Mould in a loft usually comes from one of two things: water getting in through the roof, or warm, moist air from the house condensing on the cold underside of the roof. Fix whichever is causing it first, before cleaning the mould or adding insulation, or the mould will come back.

The two causes often look alike, so the first job is to work out which one you have. This guide explains how to tell them apart, what to check, and who to call.

What causes mould in a loft?

Roof leaks (penetrating damp). The government's guidance on the health risks of damp and mould describes penetrating damp as water getting into a building from outside through defects in its walls, roofs, windows or floors. In a loft, that might be a slipped or broken tile, failed flashing around a chimney, a damaged felt or membrane, or a blocked gutter overflowing at the eaves.

Condensation. Warm air from the rooms below carries moisture up into the loft through the hatch, gaps around pipes and light fittings, or a bathroom fan that discharges into the loft instead of outside. When it meets the cold roof timbers and the underside of the felt, the water condenses.

Too little loft ventilation. The government guidance says roof spaces should be properly ventilated so the timber stays dry. Insulation pushed hard into the eaves, or vents and grilles that have been covered, can stop that airflow.

Why can new loft insulation make it worse?

Topping up loft insulation is one of the best ways to keep a home warm, but it changes the loft. The Energy Saving Trust's loft insulation advice explains that insulation slows heat escaping from the rooms below, which makes the loft space cooler. That makes condensation more likely and can worsen an existing damp problem.

The Energy Saving Trust says:

  • if your loft is already damp, get professional advice and fix the cause before installing insulation
  • don't cover any vents, grilles or airbricks when insulating
  • leave a small ventilation gap between the insulation and any boards laid on top
  • some lofts need extra vents or ventilation trays to keep air moving
  • flat roofs, homes with damp problems and lived-in lofts usually need professional advice

It also notes that with spray foam insulation, roof timbers can rot if the roof leaks or condensation occurs. For a plain-English overview of the job itself, see this guide to loft insulation and keeping the loft ventilated.

How can I tell a leak from condensation?

Neither of these is proof on its own, but they're a useful first look. Go up on a dry day and again after heavy rain, with a torch.

Signs that point to a roof leak:

  • staining or wet patches in one area, often below a chimney, valley, roof window or ridge
  • marks that get worse after rain or wind-driven rain
  • daylight visible through the roof covering
  • a damp patch on the ceiling below, as well as in the loft

Signs that point to condensation:

  • droplets or frost on the underside of the felt or membrane in cold weather, often widespread rather than in one spot
  • mould spread widely across the roof timbers rather than in one patch
  • damp around the loft hatch, the bathroom fan duct or the water tank
  • the problem is worst in winter, whatever the weather

If you're not sure, or the roof timbers look soft or stained, ask a roofer or surveyor to inspect it. Here's what a professional roof inspection should cover.

What should I fix first?

  1. Stop water getting in. Repair the roof covering, flashing and any blocked or broken gutters and downpipes. The government guidance lists missing or broken roof finishes and damaged or blocked gutters among the outside checks for damp, and says repairing the source of water, such as defective roofs and blocked gutters, can remove the source of moisture. If you need a roofer, here's what to expect from roof leak repairs.
  2. Stop moist air getting into the loft. Check that bathroom and kitchen fans vent to the outside, not into the loft. Seal around the loft hatch and gaps where pipes and cables pass through the ceiling.
  3. Restore the loft's own ventilation. Pull insulation back from the eaves so air can flow, uncover vents and grilles, and ask whether extra roof vents are needed.
  4. Then deal with the mould and any damaged timber. Small patches on dry timber can often be cleaned once the cause is fixed. Rotten timber needs a builder or roofer.
  5. Then insulate. Once the loft is dry and ventilated, add or top up the insulation without blocking the airflow.

Is loft mould dangerous?

Mould in a loft you rarely go into is less of a day-to-day health risk than mould in a bedroom, but it still needs dealing with because it shows the timber is staying damp. The government guidance warns that disturbing mould can release a lot of spores, so wear a mask, gloves and goggles, and get help for large areas. Our guide to whether black mould is dangerous covers who is most at risk.

The bigger concern is the roof itself. Timber that stays damp for long periods can rot, and that is far more expensive to put right than a slipped tile or a blocked vent.

Common questions

Is some condensation in the loft normal?

A little on very cold mornings can happen, but it should clear quickly. Condensation that drips, soaks the insulation or keeps coming back means moist air is getting in or the loft isn't ventilated enough.

Should I remove the insulation if the loft has mould?

Not necessarily. Fix the cause first, then ask whoever inspects the loft whether the insulation is wet or damaged and needs replacing, or only needs pulling back from the eaves so air can flow.

Can I clean loft mould myself?

Small areas on sound, dry timber, yes, with protective kit. Large areas, soft or crumbling timber, or mould you can't reach safely need a professional.

Who fixes loft mould in a rented home?

Damp caused by the building, including roof leaks and poor ventilation, is normally the landlord's responsibility. See who has to fix damp and mould in a rented home.

Get help with loft mould

If mould keeps returning in your loft or upstairs ceilings, a specialist can find the cause and tell you what needs a roofer and what doesn't. Get free quotes from local mould and damp specialists.

Sources

Tags

loft mouldroof leakscondensationloft insulationventilation