How to Get Rid of Black Mould and Stop It Coming Back

Mould is not a cleaning problem with a ventilation side effect. It is a ventilation problem with a cleaning side effect. Scrubbing it off without changing anything else buys you about six weeks.

Kill the existing growth

You need a biocide, not a detergent. Wiping mould with soapy water spreads spores and removes the visible staining while leaving the colony.

1. Ventilate and protect yourself. Open the window, wear gloves, and wear a mask if the area is larger than a dinner plate. Disturbing mould releases a lot of spores.
2. Do not dry-brush it. Spray first, so spores are wetted down rather than launched into the air.
3. Apply the biocide. Household bleach diluted one part to four parts water works on non-porous surfaces. Proprietary mould sprays work better on sealant and grout because they cling. Leave it in contact for at least fifteen minutes.
4. Wipe with a disposable cloth and bin it. Do not put a mouldy cloth in the washing machine with clothes.
5. Repeat once. The second application always lifts more staining than you expect.
6. Dry the surface thoroughly and leave the room ventilated for a few hours.

Then fix the cause

Mould needs three things: moisture, a surface, and still air. You can only realistically remove the first and third.

Run the extractor fan properly. During the shower and for at least fifteen minutes afterwards. Most bathroom fans are switched with the light and run for two minutes on an overrun timer, which is nowhere near enough. Fitting a longer overrun timer is a cheap electrical job.

Check the fan actually extracts. Hold a sheet of tissue against the grille. If it does not hold itself in place, the fan is clogged, the ducting is crushed, or the external flap is stuck shut. A large proportion of bathroom fans move essentially no air.

Do not dry washing on radiators in an unventilated room. A single load of wet laundry releases around two litres of water into the air. That water condenses on the coldest surface in the house, which is where the mould appears.

Leave a gap behind furniture on external walls. Wardrobes pushed flat against a cold north-facing wall create a still, cold pocket. Fifty millimetres of gap is often enough to stop it.

Keep heating steady rather than intermittent. Cold surfaces are what condensation forms on. A house heated to a lower temperature continuously has warmer wall surfaces than one heated hard for two hours in the evening.

When it is not condensation

Condensation mould appears in cold corners, on window reveals, behind furniture, and on ceilings above showers. It is generally black and speckled.

If you see a tide mark rising up a wall from the floor, staining that changes with rainfall, or plaster that is blown and crumbling, that is a different problem — penetrating or rising damp — and it needs the source found rather than the surface treated.

Anti-mould paint is worth using in bathrooms after you have fixed the ventilation. Used instead of fixing the ventilation, it delays the growth by a few months and nothing more.