Why Is My Bathroom Growing Mould?

Bathroom mould is a genuinely common problem, and it almost always comes down to one of three real causes — not bad luck or poor cleaning. Working out which one applies to your bathroom is the difference between a mould problem that keeps coming back and one that actually gets solved.

Cause 1: Poor ventilation

Every shower or bath releases a large amount of moisture into the air, and if it isn't extracted quickly, it settles on cooler surfaces — ceilings, tiles, grout, window frames — where mould takes hold. Signs this is your issue: mould appears fairly evenly across the room, particularly on the ceiling and upper walls, and gets worse in winter. See our common bathroom mistakes guide for why adequate extraction matters from the start of a renovation, not as an afterthought.

Cause 2: Condensation on cold surfaces

Related to ventilation but distinct — surfaces that are colder than the rest of the room (an external wall, single-glazed window, an unheated towel rail) attract condensation even with reasonable ventilation. Signs: mould concentrated on one specific wall or around a window frame, rather than spread across the room. Our guide on damp vs condensation covers how to tell this apart from a more serious structural issue in more general terms.

Cause 3: A waterproofing or plumbing failure

The more serious cause — water getting behind tiling or through a failed seal, rather than sitting on the surface. Signs: mould localised around a specific grout line, the base of a shower enclosure, or a floor junction; tiles that feel loose or grout that's visibly cracked; or a musty smell that doesn't clear even after cleaning. This is a genuine structural concern, not a cleaning problem — see our bathroom tiling costs guide for what proper waterproofing should have involved in the first place.

What extraction actually needs to achieve

Building regulations set a minimum extraction rate for bathrooms, and an underpowered or badly positioned extractor fan can technically be "there" while still not clearing moisture fast enough. As a rough guide, a fan should be sized to the room volume, and — this is the part most homes get wrong — it needs to keep running for a genuine 15–20 minutes after a shower or bath, not switch off the moment the light does. A humidity-triggered or timer-delay fan solves this automatically; a simple fan on the light switch relies on someone remembering to leave the light on.

What you can genuinely fix yourself

• Run the extractor fan during and for 15–20 minutes after every shower or bath.
• Wipe down wet surfaces (shower screens, tiled areas) after use — genuinely reduces the moisture available for mould to grow on.
• Check the extractor is actually working — hold a tissue near the grille; it should be pulled towards it when running.
• Keep the door open after bathing if there's no window, to let moisture disperse into the rest of the house rather than sit in the room.
• Re-seal a tired-looking silicone joint yourself — this is a genuinely DIY-accessible job, and a cracked or blackened seal is a common early entry point for cause 3 above.

When it needs a professional, not another clean

If mould keeps returning within weeks of cleaning despite reasonable ventilation habits, or it's localised around tiling/grout rather than spread across the room, that points toward cause 3 — a waterproofing or plumbing issue behind the surface, which needs a proper investigation rather than repeated surface treatment. Re-sealing a joint yourself is fine as a first step; if the mould comes straight back through a fresh seal, that's a sign water is getting in from somewhere else and it's worth having it properly assessed.

A quick self-check

• Spread across the ceiling/upper walls, worse in winter → likely ventilation.
• Concentrated on one cold wall or window frame → likely condensation.
• Localised around one grout line, floor junction or fitting, or returns straight through a fresh reseal → likely a waterproofing or plumbing issue — get it looked at properly.

Products that help, and what they don't fix

Mould-resistant silicone sealant and anti-mould bathroom paint genuinely help — they're formulated to resist mould growth on the surface for longer than standard products. What they don't do is fix an underlying ventilation, condensation or waterproofing problem; used on their own, without addressing cause 1, 2 or 3 above, they simply delay the mould's return rather than prevent it. They're worth using as part of a proper fix, not as a substitute for one.

Preventing it after a renovation

If you're renovating anyway, this is the point to fix ventilation properly rather than patch around it later — a correctly sized, humidity-triggered extractor fan with a short duct run to the outside (long, winding duct runs lose much of their effectiveness) costs relatively little to specify correctly at first-fix stage, compared with retrofitting proper ventilation into a finished room later.