Common problem
Why is there so much mould in my bathroom?
In a UK bathroom, a black line along the silicone sealant around the bath or shower is usually worn sealant: surface cleaning, and a reseal if the bead has failed (a landlord repair if you rent). Mould in the ceiling corners, above the shower, on the walls or on the window reveal usually means the room is not clearing its steam, and sometimes points to a leak. If the mirror stays fogged long after the fan has stopped, test the ventilation before you clean again, because the mould will keep coming back while the steam has nowhere to go.
Last updated Reviewed by Vincent Pamenter
In short
Most bathroom mould is condensation. A shower or bath puts a lot of water vapour into a small room, and if the extractor fan is missing, weak or not switched on, the window stays shut or the room is cold, that vapour settles on the coldest surfaces and mould grows there. A black line along the silicone sealant is usually worn sealant: a clean-and-reseal job. Mould in the ceiling corners, above the shower or on the window reveal means the room is not clearing its steam. If you rent, a fan that does not work, or a room that cannot clear its steam, is something to report to the landlord in writing; what they must do depends on where you live (see below).
- Renters and homeowners with mould on bathroom sealant, grout, walls, ceiling corners or the window reveal
- Bathrooms with no window, no extractor fan, or a fan that does not run, runs weakly or is not wired to the light switch
- Anyone who needs to show a landlord, agent or council that the mould is a ventilation failure rather than how they use the room
- Photograph four things with the date: the extractor fan running with a tissue held to the grille, the window and any trickle vent, the worst ceiling corner, and the sealant or grout
- After a shower, run the fan, keep the door shut and note how long the mirror stays fogged; write the time down each day for a week
- Do what is within your control: run the fan for the whole shower and afterwards, open the window for a few minutes, wipe wet tiles and the screen, and clean sealant mould with a fungicidal wash
- If you rent, report it in writing: say where the mould is, whether the fan works or the room has no fan or window, and attach the dated photographs
- If the mould comes back within a couple of weeks of cleaning, treat it as a ventilation or building fault and ask for the fan to be tested, repaired or replaced rather than cleaning again
- UK-wide for the causes and the evidence. What a landlord must do differs. England, private renting: section 11 of the Landlord and Tenant Act 1985 covers the structure, exterior and installations for water and sanitation, the Homes (Fitness for Human Habitation) Act 2018 applies, and the council can inspect for damp and mould under the HHSRS. England, social housing: Awaab's Law Phase 1 has applied since 27 October 2025, with working-day timescales for investigating damp and mould and beginning safety work; it does not cover private renting. Wales: landlords must keep the home in good repair and fit for human habitation under the Renting Homes (Wales) Act 2016. Scotland: the Repairing Standard applies to private lets, and Scotland's Awaab's Law (the Investigation and Commencement of Repair (Scotland) Regulations 2026) covers private and social tenancies from 6 October 2026. Northern Ireland: private tenancy repairs have their own rules; nidirect's 'Repairing your home: private tenants' page is the official starting point.
- Approved Document F (England) gives extract rates for bathrooms of 15 litres per second for an intermittent fan or 8 litres per second continuous; it applies when a bathroom or fan is installed or altered and is a benchmark, not a retrospective standard for every existing home
- A photograph shows where the mould is, not the moisture behind a wall; a soft floor by the shower, a stain on the ceiling below or mould in the next room points to a leak and needs an inspection
- Cleaning removes the growth, not the cause; if the room cannot clear its steam the mould returns
- GOV.UK lists children, older people, pregnant women and people with asthma, COPD, allergies or weakened immune systems as more vulnerable to damp and mould; this page is not medical advice
- Ventilation: Approved Document F
- Damp and mould: understanding and addressing the health risks for rented housing providers
- Landlord and Tenant Act 1985, section 11
- Homes (Fitness for Human Habitation) Act 2018
- Housing Health and Safety Rating System (HHSRS): landlord and agent guide
- Awaab's Law Phase 1: guidance for social landlords
- Fitness of homes for human habitation: guidance for landlords
- Getting repairs done as a private tenant
- Repairing Standard: statutory guidance for private landlords
- Tenants to be better protected from damp and mould (21 January 2026)
- The Investigation and Commencement of Repair (Scotland) Regulations 2026, regulation 1 (commencement: 6 October 2026)
- Repairing your home: private tenants
What it looks like
Illustrative scenes, not photographs of a real case.
Ceiling steam pattern: densest above the shower, radiating toward the extractor.
- Dense speckle above the shower cubicle
- Halo around the extractor grille
- Paint peeling at ceiling-wall junction
Under-performing or blocked extractor: a common reason a bathroom cannot clear its steam.
- Grille discoloured or dusty
- Fan spins but a tissue barely pulls to it
- Long duct run, kinks or crushed sections in the loft
Cold window reveal: the coldest surface in the room after the tile edge.
- Sealant discoloured but tile edge intact
- Condensation on glass most mornings
- Trickle vent closed or painted shut
What causes it
Condensation is one of the most common causes of damp and mould in homes (GOV.UK), and in a bathroom the usual reason is steam that is not being cleared. Rank the four causes against your bathroom's specifics.
- #1
Under-performing extractor fan
Definition. Approved Document F (England) gives 15 litres per second (l/s) for an intermittent bathroom extract fan or 8 l/s for continuous extract. It applies when a bathroom or fan is installed or altered and is a benchmark rather than a rule for every existing home. Whatever the rating, the duct run should be short, insulated and unbroken to outside.
How to tell. A tissue held under the running fan barely lifts; the mirror stays steamed long after the fan has stopped; the fan grille is discoloured with dust; ducting is long or crushed in the loft.
- #2
Cold-surface condensation on ceiling or wall
Definition. Warm humid shower air condenses on the coldest surface in the room — the external wall, the window reveal, or the flat-roof ceiling of a rear extension bathroom.
How to tell. Mould concentrated on the outside-facing wall or the reveal, not the shared wall with an internal room. Wipes off cleanly; paint is intact.
- #3
Failing sealant or grout
Definition. Silicone bead and cement grout absorb small amounts of water. Once the bead cracks or the grout softens, water enters the substrate behind and feeds mould from within.
How to tell. Black line along the bead that returns within a couple of weeks of scrubbing, a soft or cracked seal at the bath edge, or grout that darkens when wet even after cleaning.
- #4
Concealed leak behind the shower or bath
Definition. A slow leak from the shower waste, mixer connection or bath seal wets the wall build-up from behind, and mould appears on the room side long before any water is visible.
How to tell. Growth on the wall on the far side of the shower (in the next room), a soft spot on the floor beside the shower, or a damp airing cupboard sharing a wall with the bathroom.
Symptom vs likely cause
Where the mould is tells you what the problem is. Match the pattern before you buy a new extractor or a tube of sealant.
Black line only along the bath silicone bead
- Most likely cause
- Worn sealant — surface cleaning; reseal if the bead has failed (a landlord repair if you rent)
- Confidence
- High
Ceiling mould concentrated above the shower
- Most likely cause
- Under-performing or wrongly-ducted extractor
- Confidence
- High
Mould on external wall / window reveal only
- Most likely cause
- Cold-surface condensation
- Confidence
- High
Damp patch on the far side of the shower wall
- Most likely cause
- Concealed leak behind tiling
- Confidence
- Medium
Grout darkens permanently after cleaning
- Most likely cause
- Failed grout / substrate saturation
- Confidence
- Medium
Is it serious?
A bathroom that cannot clear its steam is worth acting on rather than cleaning repeatedly, and where the mould has reached the ceiling or spread to adjoining rooms, the humidity is affecting more than the bathroom. GOV.UK guidance says damp and mould should always be acted on quickly, and lists people with a pre-existing health condition (for example allergies, asthma or COPD), people with weakened immune systems, people living with a mental health condition, pregnant women and women who have recently given birth, children and young people, and older people as more vulnerable to its health effects. This page is not medical advice.
Health context: Damp and mould: understanding and addressing the health risks for rented housing providers — GOV.UK.
This is general information, not medical advice. If you're worried about your health, contact your GP or NHS 111.
What to do first
The fix is a sequence: verify the extractor first, treat the visible mould second, deal with sealant last. Cleaning first and checking the ventilation later is how a bathroom ends up being cleaned again and again.
- 1
Test extractor performance with a tissue
Hold a single sheet of loo roll flat against the grille while the fan runs. It should stick unaided. If it falls away, the fan is not moving much air.
- 2
Time the mirror
After a normal shower, close the door, run the fan, and note how long the mirror stays fogged. Do this each day for a week. If it stays fogged long after the fan has stopped, or gets worse over the week, that is worth recording.
- 3
Purge-ventilate at the window for a few minutes after every shower
Even a good extractor benefits from a short cross-flow. Open the window fully with the door open for a few minutes so the room clears and the surfaces can dry.
- 4
Clean the visible mould with a fungicidal wash
Ceiling and walls first, then tile grout, then sealant last. Ventilate throughout, and do not repaint or reseal until the surfaces are fully dry.
- 5
Cut out and replace failed silicone sealant
If the bead is cracked, discoloured through the depth or lifting away, replace it. Trying to bleach or paint over a failed sealant traps moisture behind. If you rent, a failed bead is normally a repair to report rather than a job for you.
- 6
If mould returns within a couple of weeks, have the fan tested
If the mould comes back within a couple of weeks of cleaning and the fan fails the tissue test, the fan needs testing, repairing or replacing. Keep the duct run short and straight. If you rent, ask the landlord to do this rather than cleaning again.
What NOT to do
Bathroom mould advice is the noisiest corner of the DIY aisle. Four claims worth checking before you act on them.
Myth
A bathroom fan is optional.
Why it's wrong. Approved Document F expects extract ventilation in a bathroom when one is built or altered, and gives the rates it should achieve. For an existing home, the question is whether the room is clearing its steam: a fan that is missing, weak or switched off leaves the steam to settle on the coldest surfaces.
Myth
Just leave the door open after showering.
Why it's wrong. It vents steam into the rest of the house, where it settles on the coldest surface it finds, often a bedroom wall. The steam has to leave the building, not migrate.
Myth
Paint everything with anti-mould paint.
Why it's wrong. Anti-mould paint slows regrowth on the surface but does nothing about the steam. With an under-performing extractor the mould comes back on top of it.
Myth
Bleach the grout every week and it'll be fine.
Why it's wrong. Repeated bleaching does not restore worn sealant or grout. Once grout is discoloured through the full depth, it should be raked out and replaced, not painted or bleached repeatedly.
When you need evidence or a survey
A dated record of the fan test, the mirror times and where the mould is lets a landlord, council officer or insurer see whether the pattern fits steam that is not clearing or a leak behind the wall. A fan that fails the tissue test, a mirror that stays fogged long after the fan has stopped, and mould concentrated above the shower and in the ceiling corners point to ventilation; where a fan has been measured, the 15 l/s Approved Document F benchmark for an intermittent fan (or 8 l/s continuous) gives a reference point for a new or altered bathroom. A soft floor by the shower or a stain in the next room points to a leak, which needs a panel lifted rather than a wipe-down.
This is general information, not legal advice. For advice on your specific situation, contact Shelter, Citizens Advice, or a solicitor.
Common questions people ask
How many litres per second (l/s) should a bathroom fan be?
In England, Approved Document F gives 15 l/s for an intermittent bathroom extract fan or 8 l/s for continuous extract (Volume 1, Tables 1.1 and 1.2). It applies when a bathroom or fan is installed or altered and is a benchmark rather than a rule for every existing home. Whatever the rating, short straight ducting and a fan that runs on after the shower matter as much as the number on the box.
Why does mould keep coming back on my bathroom sealant?
Because bleach removes the surface discolouration but does not restore a bead that has aged, cracked or lifted. Once the discolouration is through the depth of the silicone, the fix is to rake it out and re-bead, not to keep cleaning. If you rent, a failed bead is normally a repair to report to the landlord.
Is a mouldy bathroom a health risk?
GOV.UK guidance on damp and mould says it should always be acted on quickly, and lists people with a pre-existing health condition (for example allergies, asthma or COPD), people with weakened immune systems, people living with a mental health condition, pregnant women and women who have recently given birth, children and young people, and older people as more vulnerable. Widespread ceiling and wall mould in a small bathroom used by anyone in those groups is a reason to act rather than wait. This page is not medical advice.
Should I run my extractor fan longer after a shower?
Yes. Run the fan for the whole shower and leave it running afterwards until the mirror and tiles have cleared. Fans with a humidity sensor or a timed overrun do this for you.
Can bathroom mould spread to the bedroom?
Yes — every shower's steam has to leave the room somewhere. If the extractor cannot handle it and the door is opened, the humid air migrates to the coolest adjoining space, often a bedroom on an external wall, and condenses there.
When does bathroom mould mean a bigger problem?
When it appears on the far side of a shower wall in the next room, when the floor by the shower is soft, when a downstairs ceiling shows a stain in the same footprint, or when a landlord's inspection has replaced the fan and mould still returns. Any of those points to a leak or a fabric defect, not something cleaning will fix.
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Damp patch on the ceiling
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Cost guide
Cost to replace a bathroom extractor