The short answer
Start with the details, not the middle of the roof. Water on a flat roof travels along the deck and insulation before it drips through the ceiling, so the stain is rarely under the hole. Check upstands, outlets, edge trims and joints first, as they fail far more often than open membrane. If that finds nothing, a controlled flood test, filling sections one by one, will show where it goes in.
Why a flat roof leak shows up in the wrong place, where to look first, and how a flood test narrows it down.
Why the stain is not under the hole
On a pitched roof water runs down the slope, so a damp patch sits roughly below the fault. A flat roof behaves differently. A flat roof is a layered build: waterproof covering, perhaps insulation, a deck of boards, then joists and the ceiling. Water that gets through the covering rides along the top of the deck, soaks sideways through insulation and follows the joists until it finds a gap, a light fitting or a plasterboard joint. It can drip metres from where it entered.
That is why guessing from the stain wastes money. A patch placed over the middle of a roof because the ceiling is wet there often does nothing, and the leak carries on from an edge you never touched. Water also lingers inside the build for days, so a leak may seem to continue after rain has stopped, or appear only after a storm that blew rain sideways against a wall.
Keep a note of when it appears: after rain from one direction, only after heavy downpours, or even in dry weather. That pattern helps a roofer decide whether the cause is a poor upstand, a blocked outlet or condensation instead.
Where to look first
Work from the details inwards. The upstand, where the covering turns up a wall, is the most common weak point. Look for a lifted edge, a gap along the top where the flashing or sealant has split, and mortar that has crumbled behind it. Next, check the outlets and gutter at the low edge. A blocked outlet leaves a puddle that tests every join, and a loose connection leaks where it meets the covering.
The edge trims come next. A drip trim that has pulled away from the board lets wind-driven rain behind the covering. Then look at penetrations such as a rooflight kerb, a soil vent pipe or a flue, and the corners and laps, where the membrane is cut or folded. Our pages on a blocked flat roof outlet and a leaking edge trim cover two of these in detail.
Only after those, look at the open field: blisters, splits, punctures from a dropped tool or a branch, and worn patches where the surface has gone. Take photos as you go, from a safe position, so the roofer can see what you saw.
Telling a leak from condensation
Not every wet ceiling is a leak. Warm, moist air from a kitchen or bathroom rises into the roof and meets a cold surface, where it turns to water. This condensation can wet the underside of the deck and drip through the ceiling in a pattern that looks just like a leak, especially in winter. If the dampness comes with black mould spots, appears in cold spells rather than after rain, and spreads across large areas instead of one patch, condensation becomes the likelier cause.
Check the weather match. A true leak tends to appear within hours of rain, and it stops when the roof dries out. Condensation follows temperature and indoor humidity, so it can persist in dry weather. The two can happen together, and a roof with both gets worse faster.
The cure is different for each: a leak needs the covering mended, while condensation needs ventilation or a better insulated build. Our answer on condensation under a flat roof explains the signs, and our guide to warm roof against cold roof shows how the build matters.
Flood testing and getting it fixed
When the obvious places show nothing, a roofer can run a flood test. The outlets are plugged, a section of the roof is dammed and filled with a few centimetres of water, and someone watches the ceiling below. If it stays dry after a set time, that section is cleared and the next is flooded. Work in small areas, so the leak is narrowed to a patch a metre or two across rather than the whole roof. It needs care: the deck and structure must be able to carry the weight of the water, so it is not something to try on a weak roof or without a plan.
You can do a gentler version with a hose, working from the low edge upwards, and a helper inside who calls out when the drip begins. Start low and move up, as starting at the top muddles the answer.
Once the entry point is found, ask whether it is a repair or a sign that the whole covering has aged out. See repair or replace a flat roof for how that is judged. We shape the survey around the cause and give one written total afterwards, with photos of the fault. More on our flat roofing page.
More on this in Flat Roofing in Crawley
Two more leak-hunting questions
Can I find the leak from inside the loft or ceiling void?
Sometimes. On a flat roof there is often no loft, only a shallow void between ceiling and deck. If you can see in, look for dark staining on joists or the underside of boards, because the worst staining tends to sit nearest the entry point. Do not poke at a wet plasterboard ceiling, as it can fall.
Is it safe to climb onto a flat roof to look for the leak?
Only if it is low, firm and dry, and even then with care. A flat roof can hide soft decking beneath a sound-looking surface, and an unprotected edge is a fall risk. Photograph what you can from ladders or windows, and leave anything more to a roofer with proper access.