Diagnosing Rising Damp

How symptoms may be misinterpreted

A damp wall does not necessarily mean "rising damp"! Visible moisture contamination may be caused by one or a number of contributing factors which, to the inexperienced investigator, could be misinterpreted. Rising damp, lateral or penetrating damp and condensation are all problems of concern but result from different causes.

Skilful, intelligent analysis could save thousands of Rands as incorrect or misinterpreted diagnostic practices could lead to considerable unnecessary expenditure. Many symptoms are common to different causes.

True "rising damp" is normally associated with surface salt deposits, crumbling fretting bricks, mortar loss, bubbling flaking paint with visible surface salts, and deteriorating plaster and should not be confused with saturation from other sources or the masonry’s own natural "ambient" moisture content. A "damp" reading alone will not confirm rising damp unless other factors, including the location and direction of moisture ingress, are considered. At 5% moisture content some mortars may be considered dry whilst others at the same 5% moisture content could be considered damp, similar variances apply to clay and concrete.

Pinpointing the source of moisture is extremely important as causes may range from leaks around windows, leaking bathroom/kitchen plumbing, leaking down-pipes and/or spouting, condensation, inadequate sub-floor ventilation etc. Various methods exist in establishing moisture contents in substrates:

The gravimetric or oven drying method is the only accurate method in establishing the true moisture content of a substrate. It is a destructive method in that drillings must be taken from the wall. It is also time consuming and obviously requires the use of laboratory facilities and therefore cannot be used on site. It is by far the most accurate method available and can be used to distinguish between ground moisture and hygroscopic moisture.

The Electrical Methods
These are non destructive and readings are taken easily and quickly. In the majority of walls which are of uncertain composition, the electric method does not provide a percentage reading of the moisture content since readings may vary from material to material. Readings are usually recorded on a relative or arbitary scale which enables the pattern of moisture to be established. The electrical method does not give a direct reading of moisture content and its value depends much upon the ability of the user to interpret the results obtained by its use. In experienced hands it is a useful tool but it can give rise to confusion and incorrect diagnosis, Electronic diagnostic tools, such as the Doser DM4A, (used on calibrated setting B2), or the Protimeter Surveymaster SM or MMS, (in sonic "Search Mode"), are quick and easy to use devices that will give an immediate indication of the presence of moisture up to depths of 25mm. Diagnostic tools will not only indicate moisture but through the skilful interpretation of moisture concentrations and positions, the source and direction of moisture ingress can be deduced. Relying purely on a moisture meter to determine “rising damp” is a common contributor to misdiagnosing rising damp. Electrical moisture meters, however, have an enormous advantage for the general survey — they are clean and non-destructive. But there are also limitations — they do not quantitatively measure moisture on/in masonry substrates, such as plaster, brick, stone, mortar, etc. The majority record electrical resistance between two applied electrodes; a few measure capacitance, and more recently some measure the reflection of radio frequency emissions from the meter: moisture, together with one or two other materials, affects these properties. Thus, such meters can reflect the presence (or absence) of moisture and/or other factors which may be present.

Electronic moisture meter readings do not diagnose “rising damp”, but merely indicates the presence of moisture, unfortunately it is often presented as “evidence” that “rising damp” exists by inexperienced and misinformed assessors.