Black mould on your walls: find the cause before you clean
Black mould on a wall usually means moisture is sitting on that surface for long enough to feed it. In most homes that's condensation, from cooking, washing, breathing and drying clothes indoors, meeting a wall that's colder than the air in the room. Clean it off, and it will come back until the wall stays warmer or the air stays drier.
Where it usually shows up
Condensation mould tends to appear in the same places every winter: on cold outside walls, in the corners where two walls meet, behind furniture pushed against an outside wall, and around window reveals. Look at the pattern. A patch that follows the shape of a cold corner or a cupboard is almost always condensation. A patch that appears after heavy rain, or in one spot on a ground-floor wall, may be something else.
Scottish homes add their own twist. Older tenements and stone houses often have thick solid walls that hold the cold, so the inside face stays cold even when the room feels warm. Modern cavity walls are more forgiving, but they can still struggle with poor ventilation. Either way, the wall is the place where the moisture lands.
What you can do this week
Start with the things that reduce the moisture in the air. Use the extractor fan or open a window while you cook, shower and dry washing. Try to dry clothes outside or in a room with a window open, rather than on radiators with the door shut. Keep the furniture a few centimetres off outside walls so air can move behind it.
Clean the mould when the room is dry and well ventilated. Small patches can usually be treated with a mould-killing spray and a damp cloth, then left to dry. Wear gloves, open a window, and avoid dry brushing, which sends spores into the air. Our guide on how to get rid of mould goes through it step by step. Wipe the area again once it's dry, and check it over the next few days to see whether it comes back.
Make sure the heating works and that rooms are warm enough to keep the walls from going cold. Gentle, steady heat does more than blasting the radiator for an hour in the evening.
When it's more than condensation
Condensation is common, but some damp needs a proper repair:
- Rising damp shows as a tide mark low on ground-floor walls, often with flaky or crumbling plaster.
- Penetrating damp shows as patches that grow after heavy rain, usually below a gutter, a downpipe or a crack in the render.
- A leak shows as a patch that doesn't follow the weather, often near pipework, a sink or a bath.
If you see any of these, cleaning will only hide the problem for a while. Find the source first, or ask someone to look at it.
Your health
The NHS says mould can trigger asthma in people who are allergic to it, so the risk is real for some households and not for others. If anyone at home has asthma or a mould allergy, take the damp seriously and speak to your GP if breathing gets worse. The NHS asthma causes page lists mould among the things that can set off an allergy. Government guidance written for landlords in England, Damp and mould: understanding and addressing the health risks for rented housing providers (MHCLG, DHSC and UKHSA), says damp and mould mainly affect the airways and lungs, and can also affect the eyes and skin.
When to get a specialist
Get a damp specialist to look if the mould covers a large area, keeps returning after you've improved the ventilation, or you think the cause is damp coming in from outside or a leak. Tell us what's happening and we'll pass it to one independent local specialist to visit and quote. It's free to ask.
If you rent: report the mould to your landlord in writing, with photos and the date you first noticed it. Our tenants' guide to damp and mould sets out what your landlord should do and where to turn if nothing happens.
If you're a landlord: under Awaab's Law in Scotland, the deadlines start on the day you're told. Read our landlords page, and use the Awaab's Law deadline calculator to work out the dates.
Last checked 26 October 2026. This is general guidance, not legal or building advice.
Common questions
Is black mould on walls always dangerous?
Not always, but it's not something to ignore. The NHS says mould can trigger asthma and allergy symptoms in people who are sensitive to it, so it's worth dealing with, especially if anyone in the home has breathing problems.
Why does it come back after I clean it?
Because the moisture is still there. Cleaning removes what you can see, but if the wall stays cold and damp, the spores will settle again. Fix the cause and the mould is far less likely to return.
Should I paint over it?
Not until the wall is clean, fully dry and the cause is sorted. Paint over live mould and it will show through, usually within a season.
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