Question 1

Why does my flat roof keep leaking?

Accepted Answer

Because the repairs are being made to the field of the roof when the failure is at a detail — an upstand, a seam, the outlet or the perimeter trim. Standing water is the other reason: it means the falls were never adequate, and ponding accelerates every failure mode there is. Flat roofs leak at their edges, and repairs that treat the middle keep failing for that reason. The open covering rarely fails on its own. What fails is the junction where the covering turns up an upstand, the seam where two sheets meet, the outlet where the water is meant to leave, or the trim at the perimeter. Upstands are the classic. Where a flat roof meets a wall, the covering has to turn up a proper height — generally 150mm above the finished surface — and be terminated with a flashing or a chase at the top. What a great many domestic flat roofs actually have is a covering dressed up 50mm and finished with a bead of mastic, which lets water in the moment there is any depth on the roof at all. Ponding is the second reason, and it is a design fault rather than a defect. A flat roof should achieve a finished fall of at least 1:80, and the trade designs to 1:40 because timber and workmanship both eat into it. Standing water a day after the rain stopped means the falls are inadequate. It is not itself a leak, but it loads the deck, grows algae, holds abrasive grit and — on felt — destroys the covering. Blisters mean something specific: trapped moisture expanding. A blistered roof is telling you there is water inside the build-up, which is why patching the blister rarely ends the story. Diagnosis is harder than on a pitched roof because there is usually a ceiling directly beneath with no accessible void, so there is no loft to inspect and no way to follow the water back. The diagnosis has to come from the roof surface — splits, pulled seams, lifted trims, and the silt lines that show where the water actually goes. The honest question with a repeatedly leaking flat roof is repair or replace, and the deck decides it. One identified defect on a covering that is otherwise sound and drains properly is a repair. A deck that gives underfoot, a covering split in several places, or ponding across the whole area means the roof has ended — and on a roof this size a proper replacement is often less than people fear.

Question 2

Flat roof leaks after being repaired

Accepted Answer

Because the repairs are being made to the field of the roof when the failure is at a detail — an upstand, a seam, the outlet or the perimeter trim. Standing water is the other reason: it means the falls were never adequate, and ponding accelerates every failure mode there is. Flat roofs leak at their edges, and repairs that treat the middle keep failing for that reason. The open covering rarely fails on its own. What fails is the junction where the covering turns up an upstand, the seam where two sheets meet, the outlet where the water is meant to leave, or the trim at the perimeter. Upstands are the classic. Where a flat roof meets a wall, the covering has to turn up a proper height — generally 150mm above the finished surface — and be terminated with a flashing or a chase at the top. What a great many domestic flat roofs actually have is a covering dressed up 50mm and finished with a bead of mastic, which lets water in the moment there is any depth on the roof at all. Ponding is the second reason, and it is a design fault rather than a defect. A flat roof should achieve a finished fall of at least 1:80, and the trade designs to 1:40 because timber and workmanship both eat into it. Standing water a day after the rain stopped means the falls are inadequate. It is not itself a leak, but it loads the deck, grows algae, holds abrasive grit and — on felt — destroys the covering. Blisters mean something specific: trapped moisture expanding. A blistered roof is telling you there is water inside the build-up, which is why patching the blister rarely ends the story. Diagnosis is harder than on a pitched roof because there is usually a ceiling directly beneath with no accessible void, so there is no loft to inspect and no way to follow the water back. The diagnosis has to come from the roof surface — splits, pulled seams, lifted trims, and the silt lines that show where the water actually goes. The honest question with a repeatedly leaking flat roof is repair or replace, and the deck decides it. One identified defect on a covering that is otherwise sound and drains properly is a repair. A deck that gives underfoot, a covering split in several places, or ponding across the whole area means the roof has ended — and on a roof this size a proper replacement is often less than people fear.

Question 3

Water standing on my flat roof

Accepted Answer

Because the repairs are being made to the field of the roof when the failure is at a detail — an upstand, a seam, the outlet or the perimeter trim. Standing water is the other reason: it means the falls were never adequate, and ponding accelerates every failure mode there is. Flat roofs leak at their edges, and repairs that treat the middle keep failing for that reason. The open covering rarely fails on its own. What fails is the junction where the covering turns up an upstand, the seam where two sheets meet, the outlet where the water is meant to leave, or the trim at the perimeter. Upstands are the classic. Where a flat roof meets a wall, the covering has to turn up a proper height — generally 150mm above the finished surface — and be terminated with a flashing or a chase at the top. What a great many domestic flat roofs actually have is a covering dressed up 50mm and finished with a bead of mastic, which lets water in the moment there is any depth on the roof at all. Ponding is the second reason, and it is a design fault rather than a defect. A flat roof should achieve a finished fall of at least 1:80, and the trade designs to 1:40 because timber and workmanship both eat into it. Standing water a day after the rain stopped means the falls are inadequate. It is not itself a leak, but it loads the deck, grows algae, holds abrasive grit and — on felt — destroys the covering. Blisters mean something specific: trapped moisture expanding. A blistered roof is telling you there is water inside the build-up, which is why patching the blister rarely ends the story. Diagnosis is harder than on a pitched roof because there is usually a ceiling directly beneath with no accessible void, so there is no loft to inspect and no way to follow the water back. The diagnosis has to come from the roof surface — splits, pulled seams, lifted trims, and the silt lines that show where the water actually goes. The honest question with a repeatedly leaking flat roof is repair or replace, and the deck decides it. One identified defect on a covering that is otherwise sound and drains properly is a repair. A deck that gives underfoot, a covering split in several places, or ponding across the whole area means the roof has ended — and on a roof this size a proper replacement is often less than people fear.

Why does my flat roof keep leaking?

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