Most flat roof failures start at an edge, a junction or the deck beneath — not in the open field of the membrane. Ponding points to inadequate falls, blisters to trapped moisture, splits at upstands to movement, and interstitial damp to a cold deck with no vapour control. Each has a different remedy, so diagnose before re-covering.
- Water standing 48 hours after rain means the falls are wrong, not that the covering has failed.
- Most leaks are at upstands, outlets and junctions. The middle of the roof is rarely the problem.
- Damp below a flat roof is often condensation inside the structure, not a leak.
- Re-covering over a failed deck buys a few years. Lift and look first.
Diagnose before you re-cover
A flat roof that leaks almost always gets the same recommendation: a new covering. Sometimes that is right. Often it treats the symptom and leaves the cause, and the new membrane fails the same way.
The useful distinction is between a covering that has reached the end of its life and a covering that is failing early because of something underneath or around it. Felt that is twenty-five years old, brittle and cracked across the whole surface has simply expired. A five-year-old membrane with a split at one upstand has a cause worth finding.
| Symptom | Usual cause | What it needs |
|---|---|---|
| Water still standing 48 hours after rain | Inadequate or reversed falls | Re-fall the deck, not just re-cover |
| Blisters or bubbles under the membrane | Moisture trapped at laying, or in the deck | Find the moisture source; patch or recover |
| Split at an upstand or wall junction | Movement; no expansion allowance | Rework the detail, not the field |
| Leak near the outlet | Blocked, undersized or badly detailed outlet | Clear and re-detail; add an overflow |
| Sagging or spongy underfoot | Deck has rotted | Lift and replace the deck |
| Damp on the ceiling, no obvious leak | Interstitial condensation, cold deck | Vapour control and ventilation |
| Cracking across the whole surface | Age and UV, mostly on felt | End of life; recover |
| Lifting at the edge trim | Fixings or expansion at the drip | Re-fix and re-detail the edge |
| Leak only in wind-driven rain | Flashing or cover flashing too shallow | Raise and re-dress the flashing |
Rows one, three, four and six are the ones that recur after a re-cover, because the new membrane inherits the same fall, the same detail, the same outlet and the same lack of ventilation.
Ponding: the fall is the fault
A flat roof is not flat. It should be laid to a fall so water runs to the outlet, and the usual design target is a finished fall of around 1 in 80 — which in practice means designing to 1 in 40, because deflection, settlement and construction tolerance all eat into it.
Water standing in a pool two days after rain means that has not been achieved, either because it never was or because the deck has deflected since.
Why it matters beyond appearance:
Standing water finds every weakness. A pinhole or a marginal lap that would shed water on a falling surface will eventually admit it under a permanent head of water.
It adds load. Water is heavy, and a deck that has deflected enough to pond now carries more weight at the lowest point, which deflects it further. That is a progressive failure.
It accelerates the covering. Standing water plus UV plus freeze-thaw degrades most membranes faster than a surface that drains.
It grows things. Silt, moss and algae collect in the pool and hold yet more water.
The honest remedy is to correct the fall — tapered insulation over the existing deck is the usual way, or re-decking with firrings. Simply laying a new membrane over a ponding roof reproduces the problem with a fresh start date.
Cold deck condensation: the leak that is not a leak
This is the one most often misdiagnosed, because the symptom is damp on a ceiling and the natural conclusion is a hole in the roof.
A flat roof is built one of two ways. In a warm deck, the insulation sits above the structural deck and below the waterproofing, so the deck stays warm and no surface inside the construction is cold enough for moisture to condense on. In a cold deck, the insulation sits between the joists below the deck, so the deck itself is cold.
A cold deck only works if two things are right: a well-sealed vapour control layer on the warm side stopping moist air getting up there, and a ventilated air gap above the insulation carrying away any that does. Older cold decks frequently have neither, or have a ventilation gap that was blocked when someone topped up the insulation.
What happens then is that warm moist air from the room rises into the roof build-up, meets the cold underside of the deck, and condenses. Over a winter that soaks the deck and the insulation, rots the timber, and drips through as what looks exactly like a leak.
How to tell the difference: a leak tracks to rain and usually appears in the same spot within hours of it. Condensation appears in cold weather, is worse after a run of cold days, is often widespread rather than a single point, and can be worst in rooms that generate moisture. If a roofer has found no defect and the ceiling is still damp, this is the likely answer.
The fix is to convert to a warm deck when the roof is next re-covered — insulation above the deck, vapour control below it, no cold surface for moisture to find.
Edges, upstands and outlets
- The upstand height. Where the roof meets a wall, the membrane should turn up and be dressed well above the finished roof level, with a cover flashing chased into the brickwork over it. A shallow upstand, or one relying on mastic at the top, is where wind-driven rain gets in.
- The junction with the house. On an extension roof this abuts the main wall, and it must finish clear of the damp-proof course with the flashing properly chased — not stuck on with sealant, which is the commonest shortcut and has a life of a few years.
- The outlet. It needs to be at the low point, large enough for the roof area, with a leaf guard, and reachable to clear. A blocked outlet turns a flat roof into a tank. An overflow or a second outlet is cheap insurance.
- The edge trim and drip. Water should leave the roof clear of the fascia, not run back under the trim. Lifting edge trim is often a fixing or expansion problem rather than a membrane one.
- Penetrations. Soil vents, extract terminals and cable entries each need a proper collar. These are frequent leak points precisely because they are small and get the least attention.
- Expansion. Long runs and junctions between different materials move. A detail with no allowance for that splits, and the split appears at the junction rather than in the field.
The pattern across all six: flat roofs fail at their boundaries. When a roof is quoted, the questions worth asking are about the upstand height, the flashing method, the outlet size and the edge detail — not about the brand of membrane. The material matters less than the detailing, and the deck matters more than either.
When the deck itself has gone
If the roof feels soft or springy underfoot, or there is a visible dip, the problem is below the covering.
Most older flat roof decks are timber — boarding or chipboard on joists. Once water has been getting in, chipboard in particular swells and loses strength quickly, and the damage is often far wider than the leak that caused it because water tracks along the joists.
Re-covering over a soft deck is the cheapest option on the day and the worst one over any longer period. The new membrane is only as flat and as supported as what it sits on, so a dip stays a dip, it ponds, and the covering is working against the same movement that destroyed the last one.
What a proper job looks like: strip the covering, inspect the deck fully, replace what has gone including any affected joists, confirm or correct the falls with firrings or tapered insulation, then lay the new covering with the details done properly.
This is why a flat roof quote that seems unusually cheap is worth questioning. The cost difference between re-covering and re-decking is substantial, and the only way to know which you need is for someone to lift a section and look. A contractor who quotes a flat roof from the ground has not established which job it is.
What it costs
These are the roofing figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Minor roof repair | from £150–£400 | Slipped or broken tiles, usually one day |
| Flat roof (EPDM / GRP) | £80–£120 per m² | Warm or cold deck |
| Chimney repointing / flashing | £300–£900 | Access dependent |
| Full re-roof (typical semi) | £5,000–£9,000 | Strip, membrane, batten, re-tile |
The published flat roof figure assumes a sound deck. Re-decking, correcting falls with tapered insulation, and converting a cold deck to a warm deck are each additional, and all three are reasons a quote can legitimately come in well above the guide.
We lift a section and look at the deck before quoting, because re-covering a rotten deck is money spent twice.
Frequently asked questions
Is ponding on a flat roof a problem?
Yes, if water is still standing 48 hours after rain. It means the falls are inadequate or the deck has deflected. Standing water finds every marginal lap, adds load that deflects the deck further, and degrades the covering faster. Correct the fall rather than just re-covering over it.
What causes blisters on a flat roof?
Moisture trapped beneath the membrane — either present when it was laid, or coming from a damp deck below. Small stable blisters may be tolerable; growing ones are not, and they indicate moisture in the build-up that will rot the deck. Find the source before patching.
Why is my ceiling damp when the roof does not leak?
Often interstitial condensation in a cold deck roof. Warm moist air rises into the build-up, meets the cold underside of the deck and condenses. It appears in cold weather rather than after rain, is usually widespread rather than one spot, and the fix is converting to a warm deck.
What is the difference between a warm deck and a cold deck?
In a warm deck the insulation sits above the structural deck and below the waterproofing, so nothing inside the construction is cold enough to condense on. In a cold deck it sits between the joists below the deck, which then needs a sealed vapour control layer and a ventilated gap to work.
Can a flat roof be re-covered over the old one?
Only if the deck beneath is sound and the falls are adequate. If the roof feels soft or springy, or there is a visible dip, the deck has gone and the new covering will inherit the dip, pond, and fail the same way. The only way to know is to lift a section and look.
Where do flat roofs usually leak?
At the boundaries, not the middle: the upstand where it meets a wall, the flashing into the brickwork, the outlet, the edge trim and any penetration such as a soil vent. When comparing quotes, ask about upstand height, flashing method and outlet size rather than the brand of membrane.
Lift it and look
We check the deck, the falls and the upstand details before quoting, so you know whether you are buying a covering or a roof.