1. The surface was cleaned, not treated
Mould roots grow into plaster, paint and sealant. Bleach and most sprays only clean the surface. If the roots survive, the mould regrows as soon as the surface is damp again.
2. The moisture source is still there
Mould needs moisture. If the cause is still there, whether condensation, a leak or rain getting in, mould will always return. Cleaning without fixing the cause is a loop.
3. Weak or broken extractor fans
A fan that does not pull air, is not ducted outside, or switches off with the light leaves steam in the room. This is the top cause of bathroom ceiling mould.
4. Cold spots in the walls
Corners, the edges of ceilings and walls with missing insulation are colder than the rest of the room. Moist air condenses there first. These cold bridges are why mould often forms in neat lines or just in corners.
5. Furniture against external walls
Wardrobes and beds pushed against cold walls stop warm air reaching the wall. Leaving a gap of about 5 to 10cm makes a big difference.
6. Drying clothes indoors
One load of washing dried indoors releases a lot of water into the air. If you have to dry indoors, do it in one room with the door closed, a window open or a dehumidifier running.
7. Painting over it
Normal paint over mould traps it, and it grows through within weeks. A treated surface needs a stain block and ideally an anti mould paint. See does anti mould paint work.
What actually stops it
Kill the mould properly, fix the moisture source, improve ventilation and keep the home at a steady temperature. That is the process behind our mould removal, and why we can offer a 6 month guarantee.
Last reviewed 15 November 2026 by the Home Damp and Mould team. Read our editorial policy.