What’s causing a wet patch on my internal walls?

I have a wet patch on two internal walls between bedrooms. No mould or mildew. My house is a granite cottage built in 1918. What do you think is causing it and how can I get rid of it?

There are a number of reasons as to why the wall could be damp. Normally the location of the damp patch would assist in diagnosing the potential source and cause. For example, a damp patch at low level or close to the floor could be indicative of a problem with rising dampness which would be typical in older properties of this nature as many properties built around this time would not contain any form of damp-proof course within the wall construction.

Despite the lack of damp-proof courses within houses of this age, they can often perform reasonably well provided that there is good drainage in the area. However, dampness can rise up through the walls in localised areas, particularly if there is a concentrated but localised source of water in small areas such as an underground spring or a defective drainpipe. Other potential sources include penetrating dampness.

However, as the problem you are experiencing is with internal walls, it is most unlikely that you will have an issue with damp penetration through the external walls which usually is a common source of water as a result of defective rainwater gutters, poor roof overhangs or even holes or cracks in the external walls.

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There could, however, be issues with an internal leak such as a plumbing appliance or a pipe buried within or passing through the wall which is serving plumbing appliances. These are the typical causes of dampness and can quite easily be identified and, in turn, addressed.

Wall plaster

Another possible – and in this case highly likely – but less obvious, source of dampness is due to an inappropriate type of wall plaster. Very often, particularly as part of remedial works, localised plaster repairs are carried out to walls using “bonding” plaster.

“Bonding” plaster is a favoured material for localised plaster repairs to damaged/uneven walls because it is relatively soft and easily applied.

The problem here, however, is that such a plaster type is not suitable if there is any potential for dampness at all as the bonding plaster tends to act as a poultice which can either suction dampness out of the background wall or even absorb moisture-laden air into the wall, giving rise to the type of stain you have described. If this is the case, the only effective remedy is to take off the unsuitable plaster and replaster the damaged area with a breathable or lime-based plaster.

Whereas there are quite a few potential causes of damp staining of this nature, an experienced surveyor will be able on investigation to diagnose the most likely potential cause and to recommend an appropriate course of action to rectify the problem.

Val O'Brien is a Chartered Building Surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie