Mould in the bathroom: why it returns and how to stop it

Why bathrooms grow mould

A shower puts a large amount of water vapour into a small room. When that warm, wet air meets a cold surface, such as an outside wall, a ceiling under a loft or a window frame, it condenses into droplets. Mould spores are always present in the air. Give them a surface that stays wet for long periods, day after day, and they settle and grow.

The fix is therefore about time. A surface that is wet for twenty minutes and then dries rarely grows mould. A surface that is still damp the next morning almost always will.

Extractor fans that do not extract

Many bathrooms have a fan that runs but moves very little air. Common reasons are a blocked or crushed duct in the loft, a duct that is far too long, a fan sized for a toilet rather than a shower room, or a fan that switches off the moment the light does. Hold a single sheet of toilet paper against the grille: if the fan barely holds it, it is not clearing the room.

Replacing a weak fan with a suitable model, on an overrun timer or humidity sensor, and positioning it near the shower makes more difference than any paint or cleaner.

Sealant and grout: when cleaning stops working

Mould on silicone often grows into the sealant itself, not just on the surface. Once that happens bleach lightens it for a while but it returns within weeks. The lasting fix is to cut the old sealant out completely, clean and dry the joint, and apply fresh sanitary silicone. Grout that has turned black and porous can be raked out and regrouted, though if it is cracked or tiles sound hollow there may be water getting behind them.

Cold ceilings and condensation spots

Patches of mould in a ceiling corner, or in a line along a joist, often show where loft insulation has slipped or was never laid to the edges. That patch of ceiling stays colder than the rest, so condensation forms there first. Checking and correcting the insulation above is part of the cure.

Black mould on walls and ceilings: when to replaster or redecorate

Light surface mould on sound paint can be treated, allowed to dry, sealed and repainted with a paint made for bathrooms. If the plaster underneath is crumbling, the paint is blistering, or mould keeps returning through new coats, the surface may need cutting back and replastering. We cover both through our decorating and plastering work, and as part of a wider bathroom refit where the room needs more than a refresh.

Before choosing between resealing and redecorating, describe the pattern rather than calling every mark damp. Is the growth confined to the shower joint, spread across the ceiling, or concentrated on a cold outside corner? Does the surface dry between showers? Has fresh paint already failed in the same spot? Keep those answers with photographs of the affected area. A black silicone joint and a blistering ceiling may be in the same bathroom but need different repairs. The aim is to identify which surfaces need attention and what is keeping them wet, instead of paying to refresh the whole room while the original cause remains.

Landlords: what to fix before a tenant reports it

In a rented home, check the fan, the sealant and the ceiling at every inspection and between tenancies. A tenant can only dry a room as well as the ventilation allows. Our guide to damp and condensation in rented property explains how to tell condensation from other damp, and our landlord maintenance plans can build these checks into routine visits.