A flat roof covering choice used to be simple: it was felt, and you replaced it every 15 years. That's no longer the case. Here's the modern picture for garage roofs, extension roofs and dormer roofs.
Traditional bitumen felt: cheap and short-lived
Lifespan: 10–20 years. Cost: lowest of the three.
Three-layer torch-on bitumen felt is still the most common covering on UK garages because it's cheap and quick to lay. The downside is that 20 years is the optimistic end of its life; 10 is the realistic minimum if installed by a hurried trade. We rarely recommend felt for new work anymore — the cost difference vs EPDM is small enough that the lifespan upgrade is almost always worth it. If you're replacing existing felt on a garage and budget is tight, it's defensible. For an extension over habitable space, it isn't.
EPDM rubber: the modern default
Lifespan: 25–40 years. Cost: 20–30% more than felt.
EPDM is a single piece of synthetic rubber bonded to the roof deck. It's UV-stable, doesn't crack or shrink the way felt does, and has no seams on most domestic-sized roofs because it comes in sheet sizes up to 15m wide. For any flat roof up to about 50 square metres, this is now our default specification — it's barely more expensive than felt at the time of laying, and it lasts at least twice as long with effectively no maintenance.
The one weakness is that EPDM doesn't love being walked on regularly — it's not damaged easily, but if you need a walk-on roof terrace, GRP or a proper paving system is a better choice.
GRP (fibreglass): the durable specialist
Lifespan: 25–40 years. Cost: 50–80% more than felt.
GRP fibreglass is laid as a liquid resin onto a glass mat that bonds into a single seamless covering. It's the toughest of the three — genuinely walk-on without damage, with excellent resistance to ponding and impact. The trade-offs are cost and installation complexity: GRP needs warm-dry conditions to lay properly, which is a real constraint in Lancashire's wet shoulder seasons, and a bad installer makes a mess of it.
We use GRP for dormer roofs, balconies, and any flat roof where regular foot traffic is expected. For a standard garage or single-storey extension, EPDM gives 90% of the benefit at 60% of the cost.
What about the old "torch on" smell?
Traditional felt is laid hot with a blow torch — that's where the smell, the smoke and the small but real fire risk come from. Both EPDM and GRP are cold-applied, no torch, no smoke. For any installation on a roof that abuts a house wall (which is most extension roofs), that's a meaningful safety upgrade.
Quick decision guide
- Garage roof, tight budget, accept 15-year lifespan: felt is defensible
- Garage or extension, want a 30-year fix: EPDM
- Balcony, walk-on terrace, dormer roof: GRP
- Replacing existing felt on a small extension: EPDM (the upgrade is almost free at this scale)
- Anything over 50 square metres or commercial: single-ply membrane (separate topic — talk to us)