Last edited 24 Mar 2021

Rising damp

Rising damp.jpg

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:

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:

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 height of rising damp depends on:

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.

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