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How damp is damp?

One frequently gets requests to undertake an investigation for dampness following building surveyors reports usually prior to the sale of a house. Inevitably they have found dampness with the aid of an electrical moisture meter and requested further investigation.

On questioning the client over this reported dampness it often becomes evident that there is no visible or physical problem - decorations are in good order, no peeling or lifting of wallpaper, no staining, no eruption of plaster finish or paint film. Clearly no obvious problem. Yet it was reported damp.

So how can we sensibly define damp within a building?

A definition of damp?

A good friend and experienced colleague of mine, John Bricknell, defines damp as, "Water where is it is unwanted in quantities which may cause a problem". In the above scenario water may present yet it was not at a sufficient level to be causing spoiling or damage.

So what is the problem? Let's take a case of rising damp (yes, it does exist!!) and assume no other influencing factors such as condensation or penetrating damp.

In many cases the difficulty lies with the very high sensitivity of electrical moisture meters to very small levels of free moisture and/or contaminant salts. For example free water levels of less than a 0.1% may result in the significant moisture meter readings being obtained, yet such a level in most masonry, render and plaster substrates is unlikely to be a significant problem.

To treat or not?

The question then arises, "OK, in such cases should we treat it?" Should the wall, for example, be damp-proofed and replastered?

The answer is possibly leave it; there is clearly not sufficient moisture to be causing decorative spoiling.

However, there are two provisos:

(1) keep an eye on it
(2) ensure that wood (e.g. skirtings, joist ends, etc.) are not left in contact with any dampness, even low level residual dampness (damp wood will always be at risk to rot!!). In the latter case it would be very prudent to investigate and evaluate such timbers.

The only problem with following this approach is that when an owner with a reported 'damp' problem that can only be detected with very sensitive instruments leaves it, and then eventually tries to sell the property, the next building Surveyor is likely to say, "Some dampness in walls - get the services of a specialist company or make further investigations"

Unfortunately, it is one those situations that you cannot win in that the buyer will not except the "damp" albeit at a level which is not effectively causing a problem.


But there is a warning that cannot be ignored.

As described above the damp may not be causing a problem, ie, no visible signs, staining, etc. However, changing the decorative finish may lead to some manifestation of a decorative problem. For example, using an emulsion paint finish all may be well - no visible problems. But change, say, to a wallpaper finish then a problem may occur. In the case of wallpaper it is usually loss of adhesion in that sufficient moisture passes through into the underlying finish to cause the glue to soften and lose adhesion; the wallpaper will lift and bubble up: this is especailly the case with vinyl based wallpapers. Of course, the reverse may also be true - remove the wallpaper and use an emulsion finish and the finish might remain perfect.

Risk and reality:

If one looks at the above objectively then it is clear or that you cannot blame the building Surveyor or, indeed, any specialist damp company because they may never be sure using the instruments they have on site are reflecting low residual levels of moisture. It would certainly be difficult to make an assessment, for example, of joist ends embedded in a wall without opening up. And, of course, he would not like to make an error on behalf of the potential purchaser. There are those who seem to berate the surveyors for being so cautious because they consider the surveyors to be too worried about protecting their professional indemnity. But then who wants to be sued? It is clearly the better to be safe than sorry, and correctly exercising a duty of care to the client. And, of course, there is the potential of spoiling should the decorative finish be changed at a later date.

Where such finding do initially suggest very low levels of dampness then the risks of leaving it versus treatment should be identified and carefully considered; this is especially important in the case of timber. In reality, however, one can effectively go round and round in circles without a definitive answer. If it is one's own property then it is quite possible to follow the 'leave it' approach and be aware of the risks. But a prospective purchaser is likely to want a 'dry' house, especially if his or her surveyor has identified dampness.

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