A flat roof that leaks once gets patched. A flat roof that leaks again after the patch usually means nobody found the hole. The stain on the ceiling tells you where the water finished its journey, not where it started, and on a flat roof those two points can be several metres apart.
This guide is for homeowners with a leaking extension, dormer or garage roof, and for facilities managers looking after larger flat roofs on offices, blocks and shops. It explains why water travels before it shows, what you can safely check yourself, what each testing method can and cannot find, and when it is worth paying for electronic testing rather than another patch.
Why does a flat roof leak show up somewhere else?
A flat roof is a layered build-up. From the top down there is usually the waterproofing (felt, EPDM rubber, GRP fibreglass or a liquid coating), then insulation or a board, then the deck, which is often timber boards or plywood on a domestic roof and concrete or metal on a commercial one. Below that are the joists and the ceiling.
Water that gets through a small breach in the membrane rarely drops straight down. It spreads across the top of the deck, runs along the joints between boards, follows the fall of the roof towards the outlet or gutter, and soaks into insulation. It only appears inside once it finds a gap in the deck or a joist to run down, or once the ceiling below is saturated enough to stain. That is why a roofer who patches the membrane directly above the stain so often fixes nothing.
Two other features make it harder. Most flat roofs are not truly flat: they are laid to a gentle fall, so water under the membrane drifts downhill. And insulation can hold water for weeks, so a leak can keep dripping into a ceiling long after the rain has stopped, or appear only after days of heavy rain.
What can you check yourself before calling anyone?
Only go on a roof if you can reach it safely and the surface is dry and sound. Many problem areas can be seen from a window or a ladder at the edge without walking on the roof at all.
- Note when it leaks. Every time it rains, only in heavy or wind-driven rain, or days afterwards? A leak only in driving rain points at walls, flashings and upstands. A leak that keeps going after the rain points at water held in the roof build-up.
- Look at the edges and upstands. Where the roof meets a wall, a parapet, a rooflight or a soil pipe, the waterproofing has to turn up and be sealed. These details fail far more often than the open middle of a roof. Look for lifted flashings, cracked lead, gaps in mortar above the flashing and split or peeling membrane at the turn.
- Check the outlets and gutters. A blocked outlet lets water pond and rise above laps that were never meant to sit underwater. Clear leaves and silt and see whether the ponding goes.
- Walk the surface with your eyes. On felt, look for blisters, splits, cracked areas and seams lifting at the edges. On EPDM, look for lifted laps and punctures near where people have walked or where equipment sits. On GRP, look for hairline cracks, especially at board joints and around trims.
- Look inside. Mark the stain, and if you can get into the void or look above a loose ceiling tile, trace which way the timber is wet.
If the ceiling is sagging or bulging with water, deal with that first. Our guide to water coming through the ceiling covers isolating electrics and releasing the water safely.
It is also worth ruling out a plumbing leak. A stain under a flat roof that also sits under a bathroom, a tank or a heating run may have nothing to do with rain. If the patch spreads in dry weather, or never changes, read our guide to a damp patch on the ceiling with no obvious leak before paying for roof work.
What are the ways to find a flat roof leak?
There are four approaches in common use. They answer different questions, so it helps to know what each one is actually telling you.
| Method | What it tells you | Where it falls short |
|---|---|---|
| Visual survey | Obvious splits, failed upstands, blocked outlets, open laps | A pinhole, a lap failed underneath, or anything hidden by chippings, paving or plants |
| Flood test | Whether a section of roof leaks when water is held on it | Does not show where the breach is; loads the roof with water; slow |
| Moisture mapping | Which areas of the build-up are wet, from meter readings or thermal imaging | Wet insulation spreads well beyond the hole, so it shows the damage, not the entry point |
| Electronic leak detection (low or high voltage) | The position of breaches in the membrane itself, marked on the roof | Needs a non-conductive membrane over a conductive deck or layer; some roofs are not suitable |
The visual survey should always come first. A good share of flat roof leaks are at a detail you can see once someone looks properly. The other methods are for when the obvious has been checked and the leak carries on.
Is a flood test enough to find the leak?
A flood test means damming a section of roof, holding water on it for a period and watching the ceiling below. It is useful for one question: does this area leak or not? Done in sections, it can narrow the leak down to part of a roof.
What it will not do is point to the hole. If water appears below, you still do not know whether the breach is at the lap by the outlet or a puncture two metres uphill. It also puts a lot of weight on the roof, soaks the insulation further if the roof does leak, and on a larger roof takes a long time to work through.
How does electronic leak detection work on a flat roof?
Most roofing membranes do not conduct electricity. A concrete deck beneath them normally does, and timber or metal decks can be tested where a conductive layer was built in under the membrane. If there is a hole in the membrane, electricity can pass through it to the deck, and that is what the equipment looks for. There are two main methods.
Low voltage testing is done on a wetted roof. A wire loop is laid around the test area and a low voltage is applied between the wet surface and the deck. Where there is a breach, current flows down through it, and probes read the direction of the electrical field across the surface, which points towards the hole. Because it relies on water carrying the signal, it can often work through gravel ballast and some paving or green roof build-ups.
High voltage testing is done on a clean, dry membrane. A charged brush or wand is swept across the surface and an alarm sounds where the charge jumps through a hole to the deck. It suits exposed membranes and is good for finding pinholes and faulty seams, including on vertical upstands.
Neither method works on every roof. Some black EPDM rubber contains carbon and conducts, some older build-ups have no conductive layer under the waterproofing, and a thick insulating layer above the membrane can block the signal. That is why we ask for photographs and what you know about the roof before booking, and tell you if another approach is the better choice. You can read more about how the kit compares in our guide to leak detection equipment.
Why do roofers' patches keep failing?
Usually because the patch went where the stain was, not where the breach was. The roofer sees water in the ceiling, goes up, finds the nearest tired-looking seam or blister above it, and seals it. If the real hole is a puncture further up the fall, or a failed detail at an upstand, the water carries on. After two or three rounds of this a roof can have several patches and the same leak.
Sometimes the patch simply does not bond to the old membrane, or the leak is not coming through the roof at all but through a wall, coping or window head above it.
Locating the breach before any repair breaks that cycle. A marked-up roof plan with each breach position gives the roofer something definite to fix, and shows whether one targeted repair will do or the membrane has failed across the roof. For managing agents, the written report with photographs can go straight to the contractor, the freeholder or an insurer, and our guide to commercial water leak detection covers how we work around occupied buildings.
How much does flat roof leak detection cost?
Flat roof electronic testing is priced per roof. The main factors are the roof area, the type of membrane and what sits on top of it, since a small exposed extension roof is far quicker to test than a large ballasted commercial roof. Send us photographs, a rough size and where the water is showing inside, and we will tell you whether the roof is suitable and give you a price before anything is booked.
If the stain may be from plumbing rather than the roof, our standard leak survey starts from £150, and commercial surveys start from £300. Our no find, no fee policy applies to leaks on your own property. You can see all our published prices on the pricing page, and full details of how we test roofs on our electronic leak detection service page.
To talk through a roof that keeps leaking, call us on 020 3912 0200.
Frequently asked questions
How do I find where my flat roof is leaking from?
Start with the details: upstands, flashings, outlets, rooflights and pipe penetrations, then look for splits, blisters and lifted seams. Note when it leaks, and remember the water inside may be well downhill of the hole. If that finds nothing and the leak continues, the breach needs locating with a test rather than guessed from the stain.
Can you find a leak in a flat roof without taking it apart?
Often, yes. Electronic leak detection tests the membrane from the surface and marks the breach on the roof. Where there is deep ballast, paving or an insulating layer above the waterproofing, some of it may need moving at the suspected area.
Does electronic leak detection work on EPDM rubber roofs?
It depends on the product. Some black EPDM contains carbon and conducts electricity, which stops standard testing working. Others are suitable. We ask about the membrane before booking so you do not pay for a visit that cannot give an answer.
Why does my flat roof only leak in heavy rain?
Leaks that only show in heavy or wind-driven rain are often at upstands, flashings, copings or walls above the roof, where water is driven in sideways. Ponding around a blocked outlet can also push water over a lap.
Is water coming through the ceiling under a flat roof always the roof?
No. A bathroom, a tank, a heating pipe or condensation in a cold roof void can all cause stains under a flat roof. If the patch appears or grows in dry weather, rule out plumbing before paying for roof work.
