Black mould on walls: what causes it, and what actually stops it coming back
Cleaning it off is the easy part. The mould is telling you something about the wall, and that is what needs fixing.
Above skirting height, black mould means the wall is staying damp, usually from condensation on a cold spot, rain getting in, or a leak. Government guidance says removing surface mould will not stop it reappearing, so photograph it, clean it off promptly while you work out which cause you have, and fix the cause before you repaint.
The short version
- Mould in the top corner of an outside wall, worst in cold dry weather, is usually condensation on a cold spot. If it is an upstairs room, look in the loft above it: the insulation often stops short of the eaves.
- A damp patch that grows after rain, often with a brown edge, is water coming in from outside. Go and look at the render, the pointing and the gutter on that wall before you buy anything for the inside.
- A patch that is the same in every weather is usually a leak from a pipe, a bath seal or the flat next door.
- Clean very small amounts with a mould and mildew product rather than bleach, which is what the government guidance says, wearing a mask, gloves and goggles. A large area is a job for a professional.
- If you rent from a council or housing association in England, Awaab’s Law has applied since 27 October 2025: a significant damp and mould hazard must be investigated within 10 working days of being reported.
Diagnosis
Mould is the symptom: three ways a wall gets wet enough to grow it
Mould needs a surface that stays damp for days at a time. So the useful question is never “how do I kill this”. It is “why is this bit of wall wet when the rest of the room is not”.
The government’s guidance on the health risks of damp and mould, published in September 2023 after the death of two-year-old Awaab Ishak, sorts damp into four types. Condensation, where moisture made inside the home settles on colder parts of the building, is the most common. Penetrating damp is water getting in from outside through defects in walls, roofs, windows or floors. Traumatic damp is leaking water from pipes, overflowing baths or a burst pipe. Rising damp, moisture from the ground, is the fourth, and the same guidance warns that it is often misdiagnosed. Black mould halfway up a wall or in a ceiling corner is almost never rising damp, so this post leaves it to our separate guide on damp at the bottom of a wall.
Getting the type right matters more than any product. BRE, the building research body, says that treatment only works if the type of dampness is correctly diagnosed, and that dampness is very often misdiagnosed, which leads to more damp problems later. The same wall can have two causes at once, too: the government guidance points out that a leak or rain getting in can make condensation worse.
| Clue | Condensation | Rain getting in | A leak |
|---|---|---|---|
| Where it appears | Cold places: top corners of outside walls, behind wardrobes, around window reveals, the ceiling where it meets an outside wall. | One wall, usually the one facing the weather, often under a crack, a gutter joint or a window sill. | Near pipes, under a bathroom, beside a shower or below the flat above. |
| When it gets worse | Cold, still weather, and when the heating goes off. Often worst on clear winter mornings. | A day or two after heavy or wind-driven rain. Dries back in a dry spell. | Whenever the water is used. The weather makes no difference. |
| What it looks like | Black speckles spread across an area, no stain behind them, often with water beads on the window. | A damp patch with a brown or salty tide line, then mould on top of it. | A stain that keeps growing outwards, sometimes with bubbling paint or soft plaster. |
| The fix is | Warmer surface and less moisture in the air: insulation, ventilation, heating. | Mend the outside: render, pointing, gutters, sills, flashings. | Mend the pipe or seal, then dry the wall out. |
Condensation
Why mould picks the corners: cold spots, and the insulation that stops short
Condensation mould grows on the coldest surface in the room, and in most houses that is a predictable handful of places. Warm air carries moisture from cooking, showers, drying washing and breathing, and drops it as water on any surface cold enough. The colder a patch of wall is compared with the wall around it, the more water it collects, and the sooner mould starts.
Building people call these patches cold bridges or thermal bridges. In a case BRE investigated, a damp patch appeared on an upstairs wall just below the eaves of a house built in 2006 that met the building regulations of the day. It looked like a leak running down inside the roof. It was condensation. The wall in the patch was colder than the dew point of the room’s air, probably because of a small break in the insulation between the rafters or building debris bridging the cavity at the top of the wall. BRE lists the usual culprits as lintels, window reveals, missing insulation, mortar bridging between the inner and outer leaves of a cavity wall, and other debris in the cavity.
The eaves: the first cold spot to check
If your mould is in the top corner of an upstairs room, where the ceiling meets an outside wall, look in the loft directly above it. Loft insulation is often pulled back from the eaves, never laid right into them, or squashed flat where someone has stored boxes or walked across it. That leaves a cold strip of ceiling along the outside wall, and the mould follows it exactly.
You can usually fix this yourself if the loft is safe to get into. Push insulation down to the wall plate so the ceiling is covered right to the edge, but keep the gap at the eaves that lets air into the roof space open, because blocking it moves the condensation into the loft instead. Our posts on condensation in the loft and what your loft is telling you in winter cover that balance. Where the eaves are too tight to reach, or the whole loft is thin, it is a loft insulation job, and fitting it at the eaves without blocking the ventilation is the part that needs care.
Other cold spots worth checking
- Behind furniture on an outside wall. A wardrobe or headboard pushed flat against a cold wall stops warm air reaching it. The government guidance advises leaving a gap between furniture and external walls. Pull it away and the mould often stops.
- Window reveals and above windows. The lintel over a window is often concrete or steel, which carries heat out faster than the wall around it.
- Solid walls. A house with no cavity, common in older terraces, has colder inside wall surfaces all over. North-facing rooms in these houses suffer most. Wall insulation warms the surface, but it has to be designed so it does not move the cold spot somewhere you cannot see.
The building regulations treat this as a design question for new work. Approved Document C says an external wall resists surface condensation and mould growth if its U-value does not exceed 0.7 W/m²K at any point, and its junctions are detailed to recognised standards. That phrase is what matters. A wall can be well insulated on average and still grow mould at the one point where it is not.
Ventilation
Ventilation: what Approved Document F actually asks for
Insulation warms the cold spot. Ventilation takes the moisture away before it gets there. You usually need both, and one without the other often fails. Approved Document F, the ventilation part of the building regulations, opens by saying its aim is to protect the health of occupants, and that without adequate ventilation mould and internal air pollution might become hazardous to health.
For a home, Volume 1 of the 2021 edition sets minimum extract rates for fans that run when the room is in use. A 2026 edition has been published, takes effect on 24 March 2027 and keeps the same figures.
| Room | Minimum rate | What it means for you |
|---|---|---|
| Kitchen with a cooker hood extracting outside | 30 litres per second | A recirculating hood that blows back into the room does not count. |
| Kitchen without one | 60 litres per second | Double the rate, because nothing is catching the steam at the hob. |
| Utility room | 30 litres per second | Matters if you dry washing or run a vented dryer there. |
| Bathroom | 15 litres per second | In a room with no window that opens, the fan should keep running for at least 15 minutes after you leave. |
You cannot measure litres per second at home, but you can check the obvious. Hold a single sheet of toilet paper against the grille with the fan running. If it barely moves the paper, the fan is probably moving little air. Look for a duct that has come off inside the loft, a fan blocked with fluff, or a fan that only runs while the light is on and stops the moment you leave.
The rule that catches people out is in Section 3, on work to existing homes. Many older houses are ventilated by draughts rather than by anything designed. Approved Document F says energy efficiency work can reduce those draughts and leave a home under-ventilated, and that building work should not reduce the ventilation of a dwelling unless it can be shown the result still meets the requirement. New windows are the classic case. If the old windows had trickle vents, the new ones should have them too, no smaller, and controllable. If the old ones had none, the house must be no worse ventilated after the work than before. Mould that appears for the first time the winter after new windows, cavity fill or a sealed-up chimney is often telling you this, though cavity fill on an exposed wall can also let rain across, so check the outside as well.
Approved Document F treats ventilation equipment as a controlled fitting, so fitting or extending it is building work. The government guidance makes the same point: work to ventilation and replacement windows is controlled work, and should be done by someone competent to do it.
Rain and leaks
When the water is coming from outside, or from a pipe
If the mould sits on a damp patch that gets bigger after rain, stop looking at the inside of the wall. The fix is outside. A wall that takes in rainwater is wet and cold from within, so it grows mould even in a well-ventilated room, and no amount of extractor fan will dry it.
The government guidance lists the external checks: damaged or blocked pipes, gutters or downpipes, broken seals around windows, cracks in render, missing or broken roof coverings, and a bridged damp proof course. In practice the usual suspects on a wall are these:
- Cracked render. Rain runs into a crack, gets behind the render and cannot get out again, so the wall behind stays wet long after the surface has dried. Not every crack matters, and our guide to which render cracks matter explains which ones do. Where render has blown or cracked across a whole elevation, patching tends to chase the problem round the wall, and re-rendering is the lasting answer.
- Eroded pointing. Mortar joints that have worn back or cracked let water into brickwork, especially on the side of the house that takes the weather. Repointing in the right mortar stops it. On an older solid wall the mortar choice matters as much as the work, so read lime or cement mortar before anyone starts.
- Gutters and downpipes. A gutter that overflows at one joint, or a cracked downpipe, pours water down one strip of wall every time it rains. Look for green staining or moss on the outside directly behind the mould. Our post on gutters that overflow in heavy rain covers the usual reasons.
- Window sills and seals. A sill with no drip groove, or a failed seal around the frame, sends water into the reveal. Mould around one window and not the others is a strong hint.
If the damp only appears when rain is being blown hard against the house, that is a pattern with its own causes and its own test, and our post on water coming in only during driving rain walks through it. On exposed walls near the coast, wind-driven rain is the main load, and the elevation facing the prevailing wind is the one to check first.
Once the outside is mended, the inside does not recover on the same day. Plaster that has been wet for a long time can stay damp, carry salts that keep drawing in moisture, or come away from the wall. Give it time to dry before you redecorate, and if it has blown, see blown plaster and the hollow-sounding wall. Replastering is the last step, never the first, and it is plastering work that only lasts once the water has stopped.
The patch that ignores the weather
A damp patch that is just as bad in a dry fortnight as in a wet one is not rain and is rarely condensation. It is water from a pipe, a waste, a shower tray, a bath seal, a radiator valve or the flat above. These are often the fastest to find and the cheapest to fix, and they need a plumber, not a builder.
Look at what is on the other side of the wall or directly above it, and check whether the patch grows after one particular thing is used, such as a shower. Our post on telling a rain leak from a plumbing leak sets out the full method, and a brown stain on the ceiling covers tracing one through a ceiling.
If the water is coming from a neighbouring property, the government guidance says to work with the owner of that property, and that the local council can take action where one property is causing a nuisance to another.
Cleaning it off
How to remove black mould safely, and why bleach is not the fix
Clean the mould off promptly, while you find out why it is there. Leaving it on the wall while you investigate is not the answer: the government guidance says mould should be removed promptly and never left for long periods. It also says that removing surface mould without dealing with the cause will not stop it coming back.
Before you clean, photograph it and draw a pencil line round the edge. The guidance advises this, because a record may help identify the source, and it gives you a fixed line to compare against in a month.
The safe way to do it
- Wear a mask, gloves and goggles. The guidance says this applies whatever the size of the patch, to keep both the spores and the cleaning product off you.
- Shut the door to the rest of the house and open the window, and keep it open during and after the job.
- Use a product sold for mould and mildew, and follow the label. The guidance says these should be used in preference to bleach, for health and safety reasons.
- Wipe gently with a cloth or sponge. Do not dry-brush or sand it: the government guidance warns that disturbing mould, such as pulling up carpet or wallpaper, can release a lot of spores.
- Let the surface dry completely. Bag the cloths and wash or dispose of what you wore.
The guidance draws a clear line on size. Most people can reasonably deal with condensation and very small amounts of mould with a suitable cleaner, but larger areas should only be dealt with by qualified professionals. It also warns that soft, absorbent things such as carpets, soft furnishings and ceiling tiles may have to be thrown away, because it can be difficult or impossible to remove mould from them completely.
Why bleach on the wall is not a fix
Bleach is the first thing most people reach for, and the reason it seems to work is the reason it misleads you. It takes the black out of the stain, so the wall looks clean. Nothing about the wall has changed: it is just as cold, or just as wet from outside, as it was yesterday, so the mould returns in the same place, often in the same shape as your pencil line. The government guidance prefers mould and mildew products to bleach anyway, and if you do use bleach, do not mix it with other cleaners.
Anti-mould paint has the same limit. It is a sensible finish once the cause is fixed. Over a wall that is still wet or cold, it hides the evidence of what was wrong.
Health comes first. The government guidance says damp and mould mainly affect the airways and lungs, and can also affect the eyes and skin. Everyone is vulnerable, but people with asthma, COPD or other lung conditions, people with weakened immune systems, pregnant women, children and older people are at greater risk of more serious effects. If you or someone you live with has symptoms you are worried about, speak to a healthcare professional. Whoever you are, the guidance says larger areas of mould should only be dealt with by qualified professionals.
If you rent
If you rent: what your landlord has to do
If you rent, finding and fixing the cause is normally your landlord’s job, and the government guidance is unusually direct about whose fault it is not. It says it is “totally unreasonable” to blame damp and mould on lifestyle choices, because cooking, bathing and drying washing will always produce moisture, and that the fundamental cause will be building deficiencies, inadequate ventilation, inadequate heating or poor energy efficiency.
For social housing in England, from a council or housing association, the rules now have deadlines. The Hazards in Social Housing (Prescribed Requirements) (England) Regulations 2025, known as Awaab’s Law, came into force on 27 October 2025. The government’s guidance for tenants says significant damp and mould hazards must be investigated within 10 working days of being reported and made safe within 5 working days of the investigation finishing, with a written summary within 3 working days of the investigation, and any further work started within 12 weeks at the latest.
Private tenancies are not yet covered by those deadlines. On 9 October 2026 the government opened a consultation on extending Awaab’s Law to private rented tenancies, as the Renters’ Rights Act 2025 provides for, and it closes on 18 December 2026. Private landlords are still bound by the existing law, including the Housing Act 2004 and the requirement that a home is fit for human habitation.
Either way, report it in writing, with dated photos and your pencil line. If you are a landlord yourself, our page for landlords and letting agents explains how we work with landlords and tenants on repairs.
What to do next
What to do, in order
- Photograph the mould, draw round it in pencil, and decide which pattern in the table above it fits.
- If it looks like condensation, check the fans with a sheet of toilet paper, open the trickle vents, pull furniture off outside walls, and look in the loft above any upstairs corner for insulation that stops short of the eaves.
- If it looks like rain, go outside and look at that wall from the ground: the gutter, the downpipe, the render, the pointing and the sill. Do not climb a ladder to do it.
- If it looks like a leak, call a plumber, or tell the neighbour or the landlord above.
- Clean very small amounts yourself with a mould and mildew product, wearing a mask, gloves and goggles. Leave large areas to a professional.
- Look again after at least six weeks. That is the interval the government guidance tells landlords to allow before checking that a repair has worked. If the mould is back inside your pencil line, the cause is still there.
Plenty of this you can sort out yourself, and if a fan, a gap behind a wardrobe or a handful of insulation pushed into the eaves does it, that is the right answer.
When the cause is in the fabric of the house, such as missing insulation, cracked render, worn pointing or a failed gutter, it is building work. Our insulation work covers the cold spots, and the outside defects are rendering and repointing work we also do. Thomas Hannen comes out to look at the wall, inside and out, and you get a free written itemised quote that says what he thinks the cause is and what would fix it.
Questions
Common questions
Is black mould on walls dangerous?
It can be. Government guidance says damp and mould mainly affect the airways and lungs and can also affect the eyes and skin. Everyone is vulnerable, but people with lung conditions such as asthma or COPD, people with weakened immune systems, pregnant women, children and older people are at greater risk. Anyone worried about symptoms should speak to a healthcare professional.
Should I use bleach to remove black mould?
The government guidance says mould and mildew products should be used in preference to bleach, for health and safety reasons. Bleach also misleads you: it takes the colour out of the stain, so the wall looks fixed while the cold spot or the water getting in is still there. Do not mix bleach with other cleaners, and follow the label.
Why does black mould keep coming back on the same wall?
Because the wall is still damp. Removing surface mould does not change why it grew. If it returns in the same place, the surface is either colder than the rest of the room, often from missing insulation at the eaves or a cold lintel, or it is being wetted from outside or by a leak.
Can I paint over black mould?
Not as a fix. Clean it off first with a mould and mildew product and let it dry. Anti-mould paint is a reasonable finish once the cause has been dealt with, but painted over a wall that is still cold or wet it will not last, and it hides the evidence of what was wrong.
How much mould can I safely clean myself?
The government guidance says most people can reasonably deal with condensation and very small amounts of mould using a suitable mould and mildew cleaner, while larger areas should only be dealt with by qualified professionals. Whatever the size, wear a mask, gloves and goggles, and keep the window open during and after.
Read next
Related advice
The guides below pick up where this one leaves off, and the insulation page covers the work itself and the places we cover, if you would rather have the job looked at.
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
Free quote
Mould that keeps coming back?
Thomas comes out, looks at the wall inside and out and in the loft above it, and puts what he thinks is causing it, and what would fix it, in a free written itemised quote.