Bathroom and ceiling mould are two of the most common calls we get — and two of the most frequently mistreated, because cleaning it off deals with the symptom while leaving the cause completely untouched.
Why Bathrooms Are the Worst-Affected Room
A single shower puts a litre or more of water vapour into the air in minutes, in a room that's often the smallest and least ventilated in the house. Tiled surfaces, silicone sealant and grout are all cold, non-absorbent surfaces that condensation forms on readily — and once mould establishes in grout or silicone, it's notoriously difficult to fully remove because the mycelium grows into the porous material, not just on its surface.
If your extractor fan is disconnected, ducted into the loft space instead of outside, or switches off the moment you leave the room, the moisture from every shower has nowhere to go — which is why the mould reliably comes back within weeks of being cleaned.
Why Mould Appears on Ceilings
Warm, moist air rises, and ceilings are frequently the coldest surface in a room — especially directly below an unheated loft space, a flat roof with inadequate insulation, or a bathroom on the floor above. That combination of rising moisture and a cold surface is exactly what condensation needs.
Ceiling mould isn't always condensation, though. A roof leak, a bathroom leak from the room above, or a plumbing defect running through the ceiling void can all produce a very similar-looking patch of mould and staining — which is why the pattern and whether it correlates with rain or usage matters before assuming it's purely a ventilation issue.
Extraction and Ventilation
- Run the extractor fan for at least 15-20 minutes after a shower, not just while the shower is running
- Check the fan is ducted externally, not into the loft space where the moisture just relocates
- Fit a humidity-sensing or timer-delay fan if yours switches off the moment you leave the room
- Keep the bathroom door shut while showering and open afterwards, so extraction pulls air from the rest of the property rather than fighting an open door
- Wipe down tiles and shower screens after use — it doesn't fix ventilation, but it reduces standing moisture between uses
When It's a Roof or Plumbing Leak, Not Condensation
Condensation mould tends to track with how the room is used — worse after showers, easing off with better ventilation. A leak tends to behave differently: a stain that grows steadily regardless of ventilation changes, worsens specifically after heavy rain, or appears somewhere condensation wouldn't typically form (mid-ceiling rather than a cold corner, for instance) points towards a leak rather than airborne moisture.
See our guide on water ingress for how a suspected leak gets traced back to its actual source.
Cleaning It vs Actually Fixing It
A fungicidal wash or bleach solution kills visible mould, and mould-resistant paint slows regrowth on the surface it's applied to. Neither addresses inadequate extraction, a cold ceiling with no insulation, or a leak — the actual causes. Used after the cause is fixed, both are genuinely useful. Used instead of fixing the cause, they typically buy a few extra weeks before the mould is back.
Our full black mould removal guide covers the step-by-step process for treating the mould itself once you know what's causing it.
When to Get a Survey
If mould keeps returning within a few weeks of cleaning despite reasonable extraction, if there's a musty smell without much visible mould, or if you suspect a leak rather than condensation, it's worth getting a proper diagnosis. Our mould report from £100+VAT identifies the actual cause with moisture readings, rather than another round of cleaning and hoping.
FAQ · Frequently Asked
Common Questions
Why does mould keep coming back in my bathroom no matter how much I clean it?+
Cleaning removes the visible mould but not the cause. If the extraction is inadequate, run for too short a time, or ducted into a loft space instead of outside, the moisture that feeds the mould is still there after every shower — so it reliably returns within a few weeks of being cleaned off.
Why does mould appear on ceilings specifically?+
Warm, moist air rises and ceilings — especially in bathrooms, and rooms below a cold loft space or a poorly insulated flat roof — are often the coldest surface in the room, which is exactly where airborne moisture condenses first. A leak from a roof, bathroom above, or plumbing run can cause the same symptom, so the pattern and whether it's linked to a wet room above both matter.
How long should my extractor fan run after a shower?+
At least 15-20 minutes after you finish, to clear the moisture-laden air rather than just the steam you can see. Many extractor fans have a timer or humidity sensor for exactly this reason — if yours switches off the moment you leave the room, it's very likely under-ventilating the space.
How do I know if bathroom mould is condensation or a leak?+
Condensation mould typically follows cold surfaces — tile grout, silicone, the ceiling above the shower, external corners — and tracks with how the room is used and ventilated. A leak tends to produce a more localised, often growing, brown-tinged stain that doesn't correlate with shower use and may worsen over time regardless of ventilation.
Will a mould-resistant paint fix the problem permanently?+
It slows regrowth on the surface it's applied to, but it doesn't address inadequate ventilation, a leak, or thermal bridging — the actual causes. Used after the real cause is fixed, it's a reasonable extra layer of protection; used instead of fixing the cause, it typically buys a few extra months at best.
When should I get a professional survey instead of just re-cleaning?+
If mould returns within a few weeks of cleaning despite reasonable ventilation, if there's a musty smell without visible mould, or if you suspect a leak rather than condensation, a survey identifies the actual cause with moisture readings rather than another round of guesswork and bleach.
