It became a much more common problem as insulation levels rose and houses got tighter. The loft got colder while the amount of moisture reaching it stayed the same or increased.
Where the moisture comes in
Loft hatches without seals, downlighters cut through the ceiling, bathroom extractor ducts terminating in the loft rather than through the roof or wall, and service penetrations around soil pipes and cables.
A single bathroom fan discharging into a roof space can put litres of water a week into the timber. It is the first thing we look for.
- Unsealed loft hatch
- Extractor fans ducted into, not through, the roof
- Downlighters and service penetrations in the ceiling
- Blocked or absent eaves ventilation
Ventilation that works
A cold roof needs air moving through it. Insulation pushed hard into the eaves is the classic blocker; eaves trays or over-fascia vents restore the path, and high-level ridge or tile vents give the air somewhere to leave.
Where a bituminous felt underlay has no breathability, extra ventilation matters more, because the roof cannot dry through the membrane.
If you're converting the loft anyway
A loft conversion changes the physics completely: the roof becomes a warm roof, with insulation between and over or under the rafters and a controlled vapour layer on the warm side.
That is the moment to solve any condensation history properly rather than adding ventilation to a space that will shortly be a bedroom.
Common questions
Why is my loft wet after I insulated it?
More insulation makes the loft colder, so moist air from the house condenses more readily on the roof timbers. Ventilation and sealing air paths from below are the fixes.
Can a bathroom fan cause loft damp?
Very much so. A fan discharging into the roof space rather than through the roof or wall is one of the most common causes we find.
Do I need ridge vents?
You need a path in at the eaves and out at high level. Ridge or tile vents are the usual way to provide the outlet on an existing roof.
Will this affect a future loft conversion?
It changes the design. A converted loft becomes a warm roof with a vapour control layer, which is a different build-up from a ventilated cold roof.