Damp Patch on the Wall? Condensation, Rising Damp, or a Leak

Condensation is the most common cause of damp. It appears on cold surfaces — external walls, window reveals, behind furniture, in corners — and is worst in winter. It often comes with black mould spots and streaming windows. The fix is to reduce moisture (lids on pans, extractor fans, drying clothes outside or with a vented dryer) and keep the home evenly heated.

Water coming through the wall from outside causes patches that grow after heavy rain, often at any height. Common sources are blocked or leaking gutters, cracked render, failed pointing, and damaged flashing. Check the outside wall directly opposite the patch.

True rising damp — groundwater wicking up through masonry — is rarer than often claimed. It shows as a tide mark up to about a metre above the floor, with salts that make plaster crumble. Raised outside ground levels or soil heaped against the wall can bridge an existing damp-proof course, so check those first.

A patch that stays damp regardless of weather, or appears near pipes, a bathroom, or a radiator, may be a plumbing leak. Turn off all taps and check whether the water meter keeps moving. If it does, there is a leak somewhere, and a plumber with a moisture meter or thermal camera can trace it.

If you cannot identify the cause, consider a surveyor who does not sell damp treatments. Their report should name the cause and the fix, rather than recommending a chemical injection by default.