Problems and fixes
Mould on walls: why it appears and what to do
Written by Martin, M24/7 Bathrooms
Last updated:
Most mould on walls in UK homes is caused by condensation on cold surfaces. Cut the moisture and warm the surface, then clean and redecorate. If the plaster stays wet, crumbles or stains from within, water may be getting in and needs finding before any repair.
Black spots on walls usually come from condensation. Warm, moist air from cooking, drying washing, showers and breathing meets a cold surface and leaves water behind, and mould grows where that water sits. The usual places are the corners of bedrooms, behind wardrobes, around windows and on outside walls.
The fix starts with the moisture, not the mould. Cleaning the wall without changing the conditions only buys a few weeks. If the wall is wet even in dry weather, the plaster is crumbling or there is a tide mark, the problem may be water getting into the wall, and that cause has to be found before any plastering is done. Bathroom cases are covered in our separate guide to damp and mould in bathrooms.
Condensation or water getting in?
Condensation and water coming through the structure look similar at first glance, but they leave different clues. Working out which you have saves money, because each one needs a different answer.
| Clue | Points to condensation | Points to water getting in |
|---|---|---|
| Pattern | Spotty black mould in corners and behind furniture | A patch or tide mark that grows after rain |
| Timing | Worse in winter and in the morning | Linked to weather, or present all year |
| Surface | Paint is damp but the plaster is sound | Plaster is soft, salty or coming away |
| Windows | Wet glass and sills most mornings | Windows may be dry |
Why bedrooms and corners suffer most
Bedrooms produce a surprising amount of moisture overnight, and doors and windows are usually shut. External corners are the coldest part of a room because two outside walls meet there, and wardrobes pushed against a wall stop warm air reaching it. The result is a cold, still pocket where moisture settles and mould takes hold.
Older solid wall houses, which are common across Greater Manchester, can have colder internal surfaces than newer homes with cavity insulation. That does not mean they must have mould, but it does mean ventilation and heating matter more.
What to check before anything is repaired
A few simple checks often tell you most of what you need to know. They also make any conversation with a tradesperson far more useful.
- Do the windows run with water in the mornings?
- Are trickle vents open and extractor fans in the kitchen and bathroom working?
- Is washing dried indoors without the window open?
- Is the patch near a gutter, downpipe or cracked render outside?
- Does the mark get bigger after heavy rain?
- Is the plaster soft, bubbling or showing white salts?
Treating mould caused by condensation
For condensation mould, the long term fix is lower moisture and warmer surfaces. Use extractor fans while cooking and showering and leave them running afterwards, open trickle vents, keep furniture slightly away from outside walls, and heat rooms steadily rather than in short bursts. Then clean the affected area with a mould treatment and redecorate once the wall is dry.
If the mould has been there for a long time, the paint and the surface of the plaster can be stained beyond cleaning. A mist coat and fresh paint often deals with it, but sometimes a skim coat is the neater finish.
When the plaster needs replacing
Plaster that has stayed wet for a long time can lose its strength. It sounds hollow when tapped, crumbles at the surface, bubbles under the paint or shows salts. Painting over it will not last. Once the cause has been dealt with and the wall has had time to dry, the damaged plaster can be cut back and the wall replastered and skimmed.
That making good is plastering work M24/7 carries out. The order matters: first the cause, then the drying, then the new plaster. Plastering a wall that is still wet, or still getting wet, simply moves the problem into the new finish.
Questions
Frequently asked
What causes black mould on walls?
Usually condensation. Moist air settles on cold surfaces such as outside walls, corners and window reveals, and mould grows where the water sits.
How do I stop condensation on walls?
Reduce moisture and keep surfaces warmer: use extractor fans, open trickle vents, dry washing with ventilation, keep furniture off outside walls and heat rooms steadily.
Is mould on walls always condensation?
No. A patch that grows after rain, a tide mark, or soft and salty plaster can mean water is getting in. That cause needs finding before any repair.
Do I need to replaster after mould?
Not always. Surface mould on sound plaster can be cleaned and redecorated. Plaster that is crumbling, hollow or salty usually needs cutting back and replastering once the wall is dry.
Can you plaster over mould?
No. The mould must be treated, the cause dealt with and the wall allowed to dry first. New plaster over a wet or mouldy surface will fail.
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