Wiping mould off the underside of the roof felt or throwing out a mildewed box deals with the evidence, not the cause. Within weeks, given the same conditions, the mould is back in the same spots. A proper repair changes the conditions rather than repeating the clean-up.
It has two parts working together: stopping warm moist air getting into the loft, and making sure whatever moisture does get up there has a clear path out.
The repair sequence
Clearing the eaves has to happen before adding vent trays makes any sense — trays fitted over insulation that's already packed against the roof slope don't achieve anything. Similarly, re-routing an extract duct is pointless if the eaves are still blocked; both air leakage paths need dealing with, not just one.
- Clear insulation away from the eaves so a continuous ventilation gap is restored along the full roof slope
- Fit eaves ventilation trays if the insulation depth makes maintaining a gap by hand unreliable
- Trace the bathroom or kitchen extract duct and re-route it to terminate outside through the wall or roof
- Seal the loft hatch and fit or improve insulation on the hatch itself
- Fit fire-and-air-seal covers over any downlights penetrating the ceiling below
- Add high-level ventilation, such as tile or ridge vents, where the roof covering itself is impermeable and has no other ventilation path
Why cleaning mould alone doesn't work
Mould treatment kills the surface growth but does nothing about the moisture source feeding it. As soon as conditions return — a cold night with the extract fan running, or a loft hatch left slightly open — condensation forms again on the same cold surfaces and the mould regrows. It's treating a symptom that will keep recurring for as long as the ventilation fault exists.
Painting over affected timber with a mould-inhibiting paint has the same limitation; it protects that specific surface but doesn't stop condensation forming elsewhere in the loft, and it doesn't address moisture reaching timber that's structurally important, like rafter feet.
Working with other trades
Re-routing an extract fan duct often means coordinating with whoever fitted the bathroom or kitchen — cutting a new outlet through a wall or roof tile needs doing properly and weathered in correctly, not just punched through. If loft insulation is being topped up as part of wider energy improvements, make sure whoever installs it knows to keep the eaves clear rather than filling right up to the roof slope.
Confirming it's resolved
Check the loft again after a cold snap, since that's when condensation shows most clearly. No fresh droplets or frost on the underside of the roof covering, and no new mould growth on stored items after a few weeks, is a reasonable sign the airflow and air-leakage fixes are working.
Common questions
Do I need to remove all the insulation to fix this?+
No — usually only the insulation nearest the eaves needs pulling back slightly to restore the ventilation gap. The rest of the loft insulation can stay exactly as it is.
Is it enough to just fit an extra roof vent?+
Adding ventilation helps but works best alongside stopping warm moist air getting into the loft in the first place. Vents alone on a loft with a badly sealed hatch and an internally-terminating extract fan won't fully solve it.
How long does re-routing an extract duct usually take?+
It depends on the route needed through the wall or roof, but it's normally a straightforward job done alongside the ventilation work in the same visit.
Book a loft ventilation repair
We fix eaves ventilation, extract routing and sealing in one visit and give you a written quote before starting.