Leak or condensation
Is it a roof leak or condensation?
The short answer
Look at when the water appears. A roof leak follows rain: it shows during or soon after a shower, in one place, often with a brown tide mark. Condensation follows cold, still, damp conditions: it forms on cold surfaces such as windows, the underside of the roof and loft timbers, in patches or across a whole room, with or without rain, and it often brings black mould. Some homes have both, so check each.
The tests that separate a leak from condensation, why they get confused, and who to call for each.
The timing test
The quickest clue costs nothing. Write down when you see the water. If it turns up during rain, or within a few hours after, and fades once the roof has dried out, a leak is the likelier cause. If it shows on dry mornings after a cold night, with no rain anywhere in the forecast, think condensation.
Condensation happens when warm, moist air from cooking, showers, drying clothes and breathing touches a cold surface and turns to water. In winter the underside of a roof is cold, so moisture rising into the loft settles there. It can drip back onto the insulation and ceiling, which looks very like a leak.
Keep a simple diary for a week or two: the date, the weather and where the water showed. A pattern usually appears quickly. Leaks line up with the rain. Condensation lines up with cold nights and busy kitchens. If you can, record the outdoor temperature and whether the heating was on, because a ceiling that drips only on the coldest mornings is giving you strong evidence of its own.
Where it shows and how it looks
A leak is local. It enters at one point, often just below a chimney, valley, roof window or vent pipe, and the water then runs along a rafter before it drops, so the mark may appear some distance from the actual entry point. Leak stains tend to have a darker, ringed edge where the water has dried and been wetted again, and the plaster can feel soft.
Condensation is broader and fainter. Look for beads of water on the inside of window glass, damp patches in cold corners and behind wardrobes, and black mould spotting on a ceiling or a north-facing wall. In the loft, fine droplets or frost on the nails and the underside of the roof, and dampness across a big stretch of timber rather than one spot, point the same way.
Our guide to checking the loft for a roof leak shows what to look for with a torch.
How a roof ends up causing condensation
Condensation is often a ventilation problem in the roof and not a hole in it. BS 5250:2021 deals with moisture control in buildings, and roof ventilation advice leans on it. Marley's advice, where the underlay is the high-resistance type, is a 25 mm opening running the length of the eaves when the pitch is 15 degrees or under, falling to 10 mm on steeper roofs. If insulation has been crammed tight against the eaves, or the vents are clogged with paint, nests or debris, the damp air has nowhere to go, and it settles on the coldest timber it can find.
Extra insulation and sealed windows have made many homes warmer but also more humid inside. Spray foam can add to the problem: the RICS consumer guide warns that badly installed insulation can bring dampness, including uncontrolled condensation.
A flat roof can suffer the same issue within its build-up. Our answer on condensation under a flat roof explains it.
What to do about each one
If it looks like a leak, treat it as one. Move belongings, set a bowl under the drip, photograph the stain and arrange for the roof to be inspected. Keep electrics well away from the wet area. Our answer on whether a roof leak is an emergency sets out which signs need action sooner.
If it looks like condensation, a roofer is not the first call. Cut the moisture at its source: extractor fans in the kitchen and bathroom, lids on pans, washing dried outside where possible and trickle vents left open. Then clear the loft eaves so air can move, and check that insulation does not block the vents. Wipe off mould with a suitable cleaner and keep watching it.
If the problem comes back every winter, ask a roofer to look at the roof ventilation. A vent or an underlay detail may need correcting, and sometimes a small change at the eaves makes a bigger difference than any work on the tiles.
When an inspection settles it
If you cannot tell which it is, an inspection is the fastest way to find out. A roofer who looks at the outside and the loft can match the damp patches to the tiles above them. A cracked tile or lifted flashing exactly where the stain sits means a leak. A sound covering with wet underlay across a wide area means condensation, and the fix is ventilation, not a roof repair.
You get a plain verdict with photographs from us, and if the roof is in good order that is what the verdict says, not a list of work it does not need. Where a fault does turn up, the price arrives in writing after the visit, as a single total.
Our emergency roof repairs page describes how leaks are made safe, and our answer on roofs that only leak in wind covers the leak that vanishes in calm weather.
Related questions
Leak and condensation questions
Can condensation soak a ceiling as much as a leak?
Yes, if the roof space stays cold and damp for long enough. Moisture builds up under the roof, drips onto the insulation and can stain the ceiling below, which is why the two are so often mistaken for each other. The pattern of timing and location tells them apart.
Will a new roof cure condensation?
Not by itself. If the cause is moist air with nowhere to go, a new covering over the same ventilation and insulation gives the same result. Better extraction in the house and clear eaves vents do more than replacing sound tiles.