Loft mould is usually discovered by accident, when someone goes up for the Christmas decorations and finds black staining across the underside of the roof or on the timbers. It is common, it is usually a ventilation problem rather than a leak, and it is fixable, but the fix is mostly about airflow rather than about treating the timber.
Why lofts get damp
A loft is cold by design. Insulation at ceiling level keeps heat in the rooms below, which means the roof space above stays close to outside temperature. Any warm, moist air that gets up there from the house meets cold roof surfaces and condenses.
The moisture usually arrives from the house in ways that are easy to overlook:
- a poorly sealed or uninsulated loft hatch
- gaps around pipes, cables and downlighters in the ceiling
- an extractor fan or tumble dryer duct that discharges into the loft rather than outside
- a bathroom fan duct that has come loose from its outlet
- gaps at the top of internal walls and around soil pipes
- a hatch left open while the house is warm
Once that moisture is in the loft, it needs somewhere to go. If the ventilation path has been blocked, it condenses instead.
The most common cause: blocked ventilation
Most roofs are designed to breathe at the eaves. When loft insulation was topped up, it was often pushed right into the eaves, closing off exactly the gap the roof relies on for airflow.
The result is a well-insulated, badly ventilated loft, which is precisely the condition that produces condensation and black staining on the felt and rafters.
What to check
Go up on a dry day, with good light and something to kneel on across the joists.
- look at the underside of the roof covering for black spotting or water marks
- check the rafters and the tops of the joists
- look at the eaves and see whether insulation is blocking the airflow gap
- check whether any ventilation is fitted at the eaves, ridge or in the tiles
- find where any ducts terminate, and confirm they exit the building
- look for daylight, slipped tiles and wet patches, which indicate a leak rather than condensation
- check whether stored items and boxes are damp
- check the hatch itself for a seal and insulation
- look at the chimney breast and around any flue for staining
Telling condensation from a leak
Condensation typically produces widespread, even spotting across the underside of the roof and along the rafters, is worse in winter, and is often worse on the north-facing slope.
A leak typically produces localised staining below a specific point, water marks running down a rafter, wet insulation directly beneath, and marks that follow heavy rain rather than cold weather.
The distinction matters because they need different fixes. A leak needs the roof repairing. Condensation needs the ventilation restoring and the moisture path from the house closing.
What to do about it
- Stop moisture getting up there: seal the hatch, close gaps around pipes and cables, and make sure every duct discharges outside the building
- Restore ventilation: clear insulation back from the eaves, keep the airflow gap open, and have additional roof or eaves ventilation considered where the airflow path is inadequate
- Fix any leak identified, which is a roofing job
- Treat the affected surfaces once the cause is addressed
- Recheck the following winter to confirm it has worked
Treating before fixing the ventilation simply means doing it again next year.
Treating loft surfaces
Where treatment is appropriate on sound, accessible surfaces:
- work in daylight with good task lighting, and mind your footing on the joists
- wear protection, since lofts combine dust, insulation fibres and mould in an enclosed space with poor airflow
- ventilate the space as much as possible while working, including leaving the hatch open with the rooms below closed off
- treat visible growth on suitable surfaces following the product instructions
- allow the area to dry fully
!Mould Solve Essential Mould Treatment Kit with two sprays, FFP3 mask, gloves and cloth
The Mould Treatment Kits include protective equipment, which matters more in a loft than in most rooms. For the treatment stage, see Mould Removal Sprays.
Do not attempt to treat large areas of a roof space yourself. Extensive staining across the whole roof is a job for someone with the right equipment and access, and the ventilation issue behind it needs assessing properly.
When to get help
- staining across most of the roof space
- wet or soft timbers, rather than surface staining
- any sign of a structural problem
- suspected leak requiring roof access
- a roof with no visible ventilation provision at all
- recurring problems after ventilation work
There is more on the outside checks that matter here in autumn outdoor jobs.
FAQs
Should I close the loft hatch or leave it open to air the loft?
Keep it closed and sealed. Leaving it open lets warm, moist air from the house into the cold roof space, which usually makes condensation worse rather than better.
Can I insulate more without making it damper?
Yes, provided the eaves ventilation path is kept clear rather than stuffed full. The problem is insulation blocking airflow, not insulation itself.
My bathroom fan vents into the loft. Is that a problem?
Yes. A fan discharging into a roof space delivers a large amount of moisture directly into the coldest part of the house. It should terminate outside the building.
Is loft mould dangerous to the roof structure?
Surface growth on timber is a warning sign about conditions rather than automatically a structural problem, but persistently wet timber can be. If timbers are wet or soft rather than just marked, get them looked at.
