Use this as a checklist for a flat roof you are worried about. It covers what to look for inside and out, what each sign suggests, and where a patch stops being enough for the roof.
Flat roofs rarely fail all at once. They give notice: a stain that appears after heavy rain, a pond that stays long after it, a soft patch, a blister that was not there last year. The trouble is that each sign is easy to put off, and a roof that is ignored for a season can turn a small repair into a deck replacement.
The signs fall into three groups. Some show inside the building, where water reaches the ceiling or the walls. Some show on the surface of the roof itself. And some show in how the roof feels and sits, which is often the most serious group, since it concerns the deck rather than the covering.
The sections below run through them in turn, then sort them into what a patch can deal with and what it cannot. It is written for owners of Thanet and East Kent flat roofs on extensions, garages, porches and dormer cheeks, and it belongs to our flat roofing work. Do not climb onto a flat roof you cannot reach safely; most of what is described here can be seen from a window or the ground, or in photographs.
Signs inside the building
The first sign to catch the eye is usually indoors. Water has found its way through the membrane, along the deck and into the ceiling below. By the time it shows there, it has often been entering for some time.
What to look for:
- A brown or yellow stain on the ceiling that grows after rain.
- Paint that bubbles, peels or flakes under a flat roof.
- A damp or musty smell in the room below, especially after wet weather.
- Black mould in the corner where the ceiling meets an outside wall.
- A ceiling that bows, sags or is soft to the touch.
A stain tells you water is arriving, not where it enters. Water often runs along the deck or between layers before it falls, so the stain can sit well away from the hole. That is why the survey traces the water back across the roof instead of treating the stain as the target.
Not every stain is a leak. Condensation forms on a cold ceiling and on cold corners, and it can look similar. A leak tends to follow rain, while condensation follows cold, damp spells and shows as a fine bloom of mould. Our page on condensation under a flat roof sets the two apart. If you are unsure, a photograph of the stain on a dry day and again after rain tells you which it is.
Look at the outside walls too. A damp patch that runs down from the roof edge, or a tide mark on the brickwork under a parapet, shows water getting in at the edge or the wall junction rather than through the middle of the roof.
Signs on the surface of the roof
On the surface, the covering shows its age. Each sign points to a different weakness, and the table pairs them. Photographs taken with a phone from an upstairs window, or at the edge of the roof where it is safe to do so, are enough to see most of them.
| Sign | What it suggests |
|---|---|
| Splits or cracks | Membrane has aged or the deck has moved |
| Open or lifting laps | Seam or bond has failed |
| Blisters | Air or moisture under the covering |
| Bare, crazed or shiny patches | Surface protection worn away |
| Gaps at the walls | Upstand or flashing has pulled away |
Felt roofs often show surface wear as dull, cracked bitumen where the mineral finish has gone. An EPDM roof shows it at the seams and at corners, where the rubber is bonded. GRP shows it as fine cracks in the resin, often along the lines of the boards beneath. A lead roof shows it as splits at the joints and around fixings.
Look also at the edges. Trims that have come loose, a drip edge that no longer shelters the fascia, or an edge where the membrane has lifted all allow water to track behind the covering. Debris in the gutter and moss on the surface are signs that water is not leaving as it should.
Gutters and downpipes belong in the same check. A gutter that overflows at one point, or a downpipe that discharges nothing in rain, can leave the roof edge saturated and send water into the wall below.
Ponding and what the water is telling you
Ponding is water that stays on a flat roof after rain. A little moisture that dries within a day or so is normal in the lowest corner. A pond that lasts days, or is back at the same place after every shower, is a sign worth taking seriously.
Standing water has three effects on a flat roof:
- It adds weight to a deck that was designed with the water running off.
- It lies on one low stretch of covering through every wet spell, and wears it.
- It collects dirt and leaves that harbour moss and block outlets.
The cause varies. The outlet may be blocked with leaves and silt, which is a quick fix. The fall may be too shallow for the roof to drain, which is built into the roof. Or the deck may have dipped between its supports, so a pond has formed where there used to be a slope.
The way to tell them apart is to clear the outlet and see what happens after the next rain. If the water still stays, the problem is the fall or the deck. BS 6229:2025 sets out the fall for membrane roofs, and a trade-body summary of it gives a minimum finished fall of 1:80 on the main surface. IKO advises that single-ply roofs be designed to 1:40. Our guide to the lifespan of each flat roof material gives the typical life of each covering.
A soft, spongy or sagging roof
Of all the signs, this is the most serious. The deck is the structural base, boards, plywood or concrete, under the membrane and any insulation. When it is wet for long enough, it softens and loses strength. The roof then feels spongy underfoot, or springs back when pressed.
Other signs of a deck problem:
- A visible dip or sag between the walls, or a roof that is no longer flat to the eye.
- Ridges or ripples in the membrane, where the deck below has moved.
- A ceiling that sags, or a crack that runs along the line of a joist.
- Dark staining on the deck when seen from the loft or the room below.
A soft deck cannot be repaired by patching the covering. The covering is only as sound as the surface it is fixed to, so the wet boards must come out and be replaced, and the cause of the water found and stopped. Our guide to a sagging flat roof deck sets out what causes a dip and what putting it right involves.
If a roof feels soft, stay off it and ask for a survey. A deck weakened by water may not take a person's weight, and the risk of a fall through it is real. Photographs from outside, and from underneath, are the safe way to start. We look at the underside wherever there is access.
What a patch can fix and what it cannot
Finding a sign does not mean the roof must be replaced. The question is whether the damage is confined to one place or runs across the roof, and whether the deck beneath is still sound. A patch deals with a place; it cannot deal with a roof.
| Fault | Patch |
|---|---|
| One split at a seam | Usually enough |
| Failed seal at an outlet or pipe | Usually enough |
| Lifted edge or loose trim | Usually enough |
| Cracks over much of the surface | Not enough |
| Many blisters | Not enough |
| Soft or rotted deck | Not enough |
| Leaks that keep returning | Not enough |
Where the table says not enough, the roof needs a new membrane, and sometimes a new deck under it. Our page on whether to repair or replace a flat roof works through the choice. A roof with a single fault, firm dry boards and plenty of range left is better mended, and the survey should recommend exactly that.
If you do need a new membrane, the replacement can bring the Building Regulations into play, because Approved Document L treats renewing the waterproof layer of a flat roof as renovation. Ask building control before work starts.
Keep a dated record. Photographs of each sign, taken from the same spot every few weeks, show whether it is changing, and give the survey a history to start from. A fault that stays the same through a wet winter is a different case from one that grows.
The written price follows the survey, and it names the work the roof needs, whether that is a repair, a recovering or a deck replacement.
Age as the background to the signs
The age of the covering does not tell you whether it is failing, but it gives the signs their weight. A split on a felt roof in its twelfth year means something different from the same split on an EPDM roof in its fifth.
Typical lives from two published guides:
- Torch-on felt: 10 to 15 years from HomeBuilding, 10 to 20 years from Less.
- GRP: 25 to 30 years from HomeBuilding, 25 to 40 years from Less.
- EPDM: 25 to 30 years from HomeBuilding, 30 to 50 years from Less.
- Lead: 60 to 100 years from HomeBuilding.
Each is a typical range for a well-laid covering, and none is a promise for the roof over your head. A roof inside its range with several of the signs above is failing early, and the deck, the fall and the details are the first places to look. A roof past its range with only a minor sign may still have time, and a careful survey will tell you.
If you do not know the age of the covering, the building's records or a previous owner may. A kitchen extension built in a known year gives a fair guess. The age matters less than the condition, but it helps frame what to expect.
The best moment to act is as soon as the first sign appears. Water spreads, and a small fault caught early is a small job.
