad not to. This is the case where the high readings are accompanied by stains, salting or rot. In these case damp proofing and possibly timber treatment will be needed (though as mentioned the work could be basic maintanance rather than chemical treatment).
The reason that rising damp has become a contentious issue is that the ‘doctors’ (surveyors) are prescribing tablets and surger,y (chemical DPC’s) and such, when the symptoms are so slight, that only very sensitive equipment can find it. This is why damp proofing specialists like me are no longer trusted – there have been too many chemical DPC’s installed for insufficient reason.
I am not saying that rising damp; even technical rising damp found via a meter should be ignored, but I am saying it should be taken in the context of the situation. Expensive and disruptive intervention is not always required or justified.
Bryan Hindle CSRT CSSW AIOSH
For further reading on rising damp try these useful posts: