Mould in your loft: ventilation, insulation and roof leaks

Mould in a loft usually means damp air has got into the roof space and has nowhere to go. The usual causes are poor ventilation, gaps or thin spots in the insulation, a bathroom fan that vents into the loft, or a leak. Some of these are cheap to fix, and some need a roofer. The mould is a warning sign, so it's best to find out which one applies.

Why lofts get damp

A loft is a cold space with the warmth of the house below it and, often, the moisture from the rooms beneath rising into it. Where the insulation is thick and even, the loft is kept cold and the warm air below stays below. Where the insulation has gaps, is pushed aside, or is missing near the eaves, warm moist air reaches the timbers and the underside of the roof and condenses there.

Ventilation matters just as much. Most pitched roofs need air to move through the roof space, from vents at the eaves to the ridge. If those vents are blocked by insulation, or they were never there, the moisture from below has nowhere to go.

The bathroom fan problem

A lot of loft mould comes from bathroom extractor fans. If the fan is ducted into the loft instead of outside, every shower and bath pushes warm, wet air into the roof space. It condenses on the cold timbers and insulation, and the result is patchy mould, staining and a damp smell in the loft.

Check where the duct goes. Look from the loft side, or ask whoever did the work. A duct that's sagging, disconnected or crushed will leak moist air into the loft as well, so it needs fixing even if it's ending outside.

What you can do this week

Start with the things you can check safely from the hatch, with a torch. Don't walk on the insulation, and never go into a loft without proper lighting and a secure footing.

• Look at the insulation. Is it level, thick, and clear of the eaves? Gaps and flattened areas are common.
• Look for the eave vents, and check they aren't blocked by insulation or debris.
• Check any bathroom fan duct. Make sure it reaches outside, is tight and is not sagging.
• Look for dark patches on the timbers, drips, or stains on the underside of the roof.
• Run the bathroom fan properly when you shower, and keep the loft hatch shut so warm air doesn't rise into it.

If the mould is on timber and it's small, it can often be cleaned with a suitable product once the loft is dry and ventilated. Wear a mask and gloves. Don't let the cleaning go on for long in a poorly ventilated space. Treat the cause first.

When it's more than condensation

Roof leaks look different from condensation. Look for staining that follows the shape of a rafter or appears after rain, drips, or dark, wet patches on the roof felt or boards. Staining that's new, spreading or wet to the touch means water is getting in, and that needs a roofer.

If the damp smell is strong, the mould spreads across lots of timbers, or the insulation is wet, get it looked at soon rather than in the spring.

Your health

The NHS says mould can trigger asthma in people who are allergic to it. If anyone in the house has asthma or a mould allergy, keep the loft closed off while you're clearing it out, and get help if symptoms get worse. The NHS asthma causes page has more on the triggers. Government guidance for rented housing in England from MHCLG, DHSC and UKHSA, Damp and mould: understanding and addressing the health risks for rented housing providers, says damp and mould can affect anyone, and mainly affect the airways and lungs.

When to get a specialist

Get a damp specialist to look if the mould is spreading, the insulation or timbers look wet, or you can't find the cause yourself. If you suspect a roof leak, a roofer is the right person first. Tell us what's happening and we'll pass it to one independent specialist who covers your area. It's free to ask.

Last checked 26 October 2026. This is general guidance, not legal or building advice.