Common problem
Why is there black mould on my walls, and what should I do?
Black spots on interior walls are almost always Aspergillus or Cladosporium mould feeding on condensation that has settled on a cold surface. The surface is the clue: mould that grows on the cold external wall side, in corners, or behind furniture is a condensation problem, not a leak. Treat the moisture source, not just the stain.
Last updated Reviewed by Vincent Pamenter
What it looks like
Illustrative scenes, not photographs of a real case.
Cold-bridge corner: mould fans out from where two external walls meet the ceiling.
- Cluster densest at the corner, thinning outward
- Wall feels colder to the hand than the room average
- No staining above on the ceiling face — rules out a roof leak
Window reveal mould: cold bridge along the frame and reveal.
- Mould hugs the reveal edge, not the glass
- Condensation runs on the frame in the morning
- Silicone bead is discoloured but intact
Behind-furniture pattern: growth stops sharply at the wardrobe outline.
- Growth mirrors the furniture footprint
- Wall surface is at least 3–4°C cooler than the adjacent wall
- Air smells musty when the wardrobe is opened
What causes it
On UK internal walls, black mould is almost never a mystery contaminant — it is a symptom of a specific moisture pathway. Rank the three below against what you actually see before assuming the worst.
- #1
Surface condensation on a cold-bridge wall
Definition. Water vapour in room air condenses on a wall surface whose temperature has dropped below the dew point, most often at external corners, window reveals and lintels.
How to tell. Growth is patchy, follows corners or furniture outlines, and the affected surface is measurably colder than the surrounding wall. Wipe-testing removes the surface film cleanly.
- #2
Penetrating damp staining
Definition. Water tracks in from outside through failed pointing, cracked render, blocked cills or a leaking overflow, and darkens the plaster from within.
How to tell. The mark is a brown or yellow tide edge rather than a black spore bloom, sits on one localised patch aligned with an exterior defect, and worsens after driving rain.
- #3
Concealed leak at pipe or waste run
Definition. A slow drip from a bathroom, kitchen or heating pipe wets plaster from inside the wall build-up before any mould appears on the visible face.
How to tell. The wall feels damp when the surface is not visibly wet, staining is roughly circular and centred on a pipe run, and moisture returns even after redecoration.
Symptom vs likely cause
Quick triage. Match what you can see today to the most likely cause. Confidence is the honest starting-point — a formal reading uses hygrometer, thermal-imaging and moisture-meter data to confirm.
Fuzzy black spots in an external corner
- Most likely cause
- Cold-bridge condensation mould
- Confidence
- High
Brown tide mark, no fuzz, ~1m up on a ground-floor wall
- Most likely cause
- Rising or penetrating damp
- Confidence
- Medium
Circular damp patch centred on a pipe or radiator
- Most likely cause
- Concealed plumbing leak
- Confidence
- Medium
Mould outline matches wardrobe footprint
- Most likely cause
- Trapped-air condensation on external wall
- Confidence
- High
Streaks running down from ceiling on one wall
- Most likely cause
- Roof or gutter defect above
- Confidence
- Medium
Is it serious?
For most healthy adults, small black mould patches under a hand-span in size are not an emergency, but they should not be ignored. Sustained exposure to indoor mould is linked to worsened asthma, allergic rhinitis and respiratory infections, and infants, older adults, and anyone with an existing lung condition are at higher risk. Treat any bedroom mould larger than a dinner plate — and any mould growing where an infant sleeps — as a same-week priority.
Health context: NHS — damp and mould in the home — NHS.
This is general information, not medical advice. If you're worried about your health, contact your GP or NHS 111.
What to do first
Before you spend money on paint or a specialist, do these five things this week. They cost nothing and tell you whether you are dealing with condensation, a leak, or a compliance issue.
- 1
Photograph every patch in daylight
Wide shot for context, then a close-up. Include the corner, ceiling line, or window edge in the frame so an assessor can place the growth in the room.
- 2
Feel the wall temperature
Palm on the mouldy area, then on an internal wall in the same room. A noticeable cold difference points strongly to condensation, not a leak.
- 3
Pull furniture 5–10cm off the wall
Air needs to move across the surface. Trapped-air pockets behind wardrobes and beds are the single most common cause of bedroom mould.
- 4
Track your morning humidity for three days
A £10 hygrometer next to the bed is enough. Bedrooms sitting above 65% RH on waking are creating condensation every night.
- 5
Report the mould in writing if you rent
Email your landlord or letting agent with dated photos. Written notice starts the clock under Awaab's Law for social tenants and gives you evidence either way.
What NOT to do
The advice you find first online is usually the advice that sells the most paint. These are the four mistakes we see most in tenant photos.
Myth
Bleach kills mould permanently.
Why it's wrong. Household bleach removes the pigment but leaves the substrate wet. The mould regrows on the same spot within weeks because the underlying dew-point problem is unchanged.
Myth
Anti-mould paint solves it.
Why it's wrong. Fungicidal paint suppresses regrowth on the painted surface only. If the wall is still colder than the dew point, condensation still forms — under the paint, then behind the paint.
Myth
Open a window and it will dry out.
Why it's wrong. In UK winters, cold outside air holds very little moisture but chills the wall further. Purge-ventilation after showers and cooking helps; leaving one window ajar all day usually makes the cold-bridge worse.
Myth
It's just cosmetic — paint over it.
Why it's wrong. Painting over active mould traps live spores and moisture behind an emulsion film. When it lifts a few months later, you have a bigger patch and no evidence trail.
When you need evidence or a survey
If your landlord, insurer or a housing officer needs to act, they need dated evidence, not a phone photo in a text thread. A free Case Check turns your photographs into a structured, dated record of what is visible: the extent, the likely pattern, how confident the read is and the next step. It does not decide a hazard category; under HHSRS only a council inspector can do that, and the record is what you give them.
This is general information, not legal advice. For advice on your specific situation, contact Shelter, Citizens Advice, or a solicitor.
Common questions people ask
Is black mould on my bedroom wall dangerous?
Small isolated patches are unlikely to be an emergency for a healthy adult, but ongoing exposure — especially in a bedroom where you spend eight hours a night — can worsen asthma, trigger allergic reactions and irritate airways. Treat anything larger than a dinner plate, or any mould in a child's room, as a same-week priority.
How do I know if it's condensation mould or a leak?
Condensation mould is fuzzy, sits on the surface, spreads in cold corners and behind furniture, and wipes off. A leak leaves a stained tide-mark shape that darkens with rain or after a shower and doesn't clean off the plaster. When the two overlap, only a hygrometer plus a moisture-meter reading will separate them reliably.
Why does mould keep coming back after I clean it?
Because you have removed the pigment, not the moisture. Mould is a symptom of a cold surface meeting warm humid air; unless the wall temperature rises, or the humidity drops, spores that survive in the plaster surface will bloom again within four to eight weeks.
Can I just paint over black mould with anti-mould paint?
No — not as your first move. Fungicidal paint is a finishing layer, not a cure. Applied over active mould it traps spores and moisture; applied without fixing the cold-bridge or ventilation, condensation forms behind it and lifts the paint film within a season.
Is my landlord obliged to remove black mould?
Yes, in almost every scenario. Under the Homes (Fitness for Human Habitation) Act 2018 the property must be free of serious damp and mould hazards. Social landlords also sit under Awaab's Law working-day windows (10/3/5) for damp and mould reports made in writing from October 2025 onwards.
What kills black mould on walls for good?
Nothing kills mould permanently on its own — but three things together do: remove the visible growth with a fungicidal wash, raise the surface temperature (insulation, moving furniture off the wall, gentle background heating), and cut peak humidity by extracting steam at source. Address any one of those in isolation and it comes back.
How much wall mould is too much before I need a professional?
As a rule of thumb: anything over roughly one square metre, anything that has come back within 12 months of being cleaned, anything visible in more than one room, or anything in a rented home your landlord has not investigated — get a written assessment. It protects your health and your evidence trail.
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Keep reading
Related problem
Condensation mould in the bedroom
Related problem
Mould behind the wardrobe
Related problem
Damp patch on the ceiling
Cost guide
How much does mould removal cost in the UK?