Problems
Why damp and mould keep coming back
How to tell condensation from a leak, and what to fix before decorating again.
Repainting a mouldy bathroom wall is the most commonly repeated mistake in home maintenance. The mould returns because the cause was never addressed, only covered.
This guide explains how we work out what is actually happening, which decides whether you need better ventilation, a repair or damp proofing.
Condensation, the usual culprit
Warm wet air meets a cold surface and water forms. It shows as black spotted mould in corners, on cold external walls, around window reveals and on ceilings. It is worst in winter and in rooms with no working extraction.
Leaks behind tiles
A failed seal at a shower tray or a pinhole on a pipe produces damp in one localised area that does not improve with ventilation. Grout going dark in a single patch, or a stain on the ceiling below, is the usual first sign.
Rising and penetrating damp
This appears low on walls with a tide mark, or on a wall exposed to driving rain, and shows salting on the plaster. This is where damp proofing is the right answer rather than better ventilation.
How to tell them apart
A rough diagnostic before anyone opens a wall.
- Black spots in corners and on cold walls: condensation
- One localised damp patch near a shower or pipe: leak
- Tide mark low on the wall with salting: rising damp
- Damp worsening after heavy rain on one elevation: penetrating damp
- Damp only in winter: almost certainly condensation
Fixing it properly
Condensation needs extraction that actually clears the room, plus warmer surfaces, which often means insulated boarding on a cold external wall during a renovation. Leaks need the seal or pipe repaired and the substrate dried. Rising and penetrating damp need treating at source and replastering with the correct system.
Only then is it worth decorating, and only then will tiles stay put.
Questions
Frequently asked
Will mould resistant paint solve it?
It slows the symptom. If the wall is still cold and wet, the mould returns.
Is an extractor fan enough on its own?
Often yes for condensation, provided it is the right size, vented outside and left running long enough after showering.
Should damp be treated before a new bathroom?
Always. Treating it first is far cheaper than taking a finished bathroom back apart.
Can damp be sorted at the same time as a renovation?
Yes, and it is the ideal moment, because the walls are already open and the room is out of use.
Why does my bathroom only go mouldy in winter?
Cold surfaces. That is condensation rather than a leak, and the fix is ventilation and insulation.
Services in this guide
Related guides
Free home survey
Ready to plan your bathroom?
Talk directly to Martin about your room, priorities and budget.