Flat roofing
Why does my flat roof keep leaking?
Ponding, failed laps, upstands and trapped moisture: why flat roof leaks recur and what has to be established before repairing again.
- Evidence before expenditure
- Options, not pressure
- Updated 2026-09-16
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Recurring flat roof leaks usually come from the same short list: standing water where drainage is inadequate, failed laps or seams, upstands and edge details that were never formed properly, and moisture already trapped inside the build-up. The last of these is why patch repairs on an old covering so often move the leak rather than stopping it.
Drainage and ponding
A flat roof is not level; it should have enough fall to clear water. Where fall is inadequate, or an outlet is blocked or set too high, water sits. Standing water finds every weakness in a seam and accelerates deterioration of the surface.
Checking the roof shortly after rain tells you a great deal: where water remains, and how long it takes to clear.
Laps, seams and details
Most failures are at joints rather than in the middle of a sheet. Laps in felt, welded seams in single-ply, and joints in glass-reinforced systems all depend on workmanship. Upstands at walls, kerbs at roof lights, outlets and edge trims are the other usual suspects.
Where the covering was dressed over a kerb that is too low, or terminated without a proper edge, no amount of patching resolves it.
- Standing water more than a day after rain
- Splits or open laps along a seam
- Upstands too low, or lifted at the top edge
- Blocked, undersized or badly positioned outlets
- Blisters, ripples or soft areas underfoot
Trapped moisture
Once water has entered a warm-deck build-up it can remain there, wetting insulation and decking. A repair on the surface then stops new water entering but does nothing about the water already inside, so damp continues internally and is read as a failed repair.
Soft areas underfoot, persistent damp after dry weather, and blistering all point this way. Establishing it usually needs a core sample or moisture survey rather than a visual look.
Deciding what to do
Where the covering is otherwise sound and the failure is at an identifiable seam or detail, repair is reasonable and should last. Where moisture is trapped, or the covering has reached the end of its life, renewal of the area is the durable answer.
Repeated patching of an old covering is the most expensive route, because each attempt costs money and none of them addresses the condition.
What evidence should you look for?
These are indications, not conclusions. Each one narrows where to look; only an inspection confirms a cause.
Water still standing a day after rain.
Fall or drainage may be inadequate, which stresses every seam.
Areas that may need investigating
- fall across the roof
- outlet position and condition
- deck deflection
Roof feels soft or spongy underfoot.
Insulation or decking beneath may be wet.
Areas that may need investigating
- extent of soft areas
- core sample or moisture survey
Damp continues internally after a surface repair.
Moisture may already be inside the build-up.
Areas that may need investigating
- trapped moisture
- condition of insulation and deck
Urgency: Prompt repair
Flat roof water reaches the deck quickly and wets insulation and timber, so delay adds to the scope rather than just prolonging a nuisance.
Treat it as more urgent if you see:
- Water is entering the room below
- The deck feels soft or deflects underfoot
- A ceiling is sagging
Repair or replace?
Repair where the covering is sound and the failure is at an identifiable detail. Renew the area where moisture is trapped in the build-up or the covering has reached the end of its life — patching a saturated build-up does not resolve internal damp.
A local repair is realistic when
- A single split, lap or upstand on a sound covering
Partial renewal is worth costing when
- One bay or section with trapped moisture confined to it
Replacement becomes the better option when
- Build-up wet across the roof or covering at end of life
Further investigation is the right next step when
- Moisture content of the build-up is unknown
How we can help
- Flat RoofingFlat roofing covers the installation and repair of EPDM rubber, GRP fibreglass, TPO single ply and torch-on bitumen systems on extensions, dormers, garages, porches and commercial roofs. Failure is usually caused by ponding, poor detailing at upstands and outlets, or a system laid over an unsuitable deck.
- Roof Leak DetectionRoof leak detection traces water back from where it appears inside to where it actually enters the roof. Water frequently travels along battens, rafters and purlins before it drops, so the ceiling stain is often several metres from the defect. Detection combines internal tracing, external inspection, drone imagery and, where needed, controlled water testing.
- Roof RepairsPlanned roof repair fixes an identified defect — slipped slates, cracked tiles, failed flashings, valley or ridge problems — before it becomes water ingress. Repairs are appropriate where the covering and structure are otherwise sound and the fault is localised.
Related questions
What should you do next?
Look at the roof shortly after rain and note where water stands, then arrange an inspection that includes whether moisture is trapped in the build-up.
Last updated 2026-09-16.
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