Where Damp Comes From

Three houses in one street each have a damp patch in the same spot on the same wall. One has rain getting in, one has moisture from the ground, and one has condensation from the air in the room. Same patch, three different causes, three different fixes.

Water reaches the inside of a house by four routes: rain from outside, ground moisture from below, condensation from the air indoors, and pipes. More than one can be at work at once. Each leaves fingerprints: where the damp sits, when it gets worse, and what it leaves behind. Reading those is most of the work, and it is what lets you judge anyone’s diagnosis later.

One thing comes first. If a damp patch is on a ceiling that sags, or near a light fitting or socket, keep people away from it and don’t touch the fittings. Tracking the Leak covers what to do.