Before you start
- Open a window in the room and close the door to the rest of the house. You want air leaving, not circulating inward.
- Wear gloves, eye protection and an FFP2 mask or better. Disturbing growth releases a great deal of spore.
- Move furniture out or cover it.
- Do not brush or vacuum it dry. That is the single most effective way to spread it through the house.
- If anyone in the house has asthma, a serious allergy or a compromised immune system, they should not be in there.
The method
- Mist the area lightly first so the growth is damp — damp spores do not become airborne nearly as easily.
- Wipe with a fungicidal wash, working from the outside of the patch inward so you do not spread it. Follow the product's contact time.
- Use disposable cloths and bin them immediately in a sealed bag. Do not rinse and reuse.
- Wipe again with clean water, then dry the surface thoroughly — a fan helps.
- Leave it and watch for a fortnight. If it returns in the same place, the moisture source is still live and no amount of repeat cleaning will change that.
Bleach is the popular choice and it is a poor one on porous surfaces. It bleaches the stain so the wall looks clean while the roots survive in the plaster below, and it puts more water into a wall that already has too much. A proper fungicidal wash penetrates instead of bleaching.
Then deal with the water
This is the part that decides whether you are doing it again next winter.
- If it is in corners and around windows, worse in winter — it is condensation. Ventilate, extract at source, warm the cold surfaces.
- If it is an isolated patch that darkens after rain — it is penetrating damp, and the fix is outside: wall crack repair and façade sealing.
- If it is low on the wall with a white salt band — it is rising damp: damp proofing in Cape Town.
- If it is on a ceiling under a bathroom — it is a failed wet-area membrane: shower waterproofing.
- If it is on a ceiling with a spreading stain — it is a roof leak.
When to stop and call someone
- It covers more than about a square metre
- It returns within weeks of a proper clean
- It is inside the wall build-up rather than on the surface
- The plasterboard is soft, or the plaster sounds hollow
- There is a musty smell with nothing visible
- Somebody in the house has a respiratory condition
Related reading
Start with Mould on Walls for the overview, or:
- Black Mould on Ceilings
- How to Remove Mould from a Bathroom
- Is Black Mould Dangerous?
- Mould-Resistant Paint
Common questions
What kills mould on walls permanently?
Nothing applied to the surface does, because the spores are in the air and will return to any surface that is wet enough. The only permanent fix is removing the moisture — a building repair, better ventilation, or a warmer surface.
Should I use bleach or vinegar on mould?
Neither is ideal on porous walls. Bleach removes the colour and leaves the roots; vinegar is mildly effective but slow. A proper fungicidal wash penetrates the surface and is designed for the job.
Can I paint over mould once it is cleaned?
Only once the surface is genuinely dry and the cause is resolved. Painting over a wall that is still damp traps moisture behind the film and the growth reappears through it, usually within a couple of months.
How do I stop mould coming back after cleaning?
Change one of the three conditions it needs — less water, a warmer surface, or more air movement. If the water is coming from a building fault rather than daily living, that fault has to be fixed or it returns regardless of what you do inside the room.