Damp or a Leak? How to Tell the Difference

Condensation forms where warm, moist air meets a cold surface. It shows as black mould in corners, behind furniture and around windows, worst in winter and in rooms where washing is dried. Rising damp comes up from the ground through a wall without a working damp-proof course. It leaves a tide mark up to about a metre high with salts on the plaster, and it is confined to ground floor walls. Neither of them is a plumbing problem, and neither is helped by a leak survey, so it matters to tell them apart early.

Penetrating damp comes through the building from outside, from a blocked gutter, cracked render, a failed window seal or a chimney without a cap. It gets worse after rain and dries out in a dry spell. A chimney breast that is damp in the middle, well above skirting level, is often rain coming down an unused flue or salts drawn out of old soot. A plumbing leak ignores the weather entirely. It follows the pipes, gets worse when water or heating is used, and keeps coming back after it has been dried and repainted.

The survey starts with the pattern you describe, then tests it. A moisture meter maps the damp area and shows whether it is spreading from a point or rising from the floor. A water test strip on any free water tells mains water from heating water and from rain. The thermal imaging camera shows pipe runs behind the plaster and whether the wet area sits on one of them. Where a pipe is a suspect, a pressure test on the heating or the mains, and the meter test, prove whether water is being lost at all.

The answer may be that it is not a leak, and we say so. A report that rules out plumbing is still useful, because it points you to a roofer, a builder or better ventilation instead of another round of repainting. It also matters for insurance. Damage from a sudden escape of water is usually covered by a buildings policy, and trace and access cover pays for finding the leak. Gradual damp, condensation and rising damp are usually excluded, so knowing which you have decides whether a claim is worth making.