Last edited 26 Oct 2020

Rising damp

Excess moisture is a common problem in buildings, and may be apparent from; damp patches, mould growth, mildew, salts, staining, ‘tide marks', blistering paint, bulging plaster, and so on.

The most common causes of persistent damp in buildings are:

Condensation (surface or interstitial).
Penetrating damp.
Rising damp.

Rising damp is said to be caused by capillary action drawing moisture up through porous elements of a building’s fabric.

Rising damp might be apparent from:

Condensation or damp patches (typically up to 1 m above the floor).
Corrosion of metal elements such as beading.
Damp odours.
Timber decay, such as skirting boards.
Damage to surface finishes.
Tide marks and staining (typically up to 1 m above the floor).
The presence of white salts.
Health problems.

It should be noted that these problems are common to other sorts of damp, such as; cold bridges, lateral penetrating damp, surface condensation and interstitial condensation. It is claimed capillary action can only cause damp to rise approximately a meter above the source of the damp (depending on the nature of the materials, the presence of salts and the rate of evaporation), and so problems above this height probably have a different cause.

Dampness can be measured with electrical resistance meters, either on the surface, or within the building fabric itself. Generally, if the meter reading indicates that the fabric is dry, then it is dry. However, electrical resistance meters were developed for use in timber, and if the reading indicates the fabric is wet, this does not necessarily mean that it is wet, as the presence of other substances such as soluble salts will give a similar reading, and in older walls, salts may be present even where damp is not.

Carbide meters are likely to give a more accurate measure of moisture content, and this can be further enhanced by testing samples drilled from the building fabric and tested for hygroscopic moisture content. This may require appointing a specialist.

Further information about testing techniques, the cause of dampness and remedies is available from BRE Digest 245, Rising damp in walls - diagnosis and treatment.

Rising damp has been attributed to faults in, or the absence of, a damp proof course (DPC). This can be exacerbated by:

The moisture content of the building fabric itself.
Raised ground water levels.
Raised ground levels around a building.
Leaks to pipework or guttering.
Flooding.
The presence of salts in the building fabric.
Crystallisation of salts on surfaces resulting in reduced evaporation.
Cool internal temperatures or internal humidity reducing the rate of evaporation.

The height of rising damp depends on:

Temperature gradients in walls - the coldest area typically being near ground level.
The rate of evaporation from the wall.
The porosity of the wall.
The salt content of the wall and soil.
Heating in the building.

If there is a specific source of damp, this should be corrected before treating rising damp, for example, fixing leaking pipes, altering ground levels, installing drainage, and so on.

Treatment of rising damp involves creating a moisture impermeable layer within the building fabric, either by the insertion of a damp proof course, or by injection of water-repellent chemicals. Treatment generally also involves remedial work to any corroded or decayed elements of the building fabric, as well as hacking off and replacing existing plaster to a height of 1 m. This can be disruptive as it involves removing skirting, sockets, and so on.