Why bathrooms get damp
A hot shower puts a surprising volume of water vapour into a small room in a few minutes. If that vapour cannot be extracted, it condenses on the coldest surfaces - the ceiling, the window reveal, the external wall corner - and mould follows within weeks.
The pattern is diagnostic: mould concentrated on cold surfaces and in corners points to condensation, not to a leak.
Extractor sizing and placement
Building Regulations set a minimum extract rate for bathrooms, and the practical point is that an undersized or badly ducted fan will not meet it regardless of its label. Long flexible ducting runs, crushed ducting in a loft and multiple bends all cut the real extract rate sharply.
Placement matters too. A fan on the far side of the room from the shower, with the door undercut providing make-up air, moves far more moisture than a fan sited badly.
Run-on timers and humidity sensing
A fan that stops when the light goes off has done perhaps a third of its job. A run-on timer continues for several minutes after you leave. A humidity-sensing fan goes further and runs until the room is actually dry, which is the right specification for a family bathroom with back-to-back showers.
What to do in a room with no external wall
Internal bathrooms need ducted extraction to the outside, routed as directly as practical with rigid duct where possible. Recirculating a fan into a loft void is not an acceptable solution - it simply moves the moisture problem into the roof structure.