Not every wet roof space has a leak. Warm, moist air from the house rises into a cold loft, hits the underside of the covering and turns back into water. It drips onto the insulation, marks the ceiling below and looks exactly like a roof fault. Fixing the roof covering will not touch it, because the water came from inside the house.
How to tell condensation from a leak
A leak follows rain, particularly rain with wind behind it, and it concentrates in one place, a run down a rafter, a stain that grows from a point. Condensation is spread evenly across the underside of the covering, it is worse in cold, still weather, and it can appear during a dry spell.
Black mould on the underside of sarking felt and on the top face of joists is a condensation signature. So is dampness that is worst directly above a bathroom or kitchen.
Where the moisture comes from
Ordinary living produces a surprising amount of water vapour, and it gets into the loft through every gap in the ceiling: loft hatches without seals, downlighters, pipe and cable penetrations, and, most often, extractor fans ducted into the roof space rather than out through the roof or wall.
- Bathroom extract terminating in the loft instead of outside
- Tumble dryer or kitchen vent discharging into the roof void
- Uninsulated, unsealed loft hatch
- Gaps around soil pipes and cable runs into the ceiling
Why modern insulation made it worse
Adding thick loft insulation is good for heating bills and it makes the loft colder, which increases condensation risk. If the insulation has also been pushed hard into the eaves it blocks the airflow path that used to keep the space dry. The combination of a well-insulated ceiling and a blocked eaves gap is behind a large share of the condensation calls we get.
Putting it right
The fix is a combination: stop the moisture getting in, then let what does get in escape. That usually means sealing the ceiling penetrations and hatch, ducting extract fans properly to outside, restoring or fitting eaves ventilation with rafter trays to keep the airway clear, and adding ridge or tile vents where cross-flow is needed. On a re-roof we specify a breather membrane, which helps considerably compared with old bitumen sarking felt.
Common questions
Will replacing the roof stop condensation?
Only if the ventilation and vapour control are addressed as part of it. A new covering over the same blocked eaves and the same fan ducted into the loft will produce the same wet timbers.
Do I need vents if I have a breather membrane?
A breather membrane helps but it is not a substitute for eaves ventilation in most existing roofs. We look at the specific roof, its insulation and its moisture sources rather than applying a rule.
Is loft condensation damaging?
Over time yes, persistent wetting rots battens and rafter ends, corrodes fixings, ruins insulation performance and grows mould. It is worth resolving even when the ceiling below is not yet marked.
Can you diagnose it in one visit?
Usually. The pattern of wetting, the weather in the preceding days and what we find at the eaves and hatch normally tell the story on the first inspection.