Damp and mould on the walls — diagnose the source first
Mould on a wall isn't just ugly — it's a sign that extra moisture is coming from somewhere. The biggest mistake is to scrub it, paint over it, and move on; if you don't stop the source, it's back in weeks. Before anything else, your job is to work out which of three things you're dealing with: condensation on a cold wall, a hidden leak, or rising damp from the ground.
The problem
You see black, green or grey patches — usually in corners, behind sofas and wardrobes, around windows, or on a bathroom or kitchen ceiling. Sometimes there's a musty smell even when the wall looks clean, or the paint is blistered and peeling, or there's a damp patch that grows when it rains or when someone showers upstairs.
Likely causes
Condensation on a cold external wall (most common in winter)
In winter the external wall is cold from outside, and when the warm, moist air inside the room meets it, water condenses on it — exactly like a cold water bottle sweats. The result is mould in corners and behind furniture pushed against the wall, and it gets worse when windows are kept shut and there's no ventilation. Its signature: no leak, and it appears in cold dead spots (corners, behind the sofa) rather than one concentrated patch.
A leak from a pipe inside the wall
If the damp patch is concentrated, keeps growing even in summer, and is near a bathroom, kitchen or a pipe run, it's usually a leaking joint in the pipework inside the wall. The tells: damp that's permanent rather than seasonal, sometimes a dirty brown stain in the middle of it, and occasionally a faint trickling sound — or the water meter creeping up while every tap is off.
A leak from the roof or the rooftop water tank
A damp patch on the ceiling means water coming from above: the rooftop tank or its pipes, a stuck float valve letting the tank overflow, a crack in the roof waterproofing, or rainwater that isn't draining. In our region rooftop tanks are a leading cause — a failed float valve overflows the tank daily and the water finds its way down onto the ceiling below.
Rising damp at the base of the wall
If the damp and mould are only along the bottom of the wall (the first 30-100 cm from the floor) and stop at a horizontal line, it's usually rising damp: water drawn up from the ground into the blockwork because the waterproofing at the foundation is weak or missing. You'll notice peeling paint and a white crust of crystallised salts in that band. This isn't just a cleaning job — it needs a waterproofing repair, and usually a professional.
Poor or absent ventilation in the bathroom and kitchen
A bathroom fills with steam when you shower, and so does a kitchen when you cook. If there's no working extractor fan or an openable window, that steam condenses on the ceiling and cold corners and grows mould fast. This isn't a leak — it's moisture you produce every day that has no way out of the room.
Fix it — step by step
- The first and most important step: find the sourceBefore any cleaning, ask yourself: is the patch under a bathroom, next to a pipe, or on a ceiling below the roof? If so, think leak. Is it in corners and behind furniture on an external wall, worse in winter, with no pipe nearby? That's condensation. Is it only along the bottom of the wall and stops at a line? That's rising damp. Getting the diagnosis right is half the fix.Try the foil test: tape a piece of kitchen foil flat onto the dry wall and leave it 24 hours. If the room-facing side gets wet = condensation. If the wall-facing side gets wet = moisture coming from inside the wall (a leak or rising damp).
- Improve ventilation and cut the indoor humidityIf you've diagnosed condensation or bathroom/kitchen moisture: open the windows daily even for fifteen minutes, run the bathroom and kitchen extractor while you shower and cook (and for ten minutes after), and pull large furniture a little off the external wall so air can move behind it. A dehumidifier makes a big difference in a closed, damp room.Don't dry wet washing indoors in a shut room in winter — it drives humidity up dramatically and feeds the mould. If you must, crack a window or run the extractor.
- Clean the mould safely (not with a dry brush)Wear a mask and gloves, and open the windows. Spray a ready-made mould remover, a diluted bleach solution (one cup of bleach to four cups of water), or white vinegar onto the patch, leave it 10-15 minutes, and wipe with a wet cloth. Never scrub mould while it's dry — that throws its spores into the air, spreads it, and you breathe it in.Don't mix bleach with vinegar or with any ammonia cleaner — the combination releases toxic chlorine gas. Pick one only. White vinegar is gentler and better for delicate surfaces.
- Let the wall dry out completelyAfter cleaning, the wall has to dry from within, not just on the surface. Keep the room ventilated, run a fan or dehumidifier, and wait at least a day or two. Paint over a wall that's still damp and it will peel and the mould will come back faster.Touch the wall with the back of your hand — if it's still cold and damp compared with the other walls, it hasn't dried. Wait.
- Fix the root cause — not just the markThis is the real work. If it's condensation: improve insulation, heating and ventilation, and don't leave a cold, damp room shut up. If it's a pipe, roof or tank leak: the source has to be found and stopped (fix the float valve, replace the leaking joint, repair the roof waterproofing). If it's rising damp: it needs a waterproofing treatment at the foundation. Paint on its own stops no water source.If you can't reach the source, or it's a hidden leak inside the wall or a fault up on the roof — stop here and call the relevant tradesperson. Cheaper than opening half a wall on a guess.
- Repaint with an anti-mould / anti-damp paintOnce the wall is fully dry and the source is stopped, apply a sealing primer, then an anti-mould / anti-damp topcoat. These paints contain agents that resist mould growth and tolerate moisture far better than ordinary emulsion.Anti-mould paint delays the return, but it does not fix a leak. Paint over a live water source and the mould will be back and peeling within weeks — and you'll have wasted the paint and the effort.
Tools & parts you'll need
When to call a professional
Call the relevant tradesperson if: the damp is extensive or comes back after you've cleaned and painted (which means the source is still live); you suspect a hidden leak inside the wall (a plumber); the source is the roof or the rooftop water tank above the ceiling (a roofing/waterproofing specialist); rising damp at the base of the wall needs a waterproofing treatment; or there are visible cracks in the ceiling or wall alongside the damp (a possible structural issue). Heavy, widespread mould over large areas isn't a cleaning job — its source has to be treated by a specialist.
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Try photo diagnosisPeople also ask
Why does the mould come back after I've painted over it?
Because you treated the mark, not the cause. Paint hides the colour but doesn't stop the water coming from behind the wall or from condensation. As long as the moisture source is live, the damp pushes back up through the paint, peels it, and the mould returns. You have to stop the source first (leak, condensation, rising damp), let the wall dry, and only then repaint with a resistant paint.
How do I tell condensation from a leak?
Condensation appears in cold corners and behind furniture on external walls, gets worse in winter with the windows shut, and has no pipe nearby. A leak is a concentrated patch that keeps growing even in summer, usually near a bathroom, kitchen or below the roof. Try the foil test for 24 hours: wet on the outer (room) face = condensation; wet on the face touching the wall = water coming from inside the wall.
Is mould on walls bad for your health?
Yes, it can be. Mould spores irritate the airways and worsen allergy and asthma symptoms, especially in children, the elderly and anyone with a sensitive chest. That's why we recommend a mask, gloves and ventilation while cleaning, and never scrubbing mould dry so you don't throw spores into the air. Large areas are best handled by a specialist.
Bleach or white vinegar — which is better for mould?
Both work. Bleach is stronger at removing the colour and disinfecting hard surfaces like tiles and ceramic. White vinegar is gentler, penetrates porous surfaces a little deeper, and smells less harsh. The key thing: pick one only, don't mix them, and never mix bleach with any ammonia cleaner — the combination gives off a toxic gas.
What do I do about mould on the bathroom ceiling?
A bathroom ceiling is usually steam condensation, not a leak — unless there's a bathroom or tank directly above it. The fix: run the extractor while you shower and for ten minutes after, open the window, and clean the mould with a mould remover. If there's a brown patch that keeps growing or water dripping from the ceiling, that's a leak from above and you need a professional.
Does anti-mould paint solve the problem for good?
Not on its own. It delays the mould's return and gives it a harder surface to grow on, but it stops no leak and no condensation. Treat it as the final step after you've stopped the source and the wall has dried — not a substitute for fixing the cause.