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BRE Digest 245 'Rising damp in Walls: diagnosis and treatment' - A Guide or a Specification?

When evaluating dampness in buildings one of the most useful procedures is to investigate the distribution of moisture, that is the free and air dry moisture contents: the presence of free moisture indicates a source of water ingress.

The method of moisture analyses and removal of samples is given in Building Research Establishment Digest 245, 'Rising damp in walls: diagnosis and treatment'. The Digest describes the size of the drilled be used, the method of sampling and the regimes for drying and weighing in order to obtain the objective distribution of moisture. Whilst the method described in the Digest is basically for evaluating rising dampness, it is extremely useful for evaluating any source of dampness, especially 'salt damp', other than condensation.

However, recently it has become evident that some authorities insist that the methods described in the Digest must be adhered to rigidly, otherwise the results are regarded as not valid. For example, one must ignore the outer 20 mm of sample, use a 9 mm at drill as per the Digest, only take mortar samples, etc. But there is nothing in the Digest to suggest that sampling and other methods described should be that rigid in principle. As such the methodology in the Digest will need to be modified, as indeed it must for some purposes: if one didn't then the methodology would prove far too rigid for some purposes. The method is described in Digest 245 is for evaluating 'rising dampness'; frequently, an evaluation may be necessary to answer other questions.

When one decides that there is a dampness problem it is usually because because of the visual appearance of the surface, eg, spoiling/staining, and/or high surface moisture meter readings. It is important to note that the above are all manifest at the surface and are assessed literally at the surface, visually or by instruments. The question that must then be asked "Why ignore the surface when this is where the problem is manifest?". The answer is quite simply it doesn't matter where or what you sample - it depends on the question you are been asked or are trying to answer. In reality there is probably a strong argument to sample the outer 20 mm rather than discard it since this is far more likely to reflect the problem than deeper in the masonry; for example, the problem of hygroscopic salts is likely to be greater at the surface. Of course, you must eliminate at potential of surface condensation before sampling.

Another question that arises is relating to the use of a surface moisture meter. In such cases the question is often is the meter reflecting dampness or salt contamination? Clearly, in such cases it is essential that the surface is NOT ignored (say the out 10mm); after all, one wants to know if the meter is reflecting conditions at the surface, not deep within the wall! Indeed, note the title of the Digest, ''Rising damp in walls:diagnosis and treatment' - if one is looking at other factors such as described above then clearly the Digest as a whole is not applicable - but the basics of the method of determining the distribution of moisture is!

The Digest also recommends sampling mortar as this is usually 'wetter' than the surrounding brick. This is ideal because mortar is usually more consistent in its permeability /porosity than the surrounding brick. However, in practice it is not always possible to obtain all mortar samples; frequently one has to drill 'blind', that is through plasters and renders into the substrates because the client does not want the damage caused by removing plasterwork, etc, to expose mortar beds. Furthermore, going back to the question which is being asked, in one case the question was, "Are the surface moisture meter readings reflecting dampness or salt contamination?" In this case clearly it is more important to sample the surface, eg, the outer 20 mm, rather than discarding it as suggested in the BRE Digest; it is the surface that is under investigation, not deeper within the wall. After all, surface moisture meter readings are obtained from the surface and that is where the interest lies.

In many cases one could sample the plasterwork. Take, for example, very dense substrates such as granite walls, very dense brickwork, etc. Here it would be clearly difficult to drill the substrate, ie, the bricks or granite, if one wasn't allowed to expose mortar beds. In this case there would be no problem in taking plasterwork for analyses in that it would reflect quite evenly dampness in the underlying substrate or at least the pattern of dampness. Again, if there is a dampness problem it is usually manifest at the surface and this is the surface that reflects, in effect, the problem.

It must therefore be considered that the method described in the Building Research Establishment Digest forms the base of a method; the positions and materials sampled will depend on what is actually under investigation, what is the condition of the substrate, and finally what are you allowed to do in relation to exposing the substrate. Thus any type of sample can be taken provided that external factors such as condensation are eliminated and that one precisely states what samples of being taken and what the samples will reflect in relation to the distribution of moisture.


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