EPDM is a single rubber membrane with very few joints, stays flexible in cold weather and typically lasts 25 to 30 years or more. GRP is a fibreglass laminate cured in place, giving a hard seamless surface on a similar lifespan but less tolerance of movement. Felt is the cheapest and the shortest-lived. The deck condition and how much the structure moves usually decide which is right.
- Joints are where flat roofs fail, so the material with fewest joints has a real advantage.
- GRP needs a stable, dry, well-fixed deck — it is rigid and will crack if the deck moves.
- Modern torch-on felt is far better than the old pour-and-roll reputation, but still the shortest-lived.
- No membrane fixes a bad fall. If water ponds, correct the fall before re-covering.
Why flat roofs got their reputation
Flat roofs are widely distrusted, and that distrust is based on a real history rather than prejudice. For decades the standard domestic flat roof was built-up bitumen felt, laid in three layers, on a chipboard deck, with minimal fall and little thought given to ventilation. Those roofs failed, often within ten or fifteen years, and a generation of homeowners learned that flat roofs leak.
What changed is the materials. Single-ply rubber membranes and fibreglass laminates are a different proposition, and a modern flat roof on a properly built deck with an adequate fall is a long-lived piece of construction.
What has not changed is that flat roofs are less forgiving than pitched ones. A pitched roof sheds water fast and tolerates a surprising number of small defects. A flat roof holds water for a while and finds any weakness. That puts the emphasis on detailing and on the deck underneath, which is where most of the real decisions are.
The three materials, side by side
| EPDM rubber | GRP fibreglass | Torch-on felt | |
|---|---|---|---|
| Typical lifespan | 25–30 years or more | 25–30 years | 15–20 years |
| Joints | Often none on a domestic roof — one sheet | Seamless once cured; joints in the laminate | Lapped and bonded throughout |
| Flexibility | High — moves with the deck | Rigid — intolerant of deck movement | Moderate |
| Cold weather | Stays flexible | Brittle in extreme cold | Can crack if disturbed cold |
| Installation | Adhesive, no flame | Resin cured in place, needs dry conditions | Hot works — flame on site |
| Foot traffic | Soft; needs protection if walked on | Hard-wearing | Moderate |
| Repairability | Straightforward patching | Needs matching resin work | Easy to patch |
| Appearance | Matt black, utilitarian | Smooth, can be coloured, neat trims | Mineral finish, traditional look |
Published guide pricing for flat roofing on this site is shown further down. The material choice usually moves the figure less than the deck condition does.
EPDM: the forgiving one
EPDM is a synthetic rubber supplied in large sheets. On most domestic roofs — a garage, a dormer, a single-storey extension — the whole roof can be covered with one piece, which means no field joints at all. Since joints are where flat roofs fail, that is a genuine structural advantage rather than a sales point.
It stays flexible across a wide temperature range, so it accommodates the deck expanding and contracting rather than resisting it. It is installed with adhesive, so there is no flame on site, which matters on a roof attached to an occupied building and sometimes matters to insurers.
The trade-offs are honest. It is soft, so it needs protection where there will be regular foot traffic or where a ladder is habitually rested. The finish is matt black and plain — not a problem on a garage, more of a consideration on a roof overlooked from upstairs windows. And the quality of the detailing at upstands, outlets and trims is what decides its life, as with everything else.
GRP: the hard one
GRP — glass-reinforced plastic, the same family as fibreglass boat hulls — is built up in place from matting and resin, then finished with a topcoat. Once cured it is a single seamless shell, hard-wearing underfoot and able to form very neat edge trims and upstands.
It suits balconies, roofs that get walked on, and anywhere the appearance matters, since it can be pigmented and takes a tidy edge.
The caveats are about movement and conditions. GRP is rigid. If the deck beneath it flexes — because the boards are not well fixed, because the joists are at the limit of their span, or because the deck is a material that moves with moisture — the laminate will eventually crack, and a crack in GRP is a harder repair than a patch in rubber. It also needs genuinely dry, reasonably warm conditions to cure properly, which constrains when it can be laid. Both of those put weight on the deck preparation, and a GRP roof laid over an inadequate deck is money wasted.
Felt: not what it was, but still the shortest-lived
Modern torch-on felt is a considerable improvement on the material that gave flat roofs their reputation. It is a polymer-modified bitumen membrane, applied in layers with a torch, and a properly specified high-performance system on a sound deck will give a respectable service life.
It is the cheapest of the three and it is easy to repair, which keeps it relevant for outbuildings, garages and situations where budget is the governing factor.
The reasons we usually recommend something else on a roof attached to the house: it has the shortest life of the three, it is laid in overlapping sheets so it has the most joints, and installation involves an open flame on your roof, which carries a real fire risk on a building with a timber structure. Where felt is the right answer for budget reasons we will say so, but the whole-life cost comparison usually favours EPDM.
The deck matters more than the membrane
This is the point most comparison articles skip, and it is the one that decides outcomes.
Whatever goes on top, it is only as good as what is underneath. A membrane laid over a soft, delaminating chipboard deck will fail at the deck, not the membrane, and no warranty covers that.
Things worth establishing before choosing a material:
- Is the deck sound? Soft or springy underfoot means it is coming off. Budget for that rather than hoping.
- What is the fall? A flat roof should be laid to fall so water runs to an outlet. Design guidance is to aim for 1:40 so that at least 1:80 is achieved after deflection. Ponding is not automatically a leak but it accelerates ageing and finds weaknesses.
- Warm deck or cold deck? A warm deck puts insulation above the structure and is the modern approach. A cold deck has insulation between the joists and needs a ventilated void above it — and an unventilated cold deck is a condensation problem waiting to happen.
- Where does the water actually go? Outlets, gutters and the route off the roof. A perfect membrane above a blocked outlet is still a pond.
- Upstands and abutments. Where the flat roof meets a wall needs a properly dressed upstand to an adequate height, tucked in and flashed. More flat roofs leak here than anywhere in the field.
Which to choose
- Garage or outbuilding, budget-led: felt is defensible, EPDM is better value over its life if the budget stretches.
- Extension or dormer on the house: EPDM, in most cases. The joint-free sheet and the absence of hot works are both worth having over a habitable room.
- Balcony or a roof that gets walked on: GRP, for its hardness — provided the deck is sound and stable.
- A roof visible from above: GRP for appearance, or EPDM with a better trim detail.
- A deck you suspect moves: EPDM. Rigid laminate over a moving deck cracks.
- Any of the above with ponding: fix the fall first. The material choice is secondary to that.
What flat roofing costs
These are the roofing figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Flat roof (EPDM / GRP) | £80–£120 per m² | Warm or cold deck |
| Minor roof repair | from £150–£400 | Slipped or broken tiles, usually one day |
| Full re-roof (typical semi) | £5,000–£9,000 | Strip, membrane, batten, re-tile |
| Larger / detached re-roof | £9,000–£12,000+ | Scaffolding included |
Deck replacement, insulation upgrades and correcting a fall are additional to the covering rate, because they are separate operations. We survey the deck before quoting so those are priced rather than discovered.
We check the deck, the fall and the outlets before recommending a covering, because that is what decides how long the new one lasts.
Frequently asked questions
Which lasts longer, EPDM or GRP?
They are broadly comparable, both typically in the 25 to 30 year range when properly installed on a sound deck. The difference is how they fail: EPDM stays flexible and tends to age gracefully, while GRP is rigid and is more likely to crack if the deck moves. On a stable deck either should see out a couple of decades.
Is EPDM better than felt?
For a roof attached to the house, generally yes. It lasts longer, it typically has no field joints on a domestic-sized roof, and it is installed without a flame. Felt remains a reasonable budget choice for a detached garage or outbuilding, and modern torch-on systems are much better than the material that gave felt its reputation.
Can a new flat roof go over the old one?
Sometimes, but it is usually a false economy. Overlaying traps whatever moisture is in the old build-up, hides the condition of the deck, and adds weight. It also means the new covering is only as good as what it is stuck to. Unless the existing roof is genuinely sound and dry, stripping back to the deck is the right call.
Why does my flat roof pond after rain?
Because it has lost its fall, or never had enough. Flat roofs should be laid to a fall so water runs to an outlet — design guidance aims for 1:40 so at least 1:80 survives deflection. Decks sag and joists deflect over time. Re-covering without correcting the fall simply repeats the problem.
Do flat roofs need ventilation?
A cold deck does — insulation between the joists with a ventilated void above it, or moisture condenses on the underside of the deck and rots it. A warm deck, with insulation above the structure, does not need that ventilation and is the modern approach. Which you have changes the specification considerably.
Can I walk on a flat roof?
Occasionally for maintenance, on any of the three, but regular traffic needs designing for. GRP is the hardest wearing. EPDM is soft and needs walkway protection where people will use it routinely. If the roof is intended as a balcony or terrace, say so at the quote stage — it changes both the covering and the deck.
Get the deck checked, not just the covering
We survey the deck, the fall and the outlets, and recommend a covering that suits the structure rather than the one we have in the van. Free inspection, fixed written quote.