Damp patch on a wall: condensation, a leak or rain getting in?

A damp patch on a wall can come from condensation, a plumbing leak or rain entering through the building. Its position is a clue, but it is not a diagnosis. Start with what you can see and when the patch changes, then look for a route by which water could reach that spot. The right repair depends on the source of the moisture, so avoid covering the mark before you understand it.

Could it be condensation?

Condensation forms when water vapour in the air meets a cold surface and turns into liquid. Windows make this easy to spot, but it also happens on external walls, in corners and behind furniture. A patch that appears alongside window condensation or mould in a colder corner may fit this pattern. Cooking, bathing and drying laundry add moisture to indoor air; poor ventilation gives it fewer ways to escape.

Look around the affected room rather than concentrating only on the mark. Are air vents blocked? Does the bathroom or kitchen extractor run and draw air out? Is furniture pushed hard against an external wall? These checks help you decide whether moisture is being made indoors and collecting at a cold spot. They do not rule out another source of water, especially if the patch persists after ventilation and heating have been considered.

Could water be leaking inside?

A leak can be less obvious than a dripping pipe. RICS lists plumbing leaks below baths, failed shower seals and leaking water supply pipes among possible sources of damp. Check the rooms on the other side of the wall and above it, where you can do so safely. Look for a relationship between the patch and the use of a bath, shower or sink. A concealed leak needs a repair to its source, followed by drying of affected materials.

The nearest visible mark need not be where water first entered. RICS describes penetrating damp as water moving through a wall, ceiling, roof or floor. That is a useful reason to inspect the building around the patch, not just the plaster immediately beneath it. If the affected area is growing or the route is hidden, a suitably experienced building surveyor can investigate without assuming the first likely explanation is correct.

Could rain be getting through?

Rain can enter through roofs, walls, chimneys and openings around windows and doors. Water overflowing from a gutter or downpipe may also wet a wall. From a safe position, look outside during rain: is a gutter spilling, is a downpipe leaking, or is water striking an area of damaged pointing or render? Note whether the patch grows after wet weather. Keep clear of ladders and roofs unless the work is being done by someone equipped for it.

Do not assume a quick coating will solve rain penetration in an older wall. The Society for the Protection of Ancient Buildings warns that hard cement pointing, render and other impervious treatments can trap moisture in older buildings. Begin with straightforward maintenance such as clearing a blocked rainwater route and repairing known defects. If a wall is exposed or earlier repairs may have changed how it dries, ask a suitably qualified surveyor to assess the building before specifying treatment.

Yorkshire and Lancashire gables: trace the rain first

An exposed hillside gable on a stone terrace can take the weather directly. If the damp patch sits below a chimney stack or near an outside corner, inspect the chimney, roof edge, pointing, gutter and downpipe from a safe vantage point. The pattern after driving rain can help narrow the search. Old pointing may be relevant, but so might an overflowing gutter higher up; finding which part is actually letting water in matters more than naming a type of damp.

Stone walls can also have moisture on their inner surface from condensation, especially where a room corner stays cold. That is why a patch on a gable should lead to checks both outside and indoors. Keep notes on weather, use of the room and any visible drips. If there is no clear defect or the problem returns after a repair, ask for an independent survey that considers rain, internal moisture and plumbing together.

Make the next step fit the cause

If you find a leak or blocked gutter, arrange to have that defect repaired. If condensation appears likely, check ventilation, extraction and cold surfaces while watching whether the patch changes. If you rent, tell the landlord or managing agent what you have found, including photographs and the timing of the dampness. Where the answer remains uncertain, commission a surveyor with relevant building pathology experience. A damp meter reading alone does not establish the source in a hard wall; a useful report explains the evidence and the proposed repair.

This page gives general information, not legal or medical advice. For your own situation, use the organisations listed under "Where this comes from" at the foot of the page.