Black mould in the corners. Pink stuff in the grout. Spots on the silicone around the bath. Wipe it down on Sunday, it's back by Friday.

Bathroom mould is rarely about how often you clean. It's almost always about ventilation, surface temperature, or moisture trapped where it shouldn't be. Here's what we actually find when we go into mouldy bathrooms.

Cause 1: Cold spots

Mould grows where warm wet air condenses on a cold surface. The cold corner where the external wall meets the ceiling. The spot behind the toilet on a north-facing wall. The bottom edge of single-glazed window frames.

If your mould is always in the same specific spot — and it's a spot that would naturally be cold — the problem isn't the cleaning. It's the temperature of that surface.

Fix: Insulation behind the cold spot, sometimes thermal liner, sometimes addressing a thermal bridge in the wall structure. Painting won't fix it. Bleach won't fix it.

Cause 2: Bad ventilation

Most UK bathroom extractors are too weak, too slow, and switched off too quickly. By regs, a bathroom needs at least 15 litres/second of extract. Most cheap extractors barely manage 8.

If your fan goes off thirty seconds after you flick the light switch, your bathroom is full of warm steam for the next two hours every time you shower. That moisture lands on every cold surface. That moisture is the mould food.

Fix: Decent extractor with proper run-on time (10–15 minutes after the light goes off). Or a humidistat that runs the fan based on actual humidity levels, not on whether the light's on.

Cause 3: Failed silicone

Silicone shrinks and pulls away from surfaces over 5–10 years. Once it's split, water gets behind it into the wall or floor, soaks the substrate, and mould blooms underneath. You'll see pink/black blooms appearing through the silicone itself — which means the wood or board behind it is wet.

Fix: Strip out the old silicone, dry the substrate, re-silicone with a proper sanitary-grade product. Use the right silicone — kitchen silicone fails fast in showers.

Cause 4: Tanking failure (worst case)

If you've got a wet room or walk-in shower and you're getting damp showing through to a wall in the next room — or worse, the ceiling below — the waterproofing under your tiles has failed.

This is the expensive one. There's no surface fix. The tiles have to come off, the substrate has to dry, and the tanking has to be redone properly. Then re-tile.

Fix: Strip and re-tank. There's no shortcut. This is the price you pay when a bathroom is fitted by someone who skipped the waterproofing step.

Quick wins

Before you call anyone, try these — they fix maybe a third of bathroom mould issues:

  • Run the extractor for 15 minutes after every shower (use a plug-in timer if your existing fan won't do it).
  • Crack the window for 5 minutes after showering, even in winter.
  • Strip and replace the silicone around the bath/shower if it's more than 5 years old.
  • Wipe down the shower walls after use. Yes, every time. The professionals do this in their own homes.

If after that you're still getting mould — especially in specific spots — it's likely a structural or ventilation issue, not a cleaning issue. Give us a ring on 07516 555 377 and we'll come and have a proper look.