Why is my wall damp at the bottom?

The most misdiagnosed damp problem

Damp at the bottom of a wall is routinely labelled rising damp, and it is routinely not. The base of a wall is where several very different problems happen to look the same — and each needs a different remedy. Paying for a new damp-proof course when the cause is condensation or a bridged DPC solves nothing.

The likely candidates

Low-level damp usually comes down to one — or a combination — of the following:

• Condensation: the base of an external wall is often the coldest surface in the room, so airborne moisture condenses there first
• Bridging: render, paths, soil, debris or internal plaster crossing the damp-proof course and letting moisture bypass it
• Raised external ground levels: ground above the level of the DPC defeats it entirely
• Plumbing leaks: radiators, pipework in floors and slow supply leaks frequently show at low level first
• Rainwater splashback and defective rainwater goods wetting the base of the wall
• Genuine rising damp: ground moisture drawn up through the masonry where the DPC is absent, failed or bypassed

Clues worth noting

Does the damp worsen in winter, when windows are closed and heating cycles on and off? That points toward condensation. Does it worsen after heavy rain? That points toward penetration or splashback. Is it present all year regardless of weather, in a line up to about a metre? Rising damp moves up the list. None of these clues is conclusive on its own — but together they shape the investigation.

What a proper investigation does

A damp survey maps how the moisture is distributed, checks the level and condition of any damp-proof course, looks at external ground levels and defects, considers the internal environment and, where useful, tests for the salts that distinguish long-term rising damp from other causes. Only then is a remedy proposed.