What it looks like
The signs below are the ones homeowners most often notice first. Spotting one early makes the problem far easier and cheaper to put right.
Why it happens
Understanding the cause matters — the right fix depends on it. These are the usual culprits behind flat roof problems.
Ponding from inadequate falls
A flat roof should have a slight slope so water runs to the outlets. Where the fall is too shallow or the deck has deflected, water sits and stands — and standing water is the leading cause of premature failure.
Blistering and UV degradation
Trapped moisture or air under the membrane expands in the sun to form blisters, while years of UV exposure make older felt brittle. Both weaken the surface until it cracks.
Failed seams, joints and upstands
The weakest points of any flat roof are its joins — laps in the membrane and the upstands where it turns up against walls and around outlets. These are where splits and leaks usually begin.
Blocked outlets and debris
Leaves and grit block the outlets so water cannot drain, deepening ponding and forcing water over upstands and under flashings.
What to do
- 1
Check the outlets and clear debris
Keeping outlets and gullies clear is the single most effective thing you can do for a flat roof — it prevents the ponding that causes most failures. Do this only if the roof is safely and properly accessible.
- 2
Note where water stands and where it leaks
Mark where ponding forms and where any internal damp appears. On flat roofs the leak point and the internal stain are often closer together than on a pitched roof, which helps a roofer locate the fault.
- 3
Inspect seams, blisters and upstands
Look for splits along laps, lifting at upstands and blisters that have broken open. These details fail long before the middle of the field does.
- 4
Get the membrane repaired or recovered
Depending on age and condition, a roofer may patch the failed detail or recommend recovering the roof with a modern membrane such as EPDM or GRP, and correcting the falls so water drains properly.
When to call a professional
Some situations are worth getting a roofer to look at straight away:
- Water is ponding for days or leaking into the room below
- Seams, upstands or blisters have split open
- The deck feels soft underfoot, suggesting the timber is wet
- The existing covering is old felt at the end of its life
Safety first
Related roof types
Related guides
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