The three that matter
| EPDM rubber | GRP fibreglass | Torch-on felt | |
|---|---|---|---|
| Lifespan | 25–35 years | 25–30 years | 15–25 years |
| Typical cost /m² | £90 – £130 | £100 – £145 | £75 – £110 |
| Joints | Often none at all | Seamless once cured | Lapped and torched |
| Foot traffic | Fair | Excellent | Fair |
| Repairable | Very | Yes, needs matching | Very |
| Best for | Extensions, large simple areas | Balconies, walk-on decks, complex shapes | Garages, sheds, budget renewals |
Costs include stripping the old covering, new decking where needed, insulation to current standards, trims and disposal.
EPDM rubber
A single sheet of synthetic rubber, cut to size and bonded down. On most domestic extensions the entire roof is one piece with no joints anywhere, which removes the single biggest failure point of any flat roof.
Where it wins: big, simple rectangles. Rear extensions, flat garage roofs, dormer tops. It handles UK temperature swings without going brittle, it is genuinely low maintenance, and a puncture is a twenty-minute patch rather than a drama.
Where it doesn't: complicated shapes with lots of upstands and penetrations mean more welded detail, and the finish is matt black — fine when nobody sees it, less appealing under a bedroom window.
GRP fibreglass
Layers of glass matt saturated in resin, laid wet and cured in place into one rigid seamless shell with a coloured topcoat.
Where it wins: anything you walk on, and anything complicated. Balconies, terraces, dormer cheeks, roofs with several upstands or awkward junctions. The topcoat comes in a range of greys so it looks deliberate rather than industrial.
Where it doesn't: GRP needs dry weather and a competent installer — it is the material we most often see failing because of how it was applied rather than what it is. Laid over an existing deck with any movement in it, it will crack. Insist on new decking.
Modern torch-on felt
Forget the 1970s felt that gave the material its reputation. Modern high-performance systems are polyester-reinforced, laid in two or three bonded layers, and are a completely different product.
Where it wins: value, and awkward access. It is the most forgiving system to lay well in imperfect conditions, and it repairs easily. For a detached garage where nobody needs a 30-year answer, it is often the sensible choice.
Where it doesn't: it has the shortest life of the three, and single-layer "pour and roll" jobs are not worth having at any price.
The bit that matters more than the material
Most flat roofs we replace did not fail because someone chose the wrong material. They failed for one of these reasons, all of which are about the build-up rather than the top layer:
- No fall. A flat roof is never flat. It needs a minimum 1:80 fall so water leaves. Standing water finds every weakness eventually.
- Tired decking. Covering soft or springy boards with a lovely new membrane is money on a bonfire. The deck comes off.
- Bad upstands. The membrane must run at least 150 mm up every abutment and be properly terminated. Most leaks happen at the edges, not the middle.
- No ventilation. A cold-deck roof without a ventilated void grows condensation, and condensation rots timber from underneath while the roof looks perfect from above.
Which is the long way of saying: choose your roofer more carefully than your material. Our flat roofing page sets out exactly how we build ours.
Last reviewed August 2026 by the Bonwycks Roofing team. Prices are genuine local ranges for Crawley and the surrounding towns and are reviewed twice a year — your own quote will always be specific to your roof.