In short
There is no single best. EPDM rubber suits simple shapes with few joins, GRP fibreglass gives a hard finish with no joins, and modern felt is a proven, usually cheaper choice for garages and extensions. The FMB says a flat roof lasts 20 to 40 years depending on the membrane, and the condition of the deck underneath matters as much as the covering.
This is a job for our Flat Roofs service.
The three options
| Felt | GRP fibreglass | EPDM rubber | |
|---|---|---|---|
| What it is | Layers of bitumen felt bonded to the deck | Glass fibre matting and resin laid on site | A single sheet of synthetic rubber |
| Joins | Laps between layers | None, it is one continuous layer | Very few on a simple roof |
| Best for | Garages, extensions, dormers | Roofs you look down on, walkable areas | Simple shapes, larger roofs |
| Watch for | Needs the right number of layers and a good finish | Must be laid in dry weather on a sound deck | Detailing around edges and outlets |
How long they last
The Federation of Master Builders puts the life of a flat roof at "between 20 to 40 years, depending on the membrane". In our experience how long a roof lasts depends as much on the boards underneath and the fall as on the covering. Water that sits on a flat roof finds every weak spot.
What they cost
We do not publish our prices, because every roof is different. For a rough guide, Which? asked 11 of its Trusted Traders in November 2025. For a single garage of 6m by 3m, they quoted:
- Felt: £1,400 to £3,500.
- Other materials such as EPDM rubber or PVC: £1,950 to £4,000.
Those figures include labour and materials but not VAT. Rotten boards, access, insulation and skylights all change the price, which is why a written quote after a proper look is the only number that counts.
The part people forget: the deck
A new covering on soft, rotten boards will fail early. When we strip a flat roof, we check the deck and the joists and replace what has gone, so the new roof has a sound, level base with a proper fall. On one workshop we replaced an old polycarbonate roof with new boards and timber work, finished with four opening skylights.
How we help you choose
- We look at the roof, the deck and how you use the space.
- We recommend felt, GRP or EPDM, and tell you why.
- You get an itemised written quote, including any timber work.