Finding flat roof leaks
How do you find a leak on a flat roof?
The short answer
You find a flat roof leak by working backwards from the stain, because water travels along the deck and the ceiling before it shows. Start with the places that fail most often: the edges, the joints where the roof meets a wall, outlets and anything that pokes through the covering. Look after rain, in daylight, from a safe position. If the source is not obvious, a controlled hose test narrows it down.
Where flat roof water really comes in, what to check from the inside and outside, and when to stop looking and ring a roofer.
Why the stain is not where the hole is
A flat roof is a sandwich of covering, deck, insulation and ceiling. When the covering lets water through, it seldom drips straight down. It runs along the top of the boards or follows a joist, then soaks into the plasterboard somewhere else. The mark on your ceiling can be well away from the real entry point, sometimes across the width of the room.
So start with the stain, but treat it as a clue, not an address. Note its shape and whether it grows after rain only or also on dry, humid days. A stain that appears only in heavy rain suggests a genuine opening in the covering. One that comes and goes in cold weather and feels damp rather than wet may be condensation, which needs a different fix, and our answer on flat roof condensation explains the difference. Ceilings that have been wet for a while also tell you something: old brown rings mean repeated visits by water.
Check the inside first
If you can reach the ceiling, look at it in daylight with the lights off, then with a torch held flat against the surface so any ripples or sags show up. Press gently around the mark with a gloved hand. Soft plasterboard means water has been arriving for some time. Check whether the stain lines up with a light fitting, a wall or an outlet pipe, because these are all places where the route down is easy.
Take photographs of the stain with a coin or a tape beside it for scale, and repeat them a few days later. That record shows whether the leak is active, and it helps whoever surveys the roof. If there is a loft or void above a pitched part of the building next to the flat roof, look there too, as the water often arrives at the junction. Put a bowl or a tray under any drip, and note what time it begins and how quickly it fills.
Then the usual suspects outside
On the roof, or from a window or ladder at a safe height, go through the likely points in order. First comes the upstand, the covering turned up the wall, and the flashing that caps it, wherever the roof ends against brickwork. Sealant cracks, and lead or felt can lift. Next are the edges, where the covering turns over the trim at the drip. Then the outlets, vents, cable entries and any rooflight kerb.
After those, look at the surface itself: splits, blisters, open seams, a sunken area where water stands. Our answer on standing water on a flat roof covers why ponds point to a bigger problem. Water that sits for days finds every tiny weakness, so a leak often begins at the low spot, not at the highest point. Moss, leaves and silt packed against an outlet make that low spot wetter still.
A hose test, done carefully
When looking has not found it, a hose test can. One person is inside with a torch and a phone, one outside with a garden hose. Start at the lowest part of the roof and wet one small section at a time, for a good ten minutes each, working uphill. Wait for the person indoors to confirm whether water has appeared before moving on. Rushing it and soaking the whole roof tells you nothing.
Do this only if you can reach the roof safely without climbing onto it, and never in wind or ice. A damp deck takes a while to dry, so test in a dry spell and expect to repeat it on another day. If the stain appears while you are wetting a particular junction, you have your suspect. Mark it with chalk and leave it for the roofer, rather than smearing sealant over it and hiding the very thing the roofer needs to see.
When to hand it to a roofer
Bring in a roofer when the leak has more than one possible source, when water is coming through a light fitting, or when the ceiling is bulging. For a leak that is actively coming in, our priority is to make the roof safe first, as set out on our emergency roof repairs page. After that comes the survey, with photographs of the fault and a quote in writing.
For scale, Less (last updated March 2026) gives a typical UK range of £150 to £500 for small flat roof repairs. A leak caused by one failed junction fits that bracket. A leak that comes from age across the whole covering does not, and then the better question is the one our page on repairing or replacing a flat roof answers. The flat roofing hub covers the rest of the work.
Related questions
Flat roof leak questions
Can sealant from a tube stop a flat roof leak?
It may slow a leak for a short while if it lands on the right spot. Often it does not, because the water enters elsewhere and sealant hides the clue. If you use it as a stopgap, photograph the area first and tell the roofer what you did.
Should I wait for dry weather before looking?
Look during or just after rain, from a safe place, because that is when water marks and wet patches are clearest. Do any hose testing in a dry spell, so you know any new water has come from your test and not from the sky.