Key points
- Most flat roof leaks start at upstands, laps, outlets, edge trims or penetrations rather than in the middle of the membrane.
- Water tracks along the deck and insulation, so the drip inside is often well away from where it gets in.
- Damp that appears in cold weather regardless of rain is often condensation, which re-covering will not fix.
- Stay off the roof, protect the room and electrics, and treat any temporary fix as buying time until a proper repair.
Why is my flat roof leaking?
Usually because a joint or junction has failed, not because the whole covering has worn through. The places that let water in most often are:
- Upstands and flashings where the roof meets a wall, parapet or rooflight kerb. The membrane should turn up at least 150 mm above the finished roof, with a flashing or cover over the top. Too short an upstand, a lifted flashing or cracked pointing above it lets water run down behind.
- Laps and seams where sheets of felt, EPDM or single-ply overlap. A lap that has split, lifted or was never fully bonded is a classic leak.
- Outlets and gutters. The joint between the membrane and an outlet is a weak point, and a blocked outlet sends water over details never meant to be under water.
- Edge trims that have come loose or split at their joints.
- Penetrations such as soil pipes, flues, cable entries and satellite brackets.
- Damage and age: punctures from foot traffic or dropped tools, blisters that have burst, and cracking on old felt or asphalt.
Why does the water come through away from the leak?
Because water rarely drops straight down. Once it is under the membrane it runs along the deck, a joist, a firring or the top of the insulation until it finds a gap, often at a ceiling joint, a light fitting or the wall plate.
On a warm roof, water can travel across the vapour control layer under the insulation and appear a metre or more from where it entered. That is why cutting out the membrane directly above a stain often misses the fault entirely. The table gives a starting point for where to look:
| Where you see water inside | Where a roofer often looks first |
|---|---|
| Along the wall where the extension meets the house | Upstand, flashing, pointing or cavity tray in the wall above |
| Around a rooflight | Rooflight kerb, its upstand and the seal of the unit |
| Near the outer wall or fascia | Edge trim, drip and gutter joint |
| Near a soil pipe or flue | The collar or seal around the penetration |
| Mid-ceiling, only in heavy rain | Laps and ponded areas upslope of the stain |
Could the water be coming from above the flat roof?
Yes, and it is often missed. Water that enters higher up can run down inside a wall and come out just above the flat roof ceiling, looking exactly like a roof leak.
Common culprits are a missing or failed cavity tray where the flat roof abuts a cavity wall, cracked render, failed window sills above, defective parapet copings, and an overflowing gutter or downpipe from the main roof discharging onto the flat roof. If the roof covering is sound, the fix may be in the wall, not the roof. A good survey checks both.
How do roofers find the real entry point?
By working from the inside stain back to a likely route, then testing that route, rather than guessing. A typical leak-trace survey includes:
- An inside check of the stain, the ceiling void if accessible, and when the leak appears: heavy rain, wind from one direction or melting snow.
- A close inspection of the roof by the roofer, with safe access and edge protection, looking at upstands, laps, outlets, trims and penetrations.
- Moisture readings across the deck to map wet areas.
- Water testing in sections, wetting one detail at a time and watching inside, so the leak can be narrowed down.
- Small core samples on some roofs to check whether insulation or deck has got wet.
Our surveys and leak tracing service follows this approach. On some membranes, electronic leak detection is another way of pinpointing a hole.
Could it be condensation rather than a leak?
It could, especially on an older cold roof over a bathroom, kitchen or bedroom. Condensation forms when warm, moist air from inside reaches a cold surface within the roof and turns to water, which then drips through the ceiling just like a leak.
Signs that point to condensation rather than rain:
- Damp appears in cold spells, often on frosty mornings, whether or not it has rained.
- Staining is spread out, or follows the line of the joists, rather than one spot.
- Black mould around ceiling edges and corners.
- The roof covering looks sound and a water test finds nothing.
Re-covering the roof will not cure it. The fix is usually better ventilation of a cold roof, an effective vapour control layer, or converting to a warm roof. The warm vs cold roof guide explains why.
What should I do right now if my flat roof is leaking?
Keep yourself and the house safe first, and stay off the roof. Climbing onto a wet flat roof in bad weather is how people get hurt, and flat roofs should be treated as fragile until a competent person says otherwise.
- Catch the drips with buckets or towels and move furniture and electrical items out of the way.
- If water is near light fittings, switches or sockets, switch that circuit off at the consumer unit if you can do so safely, and get an electrician to check it.
- If the ceiling is bulging or sagging, keep people out from under it; a wet plasterboard ceiling can come down.
- Note the date, time, wind direction and weather each time it leaks, and take a dated note of any damage. It helps the roofer narrow down the cause.
- Book a visit. We offer an emergency temporary weatherproofing visit as soon as a team is available.
What is the difference between a temporary and a permanent leak repair?
A temporary repair stops water getting in for now; a permanent repair fixes the cause in a way that matches the roof system. Both have a place, but they are not the same job.
Temporary work includes a secured tarpaulin or sheet, an emergency patch with a tape or compound that suits the membrane, or clearing a blocked outlet. It is often done in poor weather, when most permanent repairs cannot be done properly, and should be treated as buying time.
Permanent work is done in dry conditions with materials compatible with the existing covering: a bonded patch in the same system, an upstand re-dressed to the right height with a new flashing, a replaced outlet, or new edge trims and drainage. Generic sealant smeared over a joint is neither; it rarely bonds well, can trap water, and often has to be cut away before a proper repair.
How much does it cost to fix a leaking flat roof?
Most single leaks need a repair rather than a new roof, and a straightforward patch repair to the membrane is a guide £150–£500. The cost of other flat roof repairs depends on what failed and how easy it is to reach. Guide prices, including VAT:
| Work | Guide price |
|---|---|
| Emergency temporary weatherproofing visit | £180–£450 |
| Survey and leak tracing | £150–£300 |
| Upstand or flashing repair | £250–£650 |
| Outlet repair | £120–£350 |
These are guide prices; the repair is priced in writing once the cause is found. If the roof needs replacing, the fixed price comes after a free survey. See the cost guide or use the guide-price tool.
When does a leak mean the roof needs replacing?
When the leak is a symptom of a roof that is failing as a whole, not a single fault. A one-off defect on an otherwise sound, dry roof can usually be repaired. Replacement is more likely if:
- it keeps leaking in different places after repairs;
- the insulation is wet or the deck feels soft or spongy;
- cracking, crazing or blistering is widespread;
- water ponds badly, so the falls need correcting anyway (see the ponding water guide).
The repair or replace guide goes through that decision. For flat roof replacement we offer a free survey and a fixed written quote with no obligation.
More questions
Why does my flat roof only leak in heavy rain or wind?
Wind-driven rain gets behind flashings and up under laps that cope with light rain, and heavy rain can overwhelm a part-blocked outlet so water rises over details. Both point to a specific junction rather than the whole roof.
Can a flat roof leak be repaired in the rain?
A temporary repair can often be made in poor weather. Most permanent repairs need a dry surface to bond properly, so they are booked for a dry spell.
How long can I leave a leaking flat roof?
As short a time as possible. Water that gets into the deck and insulation spreads, rots timber and turns a small repair into a bigger one, so a temporary fix while you arrange a permanent one is worth it.
Is damp on a flat roof ceiling always a leak?
No. On cold roofs it is often condensation, which shows up in cold weather whether or not it has rained. Re-covering will not cure it; ventilation or a warm roof upgrade usually will.