Mold in the bathroom, and how to stop it coming back.

Run the exhaust fan for at least twenty minutes after the shower ends, with the door shut, every time. Bathroom mold is nearly always condensation rather than a leak, and moisture that is not removed within the first half hour is what feeds it. Cleaning the patch treats the symptom; the fan run time treats the cause.

If there is no fan, open the window instead and shut the door, so the moist air leaves the building rather than moving into the rest of the house.

Condensation or a leak

Worth establishing early, because the fixes are unrelated.

Condensation shows on cold surfaces and in still air: the ceiling above the shower, the corner where two exterior walls meet, the grout, the window reveal, the wall behind a cabinet. It is worse in winter, worse in rooms with poor ventilation, and it appears over the whole cold surface rather than in one spot.

A leak shows in one place, is often damp to the touch when the room is otherwise dry, and stays damp between showers. Under a sink, at the base of a toilet, on the ceiling below a bathroom on the floor above, at the edge of a tray or bath, or on the wall behind a shower.

A quick test: dry everything, do not use the room for a day, and look again. Dry after a day of no use means condensation. Still damp means a leak, and a leak with no obvious source needs a different approach.

The ventilation fix, in order of what actually works

Run the fan longer. Most people run it during the shower and switch it off on the way out, which is exactly backwards: the humidity peak is after. Twenty minutes minimum, thirty is better. A timer switch or a humidity sensing fan does this without anyone remembering.

Shut the door while it runs. An open door lets the fan pull dry air from the landing instead of removing wet air from the room.

Check the fan actually moves air. Hold a square of toilet paper to the grille. If it does not hold, the fan is not working, the duct is blocked or disconnected, or it vents into the attic rather than outside. A fan discharging into a roof space is a common and expensive fault, because it moves the problem into the structure.

Clean the grille. They clog with dust and lose most of their flow.

Open the window if there is one, even a little, particularly in warm weather.

Squeegee the walls and screen after a shower. It takes thirty seconds and removes a surprising share of the water.

Leave the shower door or curtain open afterwards so it dries.

Wash and dry bath mats properly, and hang towels where air reaches them rather than folded.

Warm the room slightly. Cold surfaces are where it condenses, so a bathroom that is much colder than the rest of the house will always be the one that gets it.

Cleaning what is already there

Grout and sealant are the usual sites. Scrub with detergent first. For grout, a paste of baking soda and water and a stiff brush does most of the work.

Silicone sealant that has gone black is finished. The growth is inside the material, not on it, and no amount of scrubbing brings it back. Cut it out, clean and dry the joint thoroughly, and reseal with a sanitary grade silicone. This is a genuine afternoon job and it is the single most satisfying fix in this cluster.

Painted ceilings usually clean with detergent if the paint is sound. If the plaster underneath is soft or the paint lifts, the moisture has been getting in for a while and it is more than a surface job.

Do not use bleach on porous surfaces, and never mix it with anything containing ammonia. On grout it will lighten the stain without removing what is in it.

Keep the door shut and the fan on while you clean, and if the affected area is larger than about a yard square, check where the threshold sits before treating it as a cleaning job.

When it is not really a bathroom problem

• Growth on the ceiling below the bathroom, in the room underneath, is a leak, not condensation.
• Growth on the outside wall of the room next door means moisture is moving through the wall.
• A musty smell with nothing visible in a bathroom usually means it is behind the tiles or under the tray. The signs it is inside the wall.
• Growth returning within days of a proper clean means the moisture load is much higher than a fan can deal with, or the fan is not working.

Renting

Damp and mold in a rented bathroom is one of the most commonly disputed repairs there is, and it usually turns on whether the property has adequate ventilation or the tenant is at fault for not using it.

Report it in writing, with photographs and dates, and keep a record of the fan running or not running. How responsibility divides between a landlord and a tenant.

The short version

Twenty minutes of fan after every shower, with the door closed, is the whole fix for most bathrooms. Check the fan actually pulls. Black silicone gets cut out and replaced rather than cleaned. Still damp after a day of no use means a leak rather than condensation, and that is a different job.