Why is there mould on my bathroom ceiling?
By The Damp GuyLast updated
Because the ceiling is the coldest surface above the wettest room in the house. Warm wet air from a shower rises, and the ceiling is the first cold thing it meets. Above it is usually an unheated loft, which keeps that plasterboard colder than any wall in the room. Mould sits at the top because the moisture goes to the top.
The building regulations agree, and they put it in writing twice. Approved Document F classes a bathroom as a wet room, meaning a room used for domestic activities that "produce significant amounts of airborne moisture". It then tells whoever fits the extractor to install it "as high as is practicable in the room" and "a maximum of 400mm below the ceiling". The fan is sited at the ceiling because that is where the wet air collects.
So the location is the diagnosis. A ceiling that goes mouldy while the walls stay clean is telling you that the room makes more moisture than it clears, and that the coldest surface in it is over your head. Two questions settle most of it, and you can answer both yourself.
Question one: does the fan move air, and where does it send it
Start with the four words most people skip. Part F says extract ventilation "to the outside" should be provided in bathrooms. Outside. Not into the roof space, not into a void, not into a duct that stops halfway. A fan that discharges into the loft runs, sounds right and clears the mirror, and it has moved a bathroom's worth of water into your roof. The ventilation guide on this site covers that defect in full.
Next, find out if it actually moves air rather than noise. Government guidance asks landlords a question worth asking yourself: are the bathroom fans "fully operational and sufficiently powered to remove moisture from showering and bathing"? It never says what sufficient means. Approved Document F does, and the ventilation guide lists the minimum extract rate for a bathroom alongside every other wet room in the house.
If the house is fairly new, you may be able to get a number rather than an opinion. Air flow rates for mechanical ventilation in new dwellings must be measured, and a notice of the measured rates has to go to the building control body. That covers intermittent extract fans, not only whole-house systems. A copy of the completed commissioning sheet should be given to the owner of the dwelling, and the sheet records the measured rate against the design rate. Ask for it. A builder who has it will hand it over, and one who cannot produce it has told you something too.
Then check how long it runs for. A bathroom with no openable window should have controls that keep the fan going for at least 15 minutes after the room is vacated, and an installer checking a run-on timer is told to confirm it is set to at least 15 minutes. A fan wired to the light switch stops the moment you flick it off, which is roughly the moment the room is at its wettest.
Question two: what is directly above that ceiling
A bathroom ceiling under a loft is not a wall with a different orientation. It is the boundary between the wettest room in the house and an unheated space, and that space got colder the day somebody insulated it. The government put it in a single sentence in its own loft insulation factsheet: "Insulating your loft will keep your house warmer but make the roof space above colder." Warmer house, colder void, and your bathroom ceiling in between.
That ceiling is also leakier than it looks, and a bathroom is the worst room in the house to have a leaky one. Recessed spotlights are the usual route. NHBC's new-build standards say that where downlighters are used they should be specified and sealed to limit air leakage, and a row of them in a bathroom ceiling is a row of openings between a steamy room and a cold void, each one a cold spot on the way past. Approved Document C asks for the same discipline everywhere else on that ceiling: gaps and penetrations for pipes and wiring filled and sealed, and a loft hatch that is properly draught-sealed, all of it "to avoid excessive moisture transfer to roof voids".
If the loft above is damp, mouldy or dripping, handle the two together rather than separately. A bathroom fan and a wet roof space directly above it are usually the same story told from both ends, and the loft condensation guide on this site takes it from the roof side, including how the duct should be run once it is up there.
Cleaning it off is the easy half
You can take it off a painted ceiling in an afternoon. The government guidance is then unusually direct about what that achieves: "Simply removing surface mould will not prevent the damp and mould from reappearing." The method, the safety kit and the reason bleach is not the recommended product are all set out in the guide to mould on walls here.
Two limits are worth carrying over, because a ceiling runs into both. The guidance says larger areas of mould should only be addressed by qualified professionals, and a ceiling is a large area worked at arm's length above your head. It also says absorbent materials such as ceiling tiles may have to be thrown away if they become mouldy, since the mould cannot reliably be got out of something porous. A painted plasterboard ceiling cleans. A suspended tile does not.
On health, the guidance has its own wording, and the same guide reports it as the guidance's own, including the at-risk groups it names. That is where to read it. A symptom is a question for a GP, not for a website.
When the ceiling is not the real answer
Some bathrooms pass both questions. The fan is right, the duct goes outside, the loft is dry, and the mould comes back anyway. At that point the problem is the room rather than the equipment, and the thing to understand is the humidity threshold at which a surface starts growing mould. It is lower than most people expect, and a surface does not have to be running wet to cross it. The condensation guide here sets out where that line sits and where the figure comes from.
The windows in the same room will usually tell you first. Glass is colder than plaster, so it mists while the ceiling still looks clean, which makes it the cheapest gauge in the house. The guide to condensation on windows argues the point that follows from it: the wet surface is the reading, not the fault.
And if the ceiling darkens after heavy rain rather than after a shower, stop treating it as condensation. That pattern points at water crossing the structure from outside, which is a defect with a location rather than a humidity problem. This page cannot see your ceiling, and neither can a photograph. When the pattern fits neither question above, get someone to look at it who has no interest in selling you the treatment.