Materials guide
EPDM rubber vs GRP fibreglass: which flat roof is better?
For a flat roof today, the real choice is between EPDM rubber and GRP fibreglass — traditional felt is largely a repair material now. We install both week in, week out on garages, extensions, dormers and bay tops, and the right answer genuinely depends on the roof, not the salesman.
Here's the honest comparison we give customers at the kitchen table.
At a glance
| EPDM rubber | GRP fibreglass | |
|---|---|---|
| What it is | A single sheet of synthetic rubber, glued to the deck — usually seam-free | Resin and fibreglass matting laid wet on site, cured to a rigid seamless shell |
| Lifespan | 40–50 years; leading systems carry a 20-year manufacturer warranty | 30+ years when laid correctly with a proper topcoat |
| Cost | Usually the cheaper of the two on simple roofs | A little more — more labour and materials |
| Appearance | Matt black rubber sheet — tidy but utilitarian | Smooth coloured finish (grey is popular) — the smarter look where the roof is visible |
| Foot traffic | Fine for maintenance access; not ideal as a terrace | Hard-wearing — the pick for balconies and walkways (with a non-slip finish) |
| Weather on install day | Tolerant — can go down in cooler, damp-ish conditions | Needs dry, mild weather to cure properly — a wet forecast delays the job |
Where EPDM wins
On a straightforward garage or extension roof, EPDM is hard to argue against. One sheet covers the whole roof with no joints — and since joints are where flat roofs fail, that's a structural advantage, not a detail. It stays flexible for decades, shrugs off UK freeze-thaw cycles, and the leading membranes carry 20-year manufacturer warranties on top of our own 10-year workmanship guarantee.
- Seam-free on most domestic roofs — the fewer joints, the fewer leaks
- Quickest install, least weather-dependent
- Best value on large, simple roofs
Where GRP wins
GRP is laid wet, so it moulds around whatever the roof throws at it — upstands, corners, outlets, awkward details — and cures into one rigid, seamless shell with crisp pre-formed edge trims. It's the more handsome finish where you look down on the roof from a bedroom window, and it's the clear choice where the roof doubles as a balcony or walkway.
- Best for complex shapes and detailed roofs
- Hard-wearing surface for balconies and foot traffic
- Smart, coloured finish where appearance matters
What they cost
For a typical single garage, budget from around £1,500 for a full re-board and re-cover; extensions run higher with size and detail. GRP usually comes out 10–20% above EPDM on the same roof. On either system, most of the cost is the same regardless: stripping the old covering, making good or replacing the deck, insulation if you're upgrading to a warm roof, and edge trims — which is why a proper survey beats a guess over the phone.
Our verdict
Big, simple roof — garage, extension, shed: EPDM, for value, speed and a seam-free covering with the longest warranty. Complex shape, visible from above, or used as a balcony: GRP, for its mouldability and hard-wearing finish. Neither is 'better' universally — and any installer who pushes one system for every roof is selling what they stock, not what you need.
We install both, so the recommendation in your quote is based on your roof. Written price within 48 hours of the survey.
Common questions
How long does an EPDM rubber roof last?
Can you fit EPDM or GRP over my old felt?
Which is better for a warm-roof build-up?
Flat roof due for replacement?
Free survey, fixed written quote within 48 hours.