Flat roofs in London are covered with one of five things: built-up felt, mastic asphalt, EPDM rubber, GRP fibreglass or a single ply membrane. Almost every domestic and small commercial flat roof is one of those, and telling them apart takes seconds once you know what to look for.
It matters because the material determines the preparation, and the preparation determines whether the roof can be sealed and saved or genuinely has to come off. This guide takes each type in turn: how to recognise it, how it typically fails, and whether it can be overlaid. Pitched and metal roofs work on entirely different principles, and our overview of the roofing systems we work on covers those separately.
Why does the material matter?
The covering decides the preparation, and the deck underneath decides everything else.
That is the whole logic of a flat roof survey in one sentence, and the two halves get confused constantly.
The covering tells us what primer is needed, what has to be abraded or cleaned back, and where the weak points are likely to be. Those are specification decisions: they change the method rather than the outcome.
The deck and insulation underneath tell us whether an overlay is honest at all. A sound deck with a tired covering is a refurbishment. A saturated or deflecting deck is a replacement, regardless of what the covering is made of, because sealing over a wet deck traps the moisture and buys a few years at the cost of a worse problem later.
So identifying your covering is the first question, not the only one. It narrows the method. The survey settles the rest.
How to identify your flat roof by sight
Built-up felt
What you see: a dark surface with fine grey or green mineral chippings bonded into it, laid in strips with visible overlapping joints running across the roof. At the edges the felt turns up against the wall, usually tucked into a mortar joint or held by a metal trim. Older roofs often have loose chippings spread across them instead.
Where you find it: by far the most common covering on London extensions, garages, dormers and small commercial roofs, and the likeliest answer on any flat roof last done before the 2000s.
How it fails: at the laps. The middle of the field is usually intact long after the joints have opened up, because the joints are where the material was cut, heated and pressed together.
Can it be overlaid? Yes, and it is the most common overlay we do. The check is adhesion: local blisters and lifted laps can be cut, dried and made good first, but a covering that has lost adhesion across the whole roof, so that it moves underfoot as a sheet, has to come off.
Mastic asphalt
What you see: a smooth, seamless, matt black surface with no visible joints anywhere, and a poured, moulded look at the kerbs and upstands. Older asphalt often has a fine crazed pattern across the surface, like dried mud, and may be dressed with a solar reflective paint that has weathered patchy and white.
Where you find it: Victorian and Edwardian buildings, particularly parapet roofs, balconies and the back additions on London terraces.
How it fails: by hardening. Asphalt is laid hot and stays slightly plastic when young. As it ages it becomes brittle, and thermal movement produces the crazing you can see and, eventually, cracks at the points of greatest movement: upstands, angles, outlets and the junction with a parapet.
Can it be overlaid? Yes, and asphalt takes a liquid system particularly well once cleaned and primed. The check is whether the cracking has gone right through. Surface crazing is normal on an old asphalt roof; cracks that have opened at an upstand are made good first.
EPDM rubber
What you see: a matt black rubber sheet, usually one continuous piece over the whole roof, with a slightly soft, flexible feel and typically no joints in the middle of a domestic roof at all. The edges are dressed over a trim, and the corners have moulded or hand-formed patches.
Where you find it: modern extensions, garden rooms and re-roofed garages, mostly from the 2000s onwards.
How it fails: at the bonded details rather than in the sheet. The field of the roof usually looks fine. What lets go is the bond at upstands, the seam tape at any joint that does exist, the corner patches and the seal at outlets, especially where the adhesive was applied in cold or damp conditions.
Can it be overlaid? Yes, but this is the one needing the closest look. Adhesion to rubber depends on the primer, on the surface being cleaned of the talc and weathering products that build up on it, and on the sheet still being bonded to the deck. Where an EPDM roof has lifted and balloons in the wind, an overlay is not the answer.
GRP fibreglass
What you see: a hard, rigid surface with a slight sheen, usually mid or dark grey but sometimes green or beige. The giveaway is the edge trim: GRP uses moulded fibreglass drip trims laminated into the roof itself, so the edge looks like a formed lip rather than a folded sheet.
Where you find it: balconies, dormers, porches and small extensions from roughly the 2000s onwards. Popular because it is walkable and looks tidy when new.
How it fails: by cracking. GRP is rigid and the deck underneath it is not, so any movement in the substrate has to go somewhere. Star cracks appear around fixings, hairline cracks open along joins in the boards below, and the laminate can delaminate at trims.
Can it be overlaid? Yes, and it is straightforward once abraded and primed. The checks are delamination and star cracking, both made good first. What an overlay does not fix is a deck moving enough to keep cracking a rigid laminate, so the cause gets identified rather than just the crack.
Single ply membrane
What you see: a light grey, off-white or occasionally dark membrane in wide sheets, with narrow welded seams running in straight lines across the roof. The seams are the identifier: neat, uniform and clearly heat-welded rather than lapped and sealed.
Where you find it: commercial and industrial roofs, larger blocks of flats and newer developments. Rare on a domestic extension.
How it fails: at the welds and the fixings. A weld that was never fully formed can hold for years and then open. Older membranes also shrink slightly, putting the details under tension.
Can it be overlaid? Yes, and on a large commercial roof it is usually the sensible answer, because stripping exposes the building and stops whatever happens inside it. The checks are primer compatibility and the condition of the welds.
The five coverings at a glance
| Covering | How to spot it | Typical failure point | Overlay possible? |
|---|---|---|---|
| Built-up felt | Mineral chippings, visible lapped joints | Open laps, blisters, splits at upstands | Yes, if adhesion holds across the roof |
| Mastic asphalt | Smooth seamless black, crazed with age | Cracking at upstands, angles and outlets | Yes, cleans and primes well |
| EPDM rubber | Matt black sheet, usually one piece, flexible | Bonded details, corners, seam tape | Yes, subject to primer and bond checks |
| GRP fibreglass | Hard glossy laminate, moulded fibreglass trims | Star cracks at fixings, delamination | Yes, once abraded and primed |
| Single ply | Light grey sheets, neat welded seams | Welds and mechanical fixings | Yes, subject to compatibility check |
The pattern in that last column is the point of the table. The covering is rarely what rules out a refurbishment. What rules it out is underneath it.
What actually stops a roof being overlaid
Four things, none of which is the material on top.
A wet deck. Timber that has taken on moisture, or a concrete slab holding water, has to dry or be replaced. Sealing it traps the moisture against the structure.
Saturated insulation. On a warm roof, insulation that has absorbed water has lost most of its thermal value and will not dry under a new waterproof layer. It comes out.
Structural movement. A deck that has deflected, or joists that have settled, will keep moving after the new system is on. That needs correcting first, not covering.
Falls that never worked. A roof that ponds will pond again under any new covering unless the falls are corrected, usually with tapered insulation. The guide to ponding water on a flat roof covers that in detail.
Where none of those applies, an overlay is faster, less disruptive and considerably cheaper than a strip and replace, because the strip-out, the disposal, the waste removal and the days with your roof open all come out of the job.
How to check safely from the ground
Do not climb onto a flat roof to identify it. A deck that has quietly rotted is exactly the kind of thing you find out about by putting a foot through it, and nothing you would learn up there justifies that. What you can do safely:
- Look from an upstairs window. Most London flat roofs are overlooked by a window in the same house or the one next door, and that view shows the surface texture, the joints and the edge detail.
- Photograph the edges and the upstands. The perimeter tells you more than the middle. Get the point where the roof meets the wall, and the outlet if you can see it.
- Look at the ceiling below in dry weather. Staining present when it has not rained for a week points at trapped moisture rather than an active leak.
- Note where water sits after rain and how long it takes to clear. Photograph it the same day and again three days later.
Send those photographs with an enquiry and we can usually tell you what covering you have before anyone visits. Where the roof is out of sight entirely, a drone survey puts a camera over it without scaffolding or a ladder.
What to do once you know what you have
Identifying the covering answers the first question. Getting the deck checked answers the one that decides the money.
What you should expect back from any survey is a clear statement of which covering you have, what condition the substrate is in, and why the recommendation follows from those two things. A quote that names no material and describes no substrate check is not a survey result, it is a price.
Our flat roof repair and waterproofing service sets out how that survey works and what the guarantee covers. For the seamless system itself and the substrates it is approved for, the guide to liquid applied roof waterproofing goes through each one. And if you want the numbers first, the flat roof cost guide sets out indicative 2026 market rates and what moves the price.