How to get rid of black mould in a bathroom

Black mould in a bathroom is caused by moisture in the air condensing on a cold surface and never fully drying out — and cleaning it off, whether with bleach or a specialist spray, only ever removes what has already grown. If the humidity behind it is not addressed, it returns, usually within a matter of weeks.

Clean it properly first

A dilute bleach solution or a specialist mould spray, left to sit rather than wiped off immediately, kills the mould at the surface and in the grout it has taken hold in. This step matters and is worth doing properly — but it is the smaller half of actually solving the problem.

Then find out why it grew there

Mould needs sustained moisture to establish, which in a bathroom almost always means one of: an extractor fan that switches off with the light rather than running on after a shower, a window that is kept shut because it is painted shut or simply never opened, or a room with genuinely poor airflow — an internal bathroom with no window at all being the extreme case.

The actual fix

A humidity-triggered extractor fan, or one on a timer that runs for fifteen to twenty minutes after the light goes off, clears the moist air properly instead of stopping the instant the switch does. Where an extractor already exists but the problem persists, it is worth checking it is actually venting outside rather than into a loft space, and that any passive trickle vents in the window frames have not been painted shut over the years. Positive input ventilation is worth considering for a house where a single extractor fan is not enough — it ducts fresh, filtered air in centrally rather than relying on individual room extraction.

When it is not condensation at all

If the affected patch is not on a cold surface prone to condensation — say, low down on an external wall, or in a pattern that does not track with steam and showering — it may be penetrating damp from an external defect rather than condensation, and treating it as a ventilation problem will not help. Telling the two apart usually comes down to where the patch sits and whether it tracks with steam and showering, rather than guessing — worth having someone experienced take a look.