Guides · Damp guides
What causes damp patches on internal walls?
In short: The most common causes are water getting in from outside (a leaking gutter or downpipe, failed pointing or cracked render), a leaking pipe, condensation on a cold patch of wall, and, low down on ground-floor walls, rising or bridged damp. Where the patch is, its shape and whether it changes with the weather usually narrow it down, and a survey confirms it.
Patches that line up with something outside
If a damp patch sits behind a downpipe, below a gutter joint, under a window sill or level with a section of eroded pointing or cracked render, it is very likely penetrating damp. These patches are usually worse after heavy or wind-driven rain and can appear at any height, including upstairs. Older solid-walled houses, which are common in Hull and the East Riding, are especially prone because there is no cavity to stop water tracking through.
Patches near pipes, bathrooms and kitchens
A patch that grows steadily, regardless of the weather, near a radiator pipe, a bathroom on the other side of the wall or a kitchen sink often points to a slow plumbing leak. These are worth finding quickly because the wall keeps getting wetter.
Patches in corners and behind furniture
Damp and mould in the top corners of outside walls, around window reveals or behind wardrobes and beds is usually condensation: moisture in the air settling on the coldest surfaces. It is typically worst in winter. See damp or condensation?
Patches low down on ground-floor walls
Damp along the bottom of ground-floor walls, with a tide mark and white salts, can be rising damp, but check outside first: soil, paths or patios built up above the damp-proof course, or a rendered plinth bridging it, cause the same symptoms and are simpler to put right.
Patches in a chimney breast
Damp in a chimney breast can come from rain getting in at the top of an unused, capped-off or uncapped chimney, from failed flashing where the stack meets the roof, or from salts left in the brickwork by years of coal fires drawing moisture from the air. Each has a different fix, so it is worth diagnosing before replastering.
Finding the cause
Note where the patch is, whether it changes after rain or in cold weather, and whether anything on the outside of the wall lines up with it. Then get it checked: our damp survey costs £119, deducted from any treatment if you go ahead with our quote, and gives you a written report on the cause.
Frequently asked questions
What causes damp patches on internal walls: FAQs.
Should I paint over a damp patch?
Not until the cause is fixed and the wall has dried. Paint over a damp patch usually blisters or stains through, and it hides the evidence needed to diagnose the problem.
Why has a damp patch appeared on a wall that was fine for years?
Something has usually changed: a gutter has blocked or cracked, pointing has eroded, ground levels outside have been raised, or ventilation has been reduced, for example by new windows without trickle vents.
Can a damp patch dry out on its own?
If the cause was a one-off, such as a leak that has been fixed, yes, though a thick wall can take months. If the cause is still there, it will keep coming back.