Short answer: mould on a bathroom ceiling is almost always condensation. Hot, wet air from showers and baths rises, hits the coldest surface in the room, and condenses there. Ceilings are cold when the loft above them is poorly insulated, and the moisture stays when the extractor fan is too weak, switched off too soon or venting into the loft instead of outside. Clean it and it comes back. Fix the ventilation and the cold ceiling, and it usually doesn't.
The exception is a ceiling with a brown or yellow tide-mark, sagging, or mould that appeared suddenly after a leak upstairs. That is water coming through from above, and it needs a plumber or roofer before anything else.
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Get free quotesCondensation or a leak? Check the pattern
- Condensation mould shows up as a scatter of black or grey dots, heaviest in the corners, along the edge where the ceiling meets external walls, and above the shower. It gets worse in winter and after long showers.
- A leak usually leaves a defined stain with a darker ring, sometimes with flaking paint or a soft, bowed patch of plaster. It may appear in one place regardless of the season, and often below a bathroom, water tank, pipe run or roof valley.
If you're not sure, a damp specialist with a moisture meter can tell you in minutes. Our guide to damp surveys versus mould inspections explains which visit you need.
The five usual causes of condensation mould on a bathroom ceiling
1. The extractor fan is too weak
Building regulations guidance (Approved Document F) sets minimum extract rates. For a bathroom with an intermittent fan, that is 15 litres per second. For a continuously running fan, the minimum high (boost) rate is 8 litres per second. Many older bathroom fans, and plenty of cheap replacements, move far less than their label suggests once they're connected to a long or kinked duct.
A rough check: hold a single sheet of toilet paper against the grille with the fan running. If it isn't held firmly in place, the fan is probably underperforming. A specialist can measure the actual flow with an anemometer.
2. The fan switches off too soon
Steam keeps rising off wet tiles, towels and the shower tray long after you've turned the water off. A fan wired to the light that stops the moment you leave does very little. Look for a fan with an overrun timer (15 minutes or more is common) or a humidistat that keeps it running until humidity drops.
3. The ducting dumps air into the loft
This is surprisingly common. If the duct has come loose, was never run to an outside vent, or is long, sagging and uninsulated, warm wet air either never leaves the house or condenses inside the duct and drips back. Look in the loft: the duct should run to a roof or soffit vent, be as short and straight as possible, and be insulated where it crosses the cold loft.
4. The ceiling is cold because the loft insulation is thin or patchy
Moisture condenses on the coldest surface. If the insulation above the bathroom is thin, missing near the eaves, or has been pushed aside when someone worked in the loft, that patch of ceiling stays cold. Government guidance on damp and mould suggests loft insulation of at least 270mm. Gaps at the edges explain why mould often starts in a strip along the external wall.
5. The room never warms up
A cold bathroom with a towel rail that's barely warm gives condensation nowhere to go. Some background heat makes surfaces warmer and helps the room dry between uses.
How to fix it properly
Do it in this order. Skipping to the paint is the reason most bathroom ceilings go black again within a season.
- Diagnose. Rule out a leak, then check fan performance, overrun and ducting.
- Fix ventilation. Replace or upgrade the fan to meet at least the 15 l/s guidance, add an overrun timer or humidistat, and make sure the duct vents outside. In some homes a continuously running low-energy fan (often called dMEV) or a whole-house system works better; our guide to PIV vs MVHR vs extractor fans compares them.
- Warm the ceiling. Top up and even out loft insulation, paying attention to the edges, without blocking eaves ventilation.
- Clean the mould with a product labelled for mould, wearing gloves, eye protection and a mask. Don't dry-brush it. (Why not bleach? See does bleach kill mould?)
- Let it dry fully, then redecorate with a bathroom or anti-mould paint.
Habits that help (but won't fix a bad fan)
- Run the fan during and after every shower, and leave it running.
- Close the bathroom door while showering so steam doesn't spread to cold bedrooms.
- Open the window for ten minutes afterwards if you can, then close it.
- Squeegee or wipe down the shower screen and tiles.
- Don't dry towels or washing in a bathroom that's already struggling.
For landlords
A bathroom with a fan that doesn't meet the extract rate, a duct venting into the loft or patchy loft insulation is a building issue, not tenant behaviour. Government guidance is clear that blaming damp and mould on "lifestyle choices" is unreasonable, and the updated Housing Health and Safety Rating System in England includes baseline indicators for ventilation and a 5% limit for observable damp or mould on any wall or ceiling surface in a room. Upgrading a fan is one of the cheapest damp repairs you can make. See condensation control for landlords.
FAQs
Why is there mould on my bathroom ceiling but not the walls?
Warm air rises, so the ceiling gets the most moisture, and it's often the coldest surface if the loft insulation is thin. Tiled walls also wipe dry more easily than a painted ceiling.
Will anti-mould paint fix it?
Only after the ventilation and insulation have been sorted. On a cold, wet ceiling it slows regrowth for a while, then fails.
Do I need planning permission or building control for a new extractor fan?
Replacing a fan like-for-like is normally straightforward, but any new electrical work in a bathroom must comply with the wiring regulations, so use a qualified electrician.
How much does it cost to fix?
A replacement fan is usually the smallest part of the bill; ducting and insulation work add more. See our condensation control cost guide for typical prices.