Question 1

Is it a roof leak or condensation?

Accepted Answer

Condensation shows on cold, clear mornings rather than during rain, spreads widely across the underside of the roof instead of concentrating in one place, and comes with black mould and damp insulation. A leak follows rain and has a track running from a single entry point. It matters enormously, because a new roof will not fix condensation. This is the single most expensive misdiagnosis in domestic roofing. People have roofs re-covered, at very serious cost, to fix damp that was never in the covering — and the following winter the loft is exactly the same. Condensation happens because a household puts several litres of water into the air every day, that moist air rises into the roof space through gaps around the loft hatch, pipe runs and downlights, and it meets the cold underside of the roof. There it turns back into water. The colder the loft and the less air moves through it, the worse it is. It has become much more common because of insulation, which is the irony of it. Thicker loft insulation makes the house warmer and the loft colder, which is exactly what condensation likes — and insulation pushed hard into the eaves usually blocks the ventilation gap that was keeping the roof space dry. The symptoms are recognisable once you know them. Water droplets on the underside of the felt, often over a wide area rather than in one spot. Black mould on the sarking and the rafters. Damp, matted, ineffective insulation. Drips off nail points on a cold morning with no rain for days. Old bitumen felt makes it worse, because it is effectively vapour-closed and moisture that gets past it cannot get out. A leak behaves differently in every respect. It follows rain. It concentrates at one point with a visible track running from it. It gets worse in a downpour and stops when the weather does. And it is usually traceable to one of a short list of junctions rather than being spread across the whole roof. Getting this right first is the cheapest thing in the whole process. Ventilation — eaves vents with rafter trays to keep the insulation clear, plus ridge or tile vents at high level — costs a small fraction of a re-roof and fixes the great majority of damp lofts permanently. It should be ruled in or out before anybody quotes for a covering.

Question 2

Damp in my loft but no leak

Accepted Answer

Condensation shows on cold, clear mornings rather than during rain, spreads widely across the underside of the roof instead of concentrating in one place, and comes with black mould and damp insulation. A leak follows rain and has a track running from a single entry point. It matters enormously, because a new roof will not fix condensation. This is the single most expensive misdiagnosis in domestic roofing. People have roofs re-covered, at very serious cost, to fix damp that was never in the covering — and the following winter the loft is exactly the same. Condensation happens because a household puts several litres of water into the air every day, that moist air rises into the roof space through gaps around the loft hatch, pipe runs and downlights, and it meets the cold underside of the roof. There it turns back into water. The colder the loft and the less air moves through it, the worse it is. It has become much more common because of insulation, which is the irony of it. Thicker loft insulation makes the house warmer and the loft colder, which is exactly what condensation likes — and insulation pushed hard into the eaves usually blocks the ventilation gap that was keeping the roof space dry. The symptoms are recognisable once you know them. Water droplets on the underside of the felt, often over a wide area rather than in one spot. Black mould on the sarking and the rafters. Damp, matted, ineffective insulation. Drips off nail points on a cold morning with no rain for days. Old bitumen felt makes it worse, because it is effectively vapour-closed and moisture that gets past it cannot get out. A leak behaves differently in every respect. It follows rain. It concentrates at one point with a visible track running from it. It gets worse in a downpour and stops when the weather does. And it is usually traceable to one of a short list of junctions rather than being spread across the whole roof. Getting this right first is the cheapest thing in the whole process. Ventilation — eaves vents with rafter trays to keep the insulation clear, plus ridge or tile vents at high level — costs a small fraction of a re-roof and fixes the great majority of damp lofts permanently. It should be ruled in or out before anybody quotes for a covering.

Question 3

How do I tell condensation from a leak?

Accepted Answer

Condensation shows on cold, clear mornings rather than during rain, spreads widely across the underside of the roof instead of concentrating in one place, and comes with black mould and damp insulation. A leak follows rain and has a track running from a single entry point. It matters enormously, because a new roof will not fix condensation. This is the single most expensive misdiagnosis in domestic roofing. People have roofs re-covered, at very serious cost, to fix damp that was never in the covering — and the following winter the loft is exactly the same. Condensation happens because a household puts several litres of water into the air every day, that moist air rises into the roof space through gaps around the loft hatch, pipe runs and downlights, and it meets the cold underside of the roof. There it turns back into water. The colder the loft and the less air moves through it, the worse it is. It has become much more common because of insulation, which is the irony of it. Thicker loft insulation makes the house warmer and the loft colder, which is exactly what condensation likes — and insulation pushed hard into the eaves usually blocks the ventilation gap that was keeping the roof space dry. The symptoms are recognisable once you know them. Water droplets on the underside of the felt, often over a wide area rather than in one spot. Black mould on the sarking and the rafters. Damp, matted, ineffective insulation. Drips off nail points on a cold morning with no rain for days. Old bitumen felt makes it worse, because it is effectively vapour-closed and moisture that gets past it cannot get out. A leak behaves differently in every respect. It follows rain. It concentrates at one point with a visible track running from it. It gets worse in a downpour and stops when the weather does. And it is usually traceable to one of a short list of junctions rather than being spread across the whole roof. Getting this right first is the cheapest thing in the whole process. Ventilation — eaves vents with rafter trays to keep the insulation clear, plus ridge or tile vents at high level — costs a small fraction of a re-roof and fixes the great majority of damp lofts permanently. It should be ruled in or out before anybody quotes for a covering.

Is it a roof leak or condensation?

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