The short answer
Flat roofs leak where the waterproof layer is broken or where water cannot leave. The usual entry points are splits and blisters in the covering, failed upstands and flashings at walls, blocked or damaged outlets, and worn seams. The stain on your ceiling is often some distance from the hole, because water runs along the deck before it drops. A survey traces the route back.
The places a flat roof lets go
A flat roof is a shallow tray. It keeps rain out with a single waterproof covering, and it needs to shed water towards an outlet or gutter. Anything that breaks the covering, or stops water leaving, can cause a leak.
Splits and punctures in the covering are the obvious cause. Felt dries and cracks with age, rubber can be pierced, and fibreglass can crack at stress points. Our answers on GRP roof cracking and EPDM repair cover those materials.
Upstands are the next candidate. These are the vertical edges where the covering climbs the face of a wall, chimney or kerb. They are held at the top by a flashing, a strip that bridges the join, and when the flashing lifts, the sealant fails or the mortar behind it crumbles, water runs behind the covering. Lead flashings are made to BS 6915 under Approved Document C.
Outlets are the third. A blocked or cracked outlet lets water pool, and the pool soaks through any weakness. Joints and seams between sheets, and the details around pipes and vents, are the others.
Why the stain is not under the hole
Water does not simply drop straight down through a roof. It enters at a split, runs along the surface of the deck or through insulation, follows a joist, and drops where the ceiling is weakest. The patch indoors can sit well away from the entry point, even across the room.
That is why guessing and smearing sealant over the nearest crack so often fails. The real hole is elsewhere, and the sealant only adds a layer that hides it.
Tracing the path is a process. We note where the stain is and when it appears, whether it follows rain or wind direction, and what lies above it. Then we inspect the covering, upstands, outlets and edges in that area and uphill from it, looking for splits, lifted edges and ponding.
Sometimes the leak is not a leak. Moisture forming inside the roof drips in cold weather without any rain, and our answer on condensation under a flat roof explains how to tell them apart. The test is simple: if the drips follow the heating and not the rain, the covering may be sound.
When water has soaked the deck, the damage can travel further than the stain suggests.
Making it safe first
While the cause is being found, the priority is to stop damage getting worse. If water is coming through the ceiling, move furniture and electrics out of the way. Put a bucket under the drip, and keep people out from beneath a ceiling that is sagging with water.
Switch off lights and circuits in the affected room if water is near fittings, and take advice from an electrician before using them again.
On the roof, a temporary measure such as a patch or a protective sheet can slow the water. Making the roof safe comes first, then the diagnosis, and the temporary work is not the repair. Please do not go up in rain or wind to look, as a wet flat roof is slippery and edges are unprotected.
If you can, take pictures of the stain and note the weather when it showed up. A pattern of dates is useful evidence. Note any recent work on the roof, such as an aerial, a solar panel or a new window, because penetrations are common causes.
Tell your insurer if the damage is significant, and keep receipts for any emergency work.
Repair, re-cover or replace
Once the cause is found, the fix usually falls into one of three kinds. A local repair deals with a split, a failed upstand or a blocked outlet, and suits a roof whose remaining surface is healthy. A re-cover puts a new covering over a sound deck. A full replacement strips the roof and replaces the deck where it has been damaged.
The choice follows what the survey shows. A young roof with a single fault is repaired. An old roof with repeated leaks, widespread splitting or a wet deck is not, and spending on patches is poor value. Our flat roofing page sets out what each route involves.
The deck matters most. If the water has been in for a long time, the boards can rot and need replacing before any covering goes on, as our answer on rotten flat roof decking explains.
Costs vary. MyJobQuote (October 2026) puts a flat roof replacement at £1,500 to £6,000, and MyJobQuote's roof repair average is around £650 in 2026. Those are typical UK figures. The written price comes once the survey is done, covering the agreed work in a single total.
