Treating the wrong kind of damp wastes money, so correct diagnosis comes first. Most cases fall into one of three types.
Condensation
By far the most common. Warm, moist air meets a cold surface and turns to water. Look for droplets on windows, black mould in corners, and damp on north-facing walls. The cause is usually poor ventilation and moisture from cooking, showering and drying clothes indoors.
Fixes: extractor fans, regular airing, keeping the heating steady, and a dehumidifier in problem rooms.
Penetrating damp
Water enters through the building from outside, for example through a leaking roof, cracked render, blocked gutters or faulty pipework. Patches often appear after rain and may be isolated rather than low down on the wall.
Fixes: repair the source: gutters, roof tiles, pointing, flashing or plumbing, then let the wall dry.
Rising damp
Groundwater travels up through masonry by capillary action, typically seen as a tide mark up to about a metre high, with peeling paint or salts on the plaster. It is less common than many people assume and is often misdiagnosed.
Fixes: a damp-proof course or injected damp proofing, applied by a qualified surveyor or contractor.
Before you spend
Get an independent survey if you are unsure, because some firms recommend treatments that are not needed. This article is general guidance only.
