6 min read
Black mould in the bathroom
Bathrooms are where we get called out most. A bathroom makes several litres of warm moisture a day and pushes it against the coldest surfaces in the house. Get the balance wrong and mould has everything it needs within a fortnight.
Why the ceiling goes first
Warm, wet air rises. When it reaches a ceiling that is colder than the air — usually because there is an unheated loft above it — the moisture condenses out onto the paint. That thin film of water is all mould needs.
This is why scrubbing the ceiling never lasts. You have removed the growth but not the daily film of water that caused it.
Grout, silicone and window reveals
Grout is porous, so spores root into it rather than sitting on top. Silicone goes black from behind when water has tracked under the bead, which is why surface cleaning does nothing once sealant has failed.
Window reveals are usually the coldest part of the room. If the black is worst around the window, you are looking at condensation, not a leak.
The three fixes that actually work
First, extraction. The fan needs to shift the air out of the building, run for at least fifteen minutes after the shower, and have the door gap to let replacement air in. Most failed bathrooms have a fan that is either too weak, ducted into the loft, or switched off with the light.
Second, heat. A bathroom that drops to twelve degrees between showers will condense whatever you do. Keeping the room gently warm costs less than repeated redecoration.
Third, proper removal and treatment. Affected grout and sealant may need replacing; suitable surfaces are cleaned and can receive a protective chemical treatment. Insulation above a cold ceiling may also need attention.
Frequently asked
Is bleach enough for bathroom mould?
No. Bleach takes the colour out but leaves the roots in porous grout and plaster, so it usually returns within weeks.
How long does a professional bathroom treatment take?
Two to four hours for a typical domestic bathroom, including cleaning, treatment and clean-up.
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