Flat roofs fail at their details far more often than in the middle of the field: upstands, outlets, junctions with walls, and around anything that penetrates the surface.
Where flat roofs actually leak
The regular offenders are the upstand where the roof meets a wall and the flashing or cover above it, the rainwater outlet and anything that has silted up around it, laps in the membrane that were never properly welded or bonded, and old mineral felt that has gone brittle and split along a joist line.
Ponding is a symptom rather than a cause. Standing water shortens the life of a covering and finds every weakness, but the falls behind it are the real issue.
- Upstands and wall flashings
- Outlets, gutters and blocked drainage paths
- Membrane laps and welded seams
- Splits over deck joints and joist lines
Patch, overlay or full recover
A single well-defined split in an otherwise sound membrane is worth patching. Once a roof has multiple patches, brittle felt or a soft deck underfoot, a recover is the cheaper option because you stop paying for repeat access.
A recover also lets us correct the falls and upgrade the insulation to a warm-roof build-up while the covering is off, which usually costs far less than doing the two jobs separately later.
Coverings we use and why
Single-ply and high-performance torch-on systems both do well on domestic extensions and dormers. GRP suits smaller, well-supported decks with simple details and gives a hard-wearing seamless finish.
The choice depends on the deck, the detailing around upstands, and whether the roof will be walked on. We'll set out the options with their expected life rather than pushing one system for every roof.
Common questions
Why can't you find the leak?
Water travels. The point of entry is often well away from the stain, so finding it means tracing the falls, the deck and the details rather than looking directly above the damp patch.
How long should a flat roof last?
A properly detailed modern covering on a sound deck with correct falls will run for decades. Poor upstand detailing and standing water are what cut that short.
Is ponding water a defect?
It signals inadequate falls or a deflecting deck. It will not necessarily leak straight away, but it accelerates ageing and finds any weak lap.
Can you insulate at the same time?
Yes. Recovering is the natural moment to move to a warm-roof build-up, which avoids condensation risk and improves the room below.