Black mould and damp rafters are usually condensation, not a leak — and the fix is ventilation, not a new roof. Why it happens and what actually stops it.
Every autumn we're called out to lofts with black mould on the rafters and damp timbers, where the owner is certain there's a leak. Very often there isn't. It's condensation, and it's a different problem with a different fix.
How To Tell The Difference
- Condensation: mould and damp spread broadly across the roof underside, worse in cold weather, often with droplets or frost on the timbers
- A leak: a defined damp patch or stain, usually tracking from one point, worse in heavy rain regardless of temperature
Why It Happens
Warm, moist air from the house rises into the loft. In older roofs it escaped freely through gaps. Seal those gaps — with more insulation, sealed hatches, or modern windows reducing airflow below — and that moisture condenses on the cold roof covering instead.
Why Insulation Can Make It Worse
This is the counterintuitive part. Adding loft insulation without checking ventilation can cause condensation, because it makes the loft colder above the insulation while blocking the airflow at the eaves. The roof holds more moisture than before.
What Actually Fixes It
- Keeping insulation clear of the eaves so air can move along the roof
- Fitting tile vents, ridge vents or slate vents where ventilation is missing
- Sealing the loft hatch so moist air isn't pumped in from below
- Where a roof is being re-covered, fitting a breathable membrane
Why It Matters
Condensation rots timber slowly. Left for years, it can mean replacing rafters or battens, not just drying out. And if it's diagnosed as a leak and "repaired", you'll have paid for a repair that changes nothing — which is why it's worth having someone look who can tell the two apart.
Want This Looked At Properly?
Free survey, photographs, and a fixed written price. No obligation and no sales pressure.
Costs and timings in this article are guides for Medway and Kent properties. Every roof is different — a survey is the only way to get an accurate figure.