Most flat roofs do not fail in a night. They show a pool that stays too long, a seam lifting at one end, a bubble in the surface or a faint brown ring on a bedroom ceiling. Each of these is a clue to a different fault, and reading them in the right order saves money, because a roofer who knows what is wrong can quote for the right job.
This guide is a field guide to symptoms. It sorts them into what you can see from a window or a ladder, what you can see inside the house and what you can feel underfoot, and it says what each sign usually points to. A few terms first: ponding is standing water on the roof, a seam is the joint between two sheets of membrane, a blister is a raised bubble in the covering, and the deck is the board surface under the covering. Once you know what you are looking at, our guide to repairing or replacing a flat roof helps with the decision that follows. Our flat roofing page covers the wider service.
Ponding: water that outstays the rain
A flat roof has a gentle fall so the water runs to an outlet or gutter. BS 6229 guidance, as set out by IKO and Rockwool, designs for a fall of 1:40 to leave a finished fall of 1:80, so a roof that is working shows only a dull sheen after a shower. A pool still sitting there on the second dry day is the sign to take seriously.
What ponding usually means
There are three common reasons. The first is a fall that was never enough, so the water has nowhere to go. The second is a deck that dips between its joists, forming a shallow bowl. The third is a blocked or badly placed outlet.
Ponding alone is not a leak, because the membrane is made to resist water. But standing water ages the covering faster, collects silt and algae and loads a deck that may already be soft. A pond that grows year on year suggests the deck is moving.
How to look
Check from an upstairs window and photograph any wet patches with a coin in shot for scale. Note whether the pool sits near an outlet, mid-roof or along an edge. Edge ponds point to a trim or fall problem; central ones to a sagging deck. Dried silt rings show where the water used to reach.
Splits, open seams and torn edges
A split is a crack running across the covering. A seam failure is a joint that has opened. Both let water straight in, and we check for them early in any survey.
Splits
On felt, a split often follows a line of weakness, such as a board edge or the junction with a wall, where the membrane has been pulled until it gave. On GRP, a hairline crack usually shows where the deck moves under the resin. On EPDM, splits are rarer, but a branch, a tool or a slipped ladder can puncture it.
Open seams and laps
Seams on EPDM are joined with adhesive or tape, and on felt with heat. Over time, a poor joint lifts at one corner and the gap runs along the seam. If a felt lap lifts with no resistance, the bond has failed. Do not pull hard, because you may turn a small fault into a large one.
Edges and trims
The edge is where covering meets trim. When a drip trim works loose or is knocked, water can run back under the edge of the membrane and into the deck. Look for gaps, rust staining, a trim that lifts when pressed or edging that has dropped lower than the roof surface.
Photograph a split with a ruler beside it. A short crack can be fixed locally; one running across much of the roof's width tells a different story.
When splits turn up across the roof, in several places, the covering is probably wearing out across its whole area, and a patch is only a stopgap.
Blisters, bubbles and crazing
A blister is a raised dome in the covering. Pressed with a gloved hand it may feel hollow, spongy or full of liquid. Blisters appear when moisture or air gets trapped between the membrane and the layer below and expands in sunshine. Our guide to flat roof blisters covers the cause in more detail.
Which blisters matter
A small, intact blister on felt is often cosmetic, and many roofs carry a few for years. A blister that has cracked open, has water in it or sits on a seam is a different matter, because it has become a point where water enters. A cluster of blisters across a large area suggests that moisture is trapped across the roof, in the insulation or the deck, and that points to a bigger job than a patch.
Crazing and lost chippings
Felt that has aged in the sun develops a network of fine cracks, a pattern known as crazing. On mineral-surfaced felt you may also see the stone chippings gathered in the gutters or at the outlets. The chippings protect the bitumen from sunlight; once they are gone, the surface weathers faster.
Lifting on EPDM
EPDM does not blister as felt does, but a rubber sheet can lift where adhesive has failed. A section that rises and falls in the wind is no longer stuck down.
Lifespan matters here. The sources quote 10 to 20 years for felt, 20 to 30 or more for EPDM and 25 to 40 for GRP. A roof within sight of those figures that shows blistering, crazing and splits all at once is not likely to improve with patching.
A soft or spongy deck underfoot
The most serious sign is the one you feel rather than see. Walk gently along the edge of the roof, where the structure is strongest, or ask a roofer to. If the surface gives under the foot, or feels like a mattress, the deck is saturated or rotten. Our page on a spongy flat roof goes deeper into what causes it.
Why a soft deck changes the job
Boards kept wet for months lose strength, swelling and delaminating or starting to rot. A new covering laid over them seals in the problem. The deck has to come out, and sometimes the joists below it need attention too.
Walking on a failing roof is risky. Do not step on a soft area to test it, do not stand on a pond and do not climb up in rain or frost. Press from a safe position, photograph what you can, and call us. Water coming in is our priority; we stop it spreading first.
Other clues to a wet deck
Look at the underside of the fascia and the soffit for dark staining, flaking paint or crumbling timber. Check upstairs ceilings for sagging plasterboard or a ceiling that has changed shape. Wet insulation can hide a leak with no visible drip, and carry water well away from where it got in.
A moisture meter and, where needed, a small test cut give a clear answer. We use both to see what lies beneath rather than guess from the surface.
Inside the house: stains, smells and peeling paint
The inside signs are often what people notice first, because they are in the places you look every day. The commonest are a brown ring on the ceiling, paint blistering on a ceiling or wall, a musty smell in a room below the roof and mould in a corner.
Reading a ceiling stain
A stain with a defined edge, brown or yellow at the border, usually means water has come through and dried, and come through again. A stain that grows after rain points to a live leak. A stain that is damp even in dry weather suggests water is held in the insulation or the deck, and is releasing slowly. The position of the stain does not always sit directly below the fault, since water can travel along joists and boards before it drops.
Condensation or leak
Not every ceiling mark is a leak. Condensation forms where warm, damp air meets a cold surface, and a cold flat roof with poor ventilation or a gap in the vapour layer can produce staining that looks like a leak. The difference is timing: a leak follows rain, condensation follows cold nights and steamy rooms. Our guide to whether modern flat roofs leak deals with that question directly.
What to do inside
Put a bucket under a drip. Move electrical items away. If the ceiling bulges, make a small hole at the lowest point so the water drains into a bucket rather than bringing the plaster down. Photograph everything with the date. If you are insured, those pictures help with any claim.
Mark the edge of a stain with a pencil and the date. A line that has moved a week later tells you more than any argument about where the water might be coming from.
Outlets, flashings and rooflights: the usual suspects
Look at every place where something goes through the covering or meets it: outlets, pipes, rooflights, vents and the junction with the house wall.
Outlets and gutters
Leaves, silt and moss build up at outlets and in gutters. A blocked outlet makes a pond; a pond puts more water on the weakest part of the roof. Clear what you can reach safely from a window or a stable ladder, and look for cracks or movement where the outlet meets the membrane.
Flashings and abutments
At a wall junction, a flashing covers the top of the membrane turn-up. Lead flashings may have lifted, slipped or been stolen; mortar around a chase may have crumbled. Our guide to flat roofs on extensions shows how an abutment is built. If you see daylight between the flashing and the wall, or a bead of sealant that has cracked, you have found a likely entry point.
Rooflights and kerbs
A rooflight kerb needs the membrane turned up on all sides and sealed at the corners. Condensation running down the inside of the glass is not a leak, but water at the foot of the kerb after rain is. Look for stains on the plasterwork around the opening and for corroded fixings.
Plants and debris
Moss and small plants rooting in silt show that the roof holds water and dirt. Roots can find their way into laps and edges. Cleaning is rarely the whole answer, but it is part of the investigation.
A look at these places twice a year costs little and catches problems early.
What to do once you have spotted a sign
Start by writing down what you found and when, with photographs. That record gives a roofer a head start and gives your insurer evidence.
Then ask for a survey rather than a quote over the phone. Flat roof faults hide, and a fix priced by phone can leave the cause untouched. At the survey we check the surface, the seams, the edges, the outlets, the penetrations and the deck, and we photograph the roof you own. The written quote that follows sets out the job in stages.
What it might cost
Costs vary by fault. MyBuilder's 2026 repair guide gives £100 to £300 for minor roof repairs and £500 to £1,000 for moderate ones. For a full covering, its flat roof guide quotes £70 to £100 per m² for bitumen felt, £90 to £130 for EPDM and £95 to £150 for GRP. Our guide to what a new flat roof costs sets out the ranges. And if the roof is a walkable terrace, read balconies and roof terraces over living space, because water in a trafficked roof travels differently.
Our work carries a 10-year workmanship guarantee in writing. The number is 01453 933613, and WhatsApp or the enquiry form both take photographs.