Amber warning over DIY checks for rising damp.
This week I received a query from a reader about the interpretation of the results of a damp meter. It would appear that he had hired a damp meter, carried out his own inspection, and then tried to interpret the results.
Damp meters are quite simple pieces of equipment. They work by sending a small electric charge between two metal probes when they are pushed into the wall. The resistance is then measured as the charge passes between the probes. If there is total resistance, the wall is deemed not to be damp.
If there is no resistance, it is deemed that the wall is damp. The idea is that an electric charge will pass through the damp wall with little or no resistance.
You can see the folly in this. Other materials used in a wall can also freely conduct electric charges; some salts embedded in plaster will conduct electricity.
If the wall has been lined in foil-backed insulating paper, the foil will conduct the electric charge and skew the results.
Add to that the presence of condensation. Condensation can be sitting on the surface of the wall and will give little resistance to the electric charge, with the result that again, the meter will deem the whole wall to be damp, even though it is only the surface.
Even if the whole wall is damp, the damp meter will not tell you what the cause of the dampness is.
It will not reveal if the condensation has soaked into the plasterwork, if it is rainwater splashing on the wall or driven by heavy wind through to the inside, or if it is dampness rising from the ground. To identify the cause and therefore determine the remedy requires a lot more skill and knowledge than simply sticking the probes of a damp meter into the wall.
This reader had hired a damp meter which had traffic lights to indicate the degree of dampness. Green was to indicate that the wall was dry, red that it was damp and amber was to indicate caution.
The query he raised with me was, `What do I do with the amber readings'?
The answer is, don't rely on the damp meter but instruct somebody who understands the causes of dampness, the interpretation of those damp-meter readings and then the remedy.
I suppose the same should be said if we invite a contractor to look at the dampness and they use the damp meter. How do we know they are interpreting the results correctly?
How do we know they have recorded the results correctly? A favourite trick of cowboy damp-proofing contractors in the past was to hold their finger across the two metal probes as they pushed it into the wall. The finger conducts electricity far better than plaster and therefore it will always give a damp reading.
On that basis, they could claim a damp-proof course needed to be installed and all the plaster replaced, when it didn't. Not a bad way to get the unsuspecting to pay up, particularly as after the work is done they can claim to have cured the invented damp and no one knows any better.
I believe the days of the cowboy damp-proofing contractor are diminishing. We are fortunate that the industry has consolidated to a smaller number of more reputable contractors. But that wouldn't stop somebody setting up in business and trying it on.
So does this help us if we think we might have rising damp? The first point is, take advice from a company with a reputation. The trade association is the British Wood Preserving and Damp-Proofing Association. Their members need to fulfil certain criteria. If you are in doubt, appoint an independent professional. Chartered surveyors have no axe to grid and will be as pleased to tell you that it isn't rising damp as they would to say it is.
Rising damp cannot be cured by simply installing a damp-proof course. That is because the water rising from the ground through the wall carries salts with it. As the water migrates to the surface of the wall, so too do the salts. While the water will evaporate through the surface of the wall, the salts are left behind in the plaster.
Some of these salts have a problem in that they absorb moisture from the atmosphere - they are called hygroscopic salts. If you leave the plaster with the salts impregnated, you will find the wall continues to show signs of dampness, even though you have stopped the flow of water up from the ground. Because of this, you need to replace the old salt-laden plaster with new plaster that has chemicals introduced into it to compete against any salts that might be left on the brick wall behind.
It is usual to remove the plaster for at least 300mm (12in) above the top line of any dampness that you find. More often than not, that can mean the bottom 900mm (3ft) of the wall plaster has to be removed and replaced.
If the dampness has been occurring for many years, it is probable that the skirting boards and the wooden architraves around the doorways will be affected by dampness and probably need replaced. This dampness could have caused wet rot, or at its worst, dry rot.
It's just as well, therefore, that you don't just look at the plaster, but also at the skirtings and any floor joists and floorboards next to the wall.
Far better to have the whole area repaired and treated at this time, while you are in a great mess, than find you have to go back at some future date and pull it all to bits again.
* Peter W Fall is the senior partner of Peter Fall Cowie, Chartered Building Surveyors. The company can be contacted on (0191) 232-7733. To request back copies of House Doctor articles, visit www.pfc.uk.com
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|Publication:||The Journal (Newcastle, England)|
|Date:||Feb 28, 2004|
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