What causes mould on walls and how to tell if you have it
Mould grows on walls when moisture sits in one place long enough for spores to take hold. The most common causes are poor ventilation, water leaks from pipes or roofs, condensation from bathrooms and kitchens, and rising damp from the ground. You will see it as black, green, or white patches, often fuzzy or slimy to the touch. It usually appears in corners, around window frames, under sinks, or anywhere air doesn't move freely.
Mould is not just unsightly — it can trigger allergies and respiratory problems, especially for children, older people, and anyone with asthma. The good news is that surface mould on walls is something you can handle yourself if you catch it early. Deep mould that has eaten into the plaster or drywall, or mould caused by a structural leak, needs a professional.
Key Takeaways
- Surface mould on walls can be cleaned with a solution of bleach and water, white vinegar, or a commercial mould cleaner, applied with a stiff brush and left to sit before wiping.
- Always wear gloves, eye protection, and a mask rated N95 or higher when cleaning mould, because spores become airborne when disturbed.
- After cleaning, reduce moisture by improving ventilation, fixing leaks, and using a dehumidifier if condensation is the problem.
- If mould returns within weeks, spreads across a large area, or appears after a visible water leak, call a surveyor or damp specialist to rule out structural damage.
- Mould inside walls or behind plaster requires professional removal and investigation of the moisture source.
Preparing the room and protecting yourself
Before you start, open all windows and doors to create airflow. If the mould covers more than one square metre (about 10 square feet), or if you have asthma or a weakened immune system, wear a respirator mask rated N95 or P100 — a cloth mask or surgical mask will not stop mould spores. Wear disposable gloves and eye protection, because bleach and vinegar can splash.
Move furniture and belongings away from the wall. Lay newspaper or a drop cloth on the floor to catch drips and spores. Do not use a vacuum cleaner on mouldy surfaces — it spreads spores into the air and can damage the machine. If you need to move items that have mould on them, seal them in a plastic bag first.
Cleaning surface mould with bleach solution
For non-porous surfaces like tiles, painted walls, and plastic, a bleach solution is fast and effective. Mix one part household bleach (usually 5% sodium hypochlorite) with four parts water in a spray bottle or bucket. Spray or brush the solution onto the mouldy area and let it sit for 10 to 15 minutes. Do not rinse when ready — the bleach needs time to kill the spores.
After the bleach has sat, scrub the area with a stiff-bristled brush to remove the dead mould. Wipe down with a clean, damp cloth and dry thoroughly with a towel or by opening windows. Bleach is harsh on some materials — test it on a hidden corner first if you are unsure. Never mix bleach with ammonia or other cleaners, as this creates toxic fumes.
Using white vinegar or commercial mould cleaners
White vinegar is a gentler alternative to bleach and works well on porous surfaces like unpainted plaster and drywall. Spray undiluted white vinegar (5% acidity) directly onto the mould, let it sit for one hour, then scrub with a brush and wipe clean. Vinegar does not kill mould as quickly as bleach, but it is less likely to damage the surface or leave harsh chemical residue.
Commercial mould cleaners are available at hardware stores and supermarkets — look for products containing hydrogen peroxide, tea tree oil, or borate compounds. Follow the instructions on the bottle exactly, as strength and contact time vary by product. These are often safer to use indoors than bleach and work on a wider range of surfaces. Some people prefer them because they are less likely to bleach the paint or wall colour.
Drying the wall and preventing mould from returning
After cleaning, the most important step is removing the moisture that allowed mould to grow in the first place. Open windows for at least 24 hours to let the wall dry completely. If the wall is still damp after that, use a dehumidifier or fan to speed up drying. A dehumidifier pulls moisture from the air; a fan moves air across the surface so it evaporates faster.
Check for the source of the moisture. If mould appeared after a shower or in a steamy bathroom, improve ventilation by running the extractor fan during and for 20 minutes after bathing, or opening a window. If it appeared after rain or near a pipe, look for leaks — water stains on the ceiling or wall, soft plaster, or a musty smell all point to a leak that needs fixing. If you find a leak, repair it before the mould returns.
When to call a professional damp specialist
Call a surveyor or damp specialist if mould returns within a few weeks of cleaning, if it covers an area larger than one square metre, or if it appears in multiple rooms. These signs suggest the moisture problem is bigger than surface condensation — it could be rising damp from the ground, a hidden leak in the walls, or poor building ventilation that you cannot fix alone.
A damp specialist will use a moisture meter to find where water is entering the wall, identify whether the problem is condensation, a leak, or structural damp, and recommend the right fix. This might be repairing gutters, installing a damp-proof course, improving ventilation, or treating the wall with a damp-proofing product. Treating the symptom without fixing the cause means mould will come back.
Mould inside walls and hidden spaces
If you suspect mould is growing inside the wall cavity or behind plaster, do not try to remove it yourself. Mould inside walls is usually caused by water trapped between the inner and outer surfaces — from a leak, condensation, or poor insulation. Disturbing it releases a large number of spores into your home, and you cannot clean it thoroughly without opening the wall.
A damp specialist or surveyor can use a moisture meter and thermal imaging to find where the water is coming from, then recommend whether the wall needs to be opened, the insulation replaced, or the building ventilated differently. This is a job that requires professional equipment and knowledge of building codes, so budget for a survey before you plan any repairs.
Frequently Asked Questions
Is mould dangerous to my health?
Surface mould can trigger allergies, asthma attacks, and respiratory irritation, especially in children and people with weakened immune systems. Toxic mould species are rare in homes. The main risk is breathing spores when you disturb the mould, which is why wearing a mask during cleaning is important. If mould is widespread or you have health concerns, ask a doctor before cleaning it yourself.
Can I paint over mould to hide it?
No. Paint will not kill mould, and mould will continue growing underneath the paint. The moisture problem that caused the mould will still be there, so the mould will spread and eventually break through the paint. Always clean and dry the wall first, fix the moisture source, and only then paint.
What is the difference between mould and mildew?
Mildew is a type of mould — a flat, powdery fungus that grows on surfaces. Mould is a broader term for any fungal growth, including the fuzzy, slimy kinds. Both are cleaned the same way: with bleach, vinegar, or commercial cleaners, followed by drying and moisture control.
How long does it take for mould to come back after cleaning?
If you have fixed the moisture problem, mould should not return. If it comes back within weeks or months, the moisture source is still there — a leak, poor ventilation, or structural damp. This is a sign you need professional help to find and fix the root cause.
Can I use a bleach pen or spray bottle instead of mixing my own solution?
Yes. Bleach pens and spray bottles are pre-mixed and convenient. Check the label to make sure it is designed for mould (some are for general cleaning). Spray bottles are faster for large areas; pens are better for small spots. Follow the contact time on the label — usually 10 to 15 minutes.