Black mould turns up in the same places in almost every British flat: the corners of window reveals, the ceiling above the shower, behind the wardrobe on an outside wall, along the silicone in the bathroom. You clean it, and a few weeks later it's back.
That's the bit worth understanding. Mould is not a cleaning problem. It's a moisture problem that shows up as a cleaning problem. Removing it without changing the conditions is a treadmill.
This guide covers removing it safely, then stopping it returning — and where the line falls if you rent.
Why it grows where it does
Mould spores are in the air everywhere, all the time. They're harmless until they land somewhere that gives them what they need to grow: moisture, a surface to feed on, and still air.
That's why it appears in such predictable spots — the coldest surfaces in the room, where warm damp air condenses, and where air doesn't circulate:
- Window reveals and frames — coldest surface in the room, and where condensation collects overnight
- Behind furniture on external walls — cold surface plus completely still air
- Bathroom ceilings and sealant — repeated steam, poor extraction
- Inside wardrobes and cupboards on outside walls — enclosed, unventilated, cold
- Corners of rooms — cold spots where two external walls meet
Every one of those is a cold surface with poor airflow. That is the whole pattern.
Removing it safely
A patch of surface mould on a windowsill or a bit of sealant is a job you can do yourself in twenty minutes. Larger areas — roughly speaking, more than about a square metre, or mould inside plaster rather than on top of paint — should be handled by someone qualified, or by your landlord.
Before you start
- Open a window and keep the door to the rest of the home shut.
- Wear rubber gloves, and a mask if you have one. Eye protection if you're working overhead.
- Never mix cleaning products — particularly bleach with anything containing ammonia, or with vinegar. The combination releases toxic gas. Pick one product and use only that.
- Don't brush or hoover dry mould. It flicks spores into the air. Everything below is done wet.
The method
- Wipe, don't scrub dry. Use a damp cloth with warm soapy water or a dedicated mould remover, and wipe gently to lift the growth off without sending it airborne.
- Treat the surface. A proper mould and mildew remover, or diluted bleach on non-porous surfaces like tiles, glass and sealant. Follow the instructions on the label — including how long to leave it on.
- Bin the cloths. Put used cloths straight in a bag and into the bin. Don't wash and reuse them, and don't put them in with your laundry.
- Dry the area thoroughly. This is the step people skip and it's the one that matters. Mould cannot regrow on a genuinely dry surface.
- Soft furnishings and porous surfaces — mouldy curtains, fabric, or badly affected plasterboard usually can't be saved by cleaning. Wash what's washable at a high temperature; replace what isn't.
A note on health: mould exposure can affect breathing, and it matters more for babies, older people, and anyone with asthma or a weakened immune system. If anyone in the home has symptoms you think are linked to damp, speak to a GP — and take it seriously rather than waiting it out. NHS guidance on damp and mould is free and worth reading.
Stopping it coming back
You've removed it. Now change the conditions, or you'll be doing this again by spring.
Cut the moisture going in
The biggest single source in most flats is drying laundry indoors — around two litres of water per wash load released into your home. Dry outside where you can. Where you can't, use one room with the door closed and the window open.
After that: lids on pans, bathroom fan on before the shower and for fifteen minutes after, door shut while it runs.
Get air moving
Short bursts of wide-open windows, ideally two on opposite sides of the flat, clear moist air far more effectively than a trickle vent left open all day — and cost you much less heat. Five to ten minutes, morning and after any steam.
Pull furniture 5cm off external walls so air can circulate behind it. This one change often solves wardrobe mould on its own.
Keep surfaces above dew point
Mould needs a cold surface. A flat heated in short hot blasts and left cold between has much colder walls than one kept at a steady low temperature, and gets noticeably more mould. Low and constant beats hot and intermittent — there's more on the running costs in our guide to heating one room cheaply.
Dry the air out, at the right scale
For a single enclosed problem spot — a wardrobe on an outside wall, a musty cupboard, a small ensuite — a compact dehumidifier does useful work and costs almost nothing to run.
For mould appearing across a whole flat, a compact unit isn't going to touch it. That needs real extraction capacity: a 12L/day dehumidifier for a 1–3 bedroom home, or a 16L/day model for a larger or badly affected property. The LED humidity readout on the 16L is genuinely useful here — you want to see the room sitting at 45–55%, rather than guessing.
Be realistic either way: a dehumidifier manages symptoms brilliantly. It does not repair a building.
If you rent, know where the line is
There's an important difference between mould caused by condensation — which responds to ventilation and heating — and mould caused by a fault in the building, which does not, and which is not your responsibility to solve.
Signs it's likely structural:
- Damp patches that appear regardless of how much you ventilate
- A tide mark rising up from the skirting board
- Damp that worsens after heavy rain
- Mould returning within days of a thorough clean
- No working extractor fan in a windowless bathroom or kitchen
- Visibly failed window seals, guttering or roofing
Under the Homes (Fitness for Human Habitation) Act 2018, rented homes in England must be free from serious damp and mould hazards throughout the tenancy. If it's structural, it's a repair, and it's the landlord's job.
What to do: report it to your landlord or letting agent in writing — email, so it's dated and saved. Take photographs with dates. Keep every reply. If nothing happens, your council's environmental health team can inspect and require the landlord to act, and that route is free to you.
Shelter and Citizens Advice both publish free, current, independent guidance on damp and disrepair, including template letters. If your home has a genuine repair problem, use them — that's a far better use of your time than buying gadgets.
The short version
- Mould is a moisture and cold-surface problem; cleaning alone never fixes it.
- Remove it wet, never dry-brushed, and never mix cleaning chemicals.
- Dry the surface properly afterwards — that step is the one that counts.
- Stop laundry drying indoors, ventilate in short bursts, pull furniture off cold walls.
- Match dehumidifier capacity to the space — compact for a cupboard, 12L+ for a flat.
- If it's structural and you rent, it's a repair — report it in writing and get help.
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