A damp patch on a wall that will not dry has a constant source of water behind it. Low on the wall points to the floor or ground, mid-height points to a pipe inside the wall, and high up points to something above. The shape narrows it further, and painting over it only hides the evidence.
You noticed a shadow on the paint a few months ago. It got bigger, developed a crusty edge, and no amount of heating or airing has shifted it. Maybe you have had it painted once and watched it bleed straight back through. A patch that refuses to dry is not a ventilation problem. Something is feeding it, and the wall has left you clues about what. This guide shows you how to read them.
Start with the height of the patch
Water moves in predictable ways: it runs down, it wicks up a short distance, and it spreads sideways along whatever it is touching. So the height of a damp patch on a wall is the most useful clue you have.
Low down, from the skirting to about a metre up
Water at the bottom of a wall is arriving from the floor or the ground. In a house with a solid floor, that often means a supply or heating pipe leaking in or under the screed, with the water soaking sideways into the base of the wall. In an older Cornish cottage with solid granite or cob walls and no damp-proof course, it can be ground moisture rising through the masonry instead. The two look similar at a glance but behave differently, and our guide on rising damp or a leak in a Cornish cottage goes through how to tell them apart.
Mid-height, somewhere between knee and shoulder
A patch that starts partway up a wall with dry plaster below it is hard to explain by anything other than water inside the wall at that level. That usually means a pipe: hot and cold feeds to a bathroom, a heating pipe chased into the plaster, or the pipework behind a boiler or cylinder. If the wall backs on to a bathroom or kitchen, the odds shorten considerably.
High up, near the ceiling
Water at the top of a wall has come from above. Downstairs that means a bathroom, a radiator or a pipe run in the floor void overhead. Upstairs it means the roof, a gutter, a flashing or a tank in the loft. Look at the ceiling too: if the stain runs across the ceiling and down the wall, the source is overhead. If the wall is wet and the ceiling is dry, something is running down inside the wall.
What the shape and the edge tell you
Height gives you the level. The outline gives you the behaviour of the water behind it.
- A tide mark with a crusty white edge. The patch has wetted and dried repeatedly and left salts behind. This is a slow, long-running source. Ground moisture and slow pipe leaks both do it.
- A round or oval patch that is darkest in the middle. Water is arriving at one point behind the plaster and soaking outwards. That is the signature of a leaking pipe or joint at roughly that spot.
- A vertical streak. Water is running down the inside of the wall from higher up. Trace it upwards, not sideways.
- A horizontal band along the whole wall. Something is wetting the wall evenly along its length: a floor-level leak spreading under the screed, or moisture from below.
- A patch that grows after rain. Think outside. A blocked gutter, cracked render, a leaking downpipe or a raised flowerbed against the wall are all common on exposed coastal properties around Cornwall.
One more observation: does it change with the heating? A patch that warms up when the radiators are on is almost certainly a heating pipe, and the boiler pressure gauge will usually agree. Our guide on why a boiler keeps losing pressure explains that link.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Low patch, tide mark, worse in winter | Ground moisture or a slow floor-level pipe leak | Meter test, then a moisture survey to separate the two |
| Mid-height, round, darkest in the centre | Pipe or joint leaking inside the wall | Note what is plumbed behind that wall, book a trace |
| High patch, ceiling also stained | Bathroom, roof or tank above | Check the room or loft above first |
| Patch warms up with the heating | Heating pipe in the wall or floor | Watch the boiler pressure, keep the system topped up |
Why painting over it makes things worse
It is tempting. A coat of stain block and emulsion makes the room look right for a few weeks. But the water behind the plaster has not gone anywhere, and it now has fewer places to evaporate from. The paint blisters or bleeds through again, and by the time you look properly the plaster has softened and any timber behind it has been wet for months.
There is an insurance angle too. Buildings policies generally cover sudden escape of water, and many will refuse a claim they consider gradual deterioration you could have acted on. A patch painted over twice is hard to present as a new problem. If you think it might be a leak, photograph it now, date the photos and leave the wall alone until it has been looked at.
Checks you can do before anyone visits
Three checks tell you whether the wall is being fed by a leak or by the building itself.
- The water meter. Turn off every tap and appliance and watch the meter for movement. If the small dial turns with nothing running, water is escaping from the supply pipework somewhere.
- The boiler pressure. A sealed heating system should hold roughly 1 to 1.5 bar when cold. Regular topping up with no visible drip means water is leaving the heating circuit, and a damp wall is a likely destination.
- The outside wall. Go out in the rain. Check for overflowing gutters, a downpipe discharging at the base of the wall, cracked render and soil or paving piled higher than the damp-proof course.
If the meter and the boiler are both steady and nothing is wrong outside, the patch is more likely ground moisture or condensation, and a damp specialist is the right call. If either one moves, read on.
How a damp patch on a wall is traced to its source
The patch you can see is the end of the water’s journey. Our water leak detection survey works backwards from it.
The first step is moisture mapping. Readings are taken in a grid across the wall and the floor beside it, not just on the visible patch. The numbers rise towards the source, and that gradient often points to a spot well away from the discoloured paint. If a damp survey has already told you the reading was high, our article on a high damp meter reading explains what that does and does not prove.
Thermal imaging comes next. A leaking hot pipe shows as a warm line behind the plaster and a cold supply leak as a cool shape, because escaping water changes the surface temperature of the wall. Where the pipe is under mains pressure, acoustic listening picks up the sound of the escape. Between the three, we can normally mark a small area and say what is behind it before any plaster is removed.
You get a written report with photographs and thermal images, which is also the evidence a trace and access claim needs. We provide leak detection across Cornwall, from terraced houses in Camborne to farmhouses on the Lizard, and a wall that will not dry is one of the most frequent reasons people call.
Cornish walls are a special case
Solid granite, cob and rubble-stone walls hold a great deal of water and give it up slowly. Cement render and gypsum plaster trap that moisture where a lime finish would let it breathe, so a patch can persist long after its cause has been fixed. Salt air and driving coastal rain add moisture from outside. None of this means the patch is just the building. It means the survey has to separate a leak from the wall’s own behaviour rather than guessing either way.
Frequently asked questions
Why does a damp patch on my wall keep coming back after I paint it?
Because the water feeding it has not stopped. Paint seals the surface, so the moisture stays in the plaster and pushes through again as blisters or a fresh stain. Until the leak, the ground moisture or the outside defect is fixed and the wall has dried through, repainting only hides the problem for a while.
How can I tell rising damp from a leak?
Rising damp sits low on the wall, rarely climbs above a metre, carries a salty tide mark and does not move the water meter or the boiler pressure. A leak can appear at any height, tends to be roundish or streaked, and usually shows up on the meter or the pressure gauge. Moisture mapping settles it.
Is a damp patch on an internal wall serious?
An internal wall has no outside face, so weather is not the cause. That leaves a pipe in or beside the wall, water tracking along the floor from another room, or something above. It is worth taking seriously, because an internal wall usually hides supply or heating pipework and the patch will grow.
Will my insurance cover a damp patch caused by a leak?
Most buildings policies cover sudden escape of water and include trace and access cover for finding the leak, though not the pipe repair itself. Gradual damage that has been left is often excluded. Photograph the patch early, avoid painting over it, and get the cause confirmed so the claim has evidence behind it.
Can a leak behind a wall be found without removing the plaster?
In most cases, yes. Moisture readings across the wall show which direction the water is coming from, thermal imaging shows warm or cold pipe lines behind the surface, and acoustic listening picks up the sound of a pressurised leak. Together they usually mark a small area, so only that section needs opening.
Find what is feeding the wall
If a damp patch on your wall has outlasted the heating, the dehumidifier and a coat of paint, we can trace what is behind it and mark the spot before anything is opened up. The report is written for your insurer as well as for you.
