Condensation in your loft: causes and fixes
Loft condensation is warm, moist air from the house meeting cold roof felt and turning to water. The tell is simple: condensation is spread across the underside of the felt and worst on cold mornings, while a leak is localised and follows the rain. Here is how to fix it, in order.
01Condensation or a leak: how to tell
Most winter calls we get about a wet loft turn out to be condensation, not a leak, and the difference decides everything you do next. A leak is a hole letting rain in from outside. Condensation is water forming on the inside, from air that was already in your house. Chase the wrong one and you can spend good money on a roof that was never broken. The pattern gives it away every time, so before you climb up, look for these tells.
| What you see | Condensation | A roof leak |
|---|---|---|
| Where the water sits | Spread across the underside of the felt | One spot, tracking down a single rafter |
| When it shows up | Worst on cold, still mornings | Follows rain and driving wind |
| Over time | Comes and goes with the weather | Worse with every downpour |
| The timber | Damp sheen or frost, widespread | Dark tide marks, rot in one area |
A quick guide, not a survey. If it is genuinely a leak, our roof repairs team can trace and fix it. Not sure which you have? A free survey settles it.
02Why lofts get damp in winter
A house makes a surprising amount of moisture: cooking, showers, drying washing on the radiator, even breathing. That warm, damp air rises, finds every gap into the loft, and meets the cold underside of the roof. There it turns back to water, exactly as it does on a cold pane of glass. In summer the loft is warm enough that it never happens. In winter the roof is cold and the house is working hard, which is why the problem is a cold-weather one. Five things usually make it worse.
03Why leaving it does harm
Condensation looks harmless, a bit of damp on a beam, so it gets ignored until spring. The trouble is that water sitting on cold timber all winter soaks the insulation, which stops it working and costs you on heating. Left long enough it feeds mould on the felt and rot in the rafters, and that is a structural repair rather than a morning's ventilation work. The sooner air is moving, the cheaper the whole thing stays.
A rough rule for how much more water vapour warm air holds than cold. That is why the damp appears on the coldest surfaces first: the underside of the roof, on the coldest mornings. Basic building physics, general guidance
04The fixes, in order
Work from the cheapest, most likely fix upward, and most lofts never need to reach the last step. First, improve ventilation: clear anything blocking the eaves so air can enter, and add soffit or tile vents low down and ridge vents high up so it can leave. That cross-flow alone solves the large majority of cases. Where the eaves boards themselves have rotted, adding vents is patching over a bigger problem, and a fascias and soffits replacement that cuts out the rotten timber is the lasting fix. Second, seal the loft hatch and any gaps around pipes and cables, so less warm, moist air gets up there in the first place. Third, check the insulation is laid up to the eaves but not jammed into them, blocking the airflow. Only where old non-breathable felt is the real culprit does the roof covering come into it, and that is a job for your next reroof, not a standalone one.
05When the felt is the problem
If your loft is well ventilated, the hatch is sealed and the eaves are clear but the underside of the felt is still wet every cold morning, the old bitumen felt itself is likely trapping the moisture. There is no quick fix for that from the loft. The proper answer is a modern breathable membrane, which lets water vapour pass out through the roof rather than pooling against the felt, and that is fitted as part of a new roof when the covering is due anyway. We will only point you that way once the cheaper steps have been ruled out.
Nine times in ten, loft condensation is a ventilation problem, not a roof problem. Fix the airflow before you spend a penny on the covering, and most lofts dry out on their own.
06Questions we hear every week
How do I tell loft condensation from a roof leak?
Why is my loft wet in winter?
Will more loft ventilation stop condensation?
Does a new roof stop loft condensation?
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