Within a season
Condensation in the Loft — Cause & Fix
If you've got damp insulation, black-spot mould on rafters, or droplets on the underside of the felt, you're probably looking at condensation — not a roof leak. The fix is ventilation, not new tiles.
What you're seeing
- Droplets on the underside of the roof felt
- Black-spot mould on rafters or roof timbers
- Damp loft insulation that wasn't damp from a leak
- Frost on the underside of the roof in winter
What it means
Warm moist air from the house is rising into a cold, sealed loft and condensing on the cold roof surface. Common after a loft conversion, a bathroom extractor venting into the loft, or installation of thicker insulation without matched ventilation.
Do this now
- Open any existing loft hatches and vents to ventilate the space
- Check that bathroom and kitchen extractors vent OUT of the building, not into the loft
- Don't lay extra insulation that blocks the eaves gap
Professional fix
Install eaves vents, ridge vents or tile vents to create cross-flow ventilation. Sometimes we add a positive-input ventilation unit. The roof itself usually stays as-is.
Typical cost
Eaves and tile vent installation typically £350–£900 depending on roof size.