Sampling for Moisture and Soluble Salt 'Profiles'

When investigating dampness it is very often essential to accurately evaluate the cause of that dampness. Where a definitive diagnosis is required, perhaps for litigation purposes, this will require sampling from which data can be obtained regarding the distribution of water within the material. It is this distribution of moisture which identifies whether there is an actual source of water ingress (free moisture) or whether the 'dampness' has some other cause ('salt damp' - hygroscopic moisture).

It is often said that moisture contents up to 5% are perfectly acceptable; this is very frequently not the case: to use such a figure as the 'acceptable' moisture content can lead to problems. The important factor is usually free moisture which indicates some form of water ingress. It is therefore necessary to identify such ingress.

The following is a guide on how to remove samples for moisture and soluble salt 'profiling'. By removing a vertical series of samples up wall a full moisture and soluble salt profile can be produced. Analyses of the samples will provide a full and detailed distribution of moisture within a wall; this, for example, can identify the source of water, its distribution, and if coupled with a soluble salt analyses, it can readily identify the presence of rising damp and if necessary the precise performance of a remedial damp-proof course. It will also identify whether it dampness is due solely to hygroscopic salts ('salt damp') and/or actual water ingress. In all it is a very powerful tool in the diagnosis of dampness.

Equipment: Slow speed drill Drill bits (masonry) 10mm - 15mm Rule Airtight containers Moisture meter with deep probes (preferred) Labels Notepad
Method: If you are going to evaluate the performance of a remedial damp-proof course then it is important that other potential factors such as leaking gutters/downpipes, internal leaks, etc, are not influencing the area of investigation. Therefore it is important to check conditions externally to eliminate the possibility of such factors. Of course it could be that one is trying to determine whether such factors are indeed responsible for water ingress. Internally, there should not be such extraneous factors other than perhaps a high floor level in an adjoining property or possibly some bridging by internal plasterwork. In the former case this will clearly have to be determined by observation, and in the latter case dampness is usually restricted to around a maximum of 200-300 mm above floor level. Again possible bridging can be determined by evaluating the structure of the floor/wall junction.
Deciding the drilling pattern: Identify the height to which dampness is apparently present. This is best achieved by the use of an electrical moisture meter. Without one however you would be best to assume at least 1.5 metres unless otherwise obvious.
If damp is present up to around 1 metre then mark positions vertically starting at 100-150 mm above floor level, then mark 250, 500, 1000, and so on up to around 1500 mm. If damp is present above one metre then mark positions 100-150, 300, 600, 900 mm, and so on, and be sure to take two samples above the height of the suspected dampness. Thus, in both cases you should collect 6 - 8 vertical samples.
Where wallpaper is present decorative spoiling can be minimised by carefully cutting out 3 sides of a square and lifting the paper away from the wall. After drilling the paper can be stuck down again thus minimising any obvious spoiling.
Ideally one should take mortar samples; these are most consistent. However, in many cases this will mean it chasing out a vertical groove a in the finish to exposed mortar beds; this is frequently not feasible due to the damage. Thus, most drilling is done 'blind', ie, directly through the render/plaster finish to take what ever is beneath, eg, brick/mortar. If this is the case ensure their is consistency in sampling and so exclude plasters/renders. As long as condensation isn't occurring then it is quite reasonable to drill wall plaster/render samples. This may also be necessary when dealing with hard stone/brick walls. A larger drill bit is useful since plasters and renders are of limited thickness, and more can easily be collected with a larger bit. In many ways there is an advantage to plaster/render sampling in that it is the surface materials in which the dampness becomes manifest to the observer.
Collecting samples: Samples should be taken by means of a slow speed drill; use a sharp masonry bit of between 10-15 mm in diameter. If collecting brick/mortar then clearly drill through the plaster/render. If collecting render/plasterwork only use this material (it is easier to use a 15 mm drill bit and take from two immediately adjacent holes to get sufficient sample). Collect samples carefully in a small airtight containers. The most convenient are plastic 35 mm film containers which are readily available from most photo shops. Small airtight containers of any type, however, can be used. Hold the container immediately beneath the drill against the substrate and collect the drillings. Collect about one-quarter container at minimum.
Immediately seal the top - make sure it is seated and fully airtight. If the samples are to be posted then also seal lids with tape - they can pop off in the post. Finally label the container showing position height and if possible, the material. Make sure the label is fixed firmly - do not write directly on the container since this can rub off.
If you have a moisture meter and deep probes then it is prudent to check holes after drilling to ensure that you have sampled above the 'damp' area; sometimes surfaces at high level appear dry but the underlying material can still be 'damp'. The use of deep probes will ensure that you have sampled to sufficient height, ie, above the level of dampness. There may be some cases where the dampness does reach full ceiling height. If this is the case then you may have to go into the room above and complete the profile.

When all samples have been removed then they can be analysed for soluble salt and moisture content, and a moisture profile produced like that shown below.

The moisture and soluble salt profile:

The basic moisture and salt profiles:

The height to which the ground water salts (chloride and nitrate) have risen effectively gives us the history of the rising damp; they will always mark the maximum height to which water has risen. Therefore we can use this fact to evaluate the performance of remedial damp-proof courses.

1. Rising damp/failed damp-proof course:

The salts mark the maximum height to which water once rose, but in this profile free water is still rising to that height. This indicates that any remedial damp-proof course has not proved effective ( remedial damp-proof courses don't fail - where a failure exists it didn't work from day 1!). If there has been no dpc installation the profile shows that rising dampness is the cause of the dampness, and the height to which it is occurring.

2. Controlled rising damp:

In this case the free water is not rising to the maximum height of the salts. Thus, as the salts mark the maximum height to which water once rose, it is quite clear that water is no longer rising to this height. We therefore have reduced or 'controlled' rising dampness.

3. Water penetration:

In the final case we have free water present above the height of the salts. This indicates that there is some form of water ingress occurring above the height that rising damp once rose to. When this situation arises then one cannot often make any comment as to the performance of any remedial damp-proof course because one doesn't know whether the free water below the height of the salts is due to continued rising damp or from the water ingress.

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© G.R.Coleman 2000