Flat Roof Options Compared: GRP Fibreglass vs EPDM Rubber vs Felt
By Leonard Walton, owner & lead roofer3 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.
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.
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.