Sources of moisture

The greatest diagnostic challenge is usually the lower part of the building, where the base of walls, floors and foundations interact, because all the sources of dampness may feature. Moisture from below the ground is the most difficult to confirm because it is impossible to see the part of the building's underground construction through which the moisture might be travelling.

Below-ground moisture sources include:

• 'free' water in the subsoil (possibly due to a high or perched water table);
• build-up from drain or pipe leaks;
• a water main breach; 
• an underground watercourse; and
• natural water table, which will be prone to fluctuation subject to the prevailing weather conditions.

Moisture can soak laterally into a cellar or basement, or it can soak up a wall from below ground. In designing water proofing schemes for basements or cellars below ground, British Standard 8102:2009 advises that it must be assumed that the surrounding water table could reach at least ¾’s of the height of the structure below ground. The latter is the classic 'rising dampness', which can be restricted to a short wall length, a particular elevation, or sometimes to numerous lengths of walls throughout a property. In this author’s experience it is more likely that rising dampness will be localised, patchy and to a variable height, rather than the uniform 'horizontal tidemark' that other authors mention.

Although it is not common practice, surveyors should consider excavating trial holes in a dampness investigation:

• to confirm the existence of a dpc that has been concealed by raised ground levels externally;
• to verify the construction of walls below ground; and
• to establish the height of the water table.

Sometimes it is possible to confirm that moisture has come from the ground by carrying out a chemical analysis. Salts testing, for example, may help you to confirm that salts found in walls or their finishes above ground could only have been derived from sources below ground. You could also trace constituents of moisture sampled above ground back to a source below ground (for example, contaminants that could only have been sourced from a nearby foul drain below ground).

It should be noted that for over 40 years the original (1962) BRE Digest 245 recognised that moisture sourced from the soil below the building resulting in rising dampness contained concentrations of chloride and nitrate salts. However, this has been shown to be unsound due to the presence of these salts in many other sources common to buildings and their use e.g. tap water, mortar additives, animal and human urine (see Common sources of moisture containing chloride and nitrate ions). The 2007 version of BRE Digest 245 has removed mention of chloride and nitrate salts and withdrawn the original figure 2 from the earlier version.

Moulds seldom develop on walls that are suffering from salt contamination caused by rising dampness, but those parts of the same wall with less concentration of salts may still exhibit mould growth typical of condensation.

However, care must be taken in knowing whether the salts being exhibited are salts from a source in the ground below the building or endemic salts from, for example, clay bricks in the form of efflorescent bloom, which could be associated with penetrating dampness and not rising dampness. See Causes, effects and solutions for more information.