Flat Roof Options Compared: GRP Fibreglass vs EPDM Rubber vs Felt
By Leonard Walton, owner & lead roofer5 min read
Replacing a flat roof on an extension, garage, dormer or porch? The three common choices in the UK are GRP fibreglass, EPDM rubber and felt (torch-on). In short: GRP gives a hard, seamless, long-life finish; EPDM is a durable single-sheet rubber that's excellent on larger roofs; and modern torch-on felt is the most budget-friendly option. Here's how they compare.
GRP vs EPDM vs felt: price, lifespan and best use
| Material | Typical price range | Expected lifespan | Best suited to | Watch-outs |
|---|---|---|---|---|
| GRP (fibreglass) | £1,200–£3,500+ | 25–30+ years | Extensions, dormers, balconies and roofs that get foot traffic | Needs dry, mild weather to cure; can show hairline cracks if the deck below flexes |
| EPDM (rubber) | £1,200–£3,500+ | 20–30+ years | Garages and larger, simple flat roofs | Softer underfoot than GRP; relies on careful bonding at edges, upstands and outlets |
| Torch-on felt | £895–£1,500 | 10–20 years | Garages, extensions and outbuildings; the proven, best-value choice | Laps between sheets need sealing properly; make sure you're quoted a modern three-layer system, not old single-layer felt |
Prices are a rough 2026 guide for a single garage or small extension. GRP and EPDM sit in the same range: the size of the roof, access and whether the deck or insulation needs work move the price more than the choice between them. A free survey gives you a fixed figure.
At a glance
| Material | Typical lifespan | Best for | Notes |
|---|---|---|---|
| GRP fibreglass | 25–30+ years | Extensions, dormers, balconies | Seamless, walkable, hard finish; needs dry conditions to install |
| EPDM rubber | 20–30+ years | Garages, larger flat roofs | One-piece sheet, very few joins, flexible in cold |
| Torch-on felt | 10–20 years | Garages, sheds, budgets | Cheapest upfront; modern 3-layer systems outperform old felt |
GRP fibreglass
GRP (glass-reinforced plastic) is laid as a wet system that cures into a single, seamless, rigid layer with no joints to fail. It's hard-wearing and walkable, which makes it ideal for balconies and roofs that get foot traffic, and it comes in a range of finishes.
- Pros: seamless, long life, walkable, neat edge trims.
- Cons: needs dry, mild weather to install correctly; can show hairline cracks if the deck below flexes, so a solid deck matters.
EPDM rubber
EPDM is a synthetic rubber membrane laid as a single sheet, often covering a whole small roof with no seams at all. It stays flexible in cold weather and copes well with movement.
- Pros: very few or no joins, long lifespan, flexible, quick to install.
- Cons: a glued/ballasted finish is less hard than GRP underfoot; quality of the bonding work matters.
Torch-on felt (modern bitumen)
Forget the old single-layer felt that blistered and split. Modern three-layer torch-on systems are far tougher and remain the most cost-effective way to cover a garage or shed.
- Pros: lowest upfront cost, proven, quick.
- Cons: shorter lifespan than GRP or EPDM; more joins than a single-sheet system.
Which should you choose?
- Extension or dormer you'll see (or walk on): GRP fibreglass.
- Garage or larger flat roof on a budget-to-mid spend: EPDM rubber.
- Lowest upfront cost, outbuilding or garage: modern torch-on felt.
If your existing roof is failing rather than just old, it's worth first checking the signs your flat roof needs replacing — a sound roof with one localised fault can often be repaired instead.
Whatever the covering, good falls (drainage), a sound deck and proper detailing at the edges and upstands matter more than the material name on the quote. A cheap material installed well will outlast a premium one installed badly.
Which we recommend and when
We don't have one favourite membrane. We recommend a system once we've checked the deck, the falls and how the roof will be used, and our quotes compare GRP, EPDM and felt side by side so you can choose with the full picture. In practice:
- Garage roofs: usually EPDM rubber. It's what we fit most often on garages locally: one sheet, minimal joins and a long service life, and a typical garage roof takes around two days.
- Extensions, dormers and roofs you'll walk on: GRP fibreglass. Its rigid, seamless finish takes foot traffic and looks neat from upstairs windows. It needs a solid deck and dry, mild weather to cure, so it isn't a job to rush through a wet spell.
- Garages, extensions and outbuildings where value matters: modern torch-on felt. Felt has covered British flat roofs for decades, and it's still a great choice. A three-layer system laid properly is tough, proven and quick to fit, and it's the best-value of the three.
- Rotten deck or water that ponds: fix that first. A new membrane over a soft deck or poor falls won't last, whichever material it is, so we put the deck and falls right as part of the job.
Sometimes none of the three is the best fit. On a leaking felt dormer roof in Beverley we used a liquid-applied system instead, a coating that cures into one continuous waterproof layer with no seams or laps. It's one of the case studies on our flat roofing page.
Frequently asked questions
What's the longest-lasting flat roof?
GRP fibreglass and quality EPDM both offer 25–30+ years. Felt is shorter at 10–20 years.
Can you walk on a flat roof?
GRP is the most walkable. EPDM and felt can take occasional access but aren't designed as everyday walkways unless specified for it.
Why does my flat roof pool water?
Flat roofs are actually laid with a slight fall. Ponding usually means the falls are inadequate or the deck has sagged — worth fixing during a recovering job.
Is GRP or EPDM better?
Neither is universally "better." GRP suits smaller, walkable roofs and a hard finish; EPDM suits larger roofs and cold-weather flexibility with minimal joins.
We install GRP, EPDM and torch-on flat roofs across Hull and the wider East Yorkshire and will recommend the right system for your roof and budget. Free quotes, fully insured.