If your flat roof needs replacing, you'll quickly encounter two options: GRP (fibreglass) and EPDM (rubber). Both are modern systems that far outperform traditional felt, but they suit different situations. This guide compares them honestly so you can make the right choice for your property.
GRP and EPDM at a Glance
| Feature | GRP Fibreglass | EPDM Rubber |
|---|---|---|
| Material type | Rigid fibreglass resin and matting | Flexible synthetic rubber membrane |
| Lifespan | 25–30 years | 25–30 years |
| Best for | Walkable areas, balconies, complex shapes | Large flat areas, garages, simple shapes |
| Typical cost (installed) | £70–£100 per m² | £55–£80 per m² |
| Joins/seams | Seamless (single moulded surface) | Glued seams on larger roofs |
| Finish | Hard, smooth, can be coloured | Soft, matt black |
| Weather resistance | Excellent — UV and frost resistant | Excellent — highly flexible in cold |
When GRP Fibreglass Is the Better Choice
GRP creates a rigid, seamless surface by layering fibreglass matting and resin directly onto the roof deck. Once cured, it forms a single waterproof shell with no joins or weak points.
- Walkable flat roofs: If you use your flat roof as a balcony, terrace, or regularly walk on it for maintenance, GRP's hard surface handles foot traffic without damage.
- Complex shapes: GRP is moulded on-site, so it handles angles, upstands, and detailing around skylights or pipes better than a sheet membrane.
- Aesthetics: GRP can be finished in different colours and has a clean, professional appearance. It suits properties where the flat roof is visible.
- Small to medium areas: Porches, bay windows, dormers, and extensions up to about 30m² are ideal for GRP.
GRP Limitations
GRP is temperature-sensitive during installation — it needs dry conditions above 5°C to cure properly. In Devon and Cornwall, this means scheduling around the weather matters. It's also more expensive than EPDM and can crack on very large uninterrupted areas if the substrate moves.
When EPDM Rubber Is the Better Choice
EPDM is a synthetic rubber membrane that's glued or mechanically fixed to the roof deck. It's extremely flexible and durable, with strong resistance to UV light and temperature extremes.
- Large flat areas: Garages, large extensions, and commercial flat roofs suit EPDM. A single sheet can cover areas up to 15m x 60m with no seams.
- Budget-conscious projects: EPDM typically costs 20–30% less than GRP for the same area, making it the cost-effective choice for larger roofs.
- Cold-weather installation: EPDM can be installed in lower temperatures than GRP, giving more flexibility with scheduling — useful in Devon and Cornwall's changeable climate.
- Simple shapes: Straightforward rectangular flat roofs with minimal penetrations are where EPDM excels.
EPDM Limitations
EPDM seams are glued rather than moulded, so on complex roofs with lots of angles and upstands, there are more potential failure points. It's also a soft membrane — sharp objects or heavy items dropped on it can puncture the surface. And the matt black finish, while functional, isn't as aesthetically pleasing as GRP.
Cost Comparison for Common Projects
| Project | GRP Cost | EPDM Cost |
|---|---|---|
| Bay window flat roof (3m²) | £800 – £1,200 | £600 – £900 |
| Porch roof (5m²) | £1,200 – £1,800 | £900 – £1,400 |
| Single garage (15m²) | £2,500 – £3,500 | £1,800 – £2,800 |
| Extension roof (25m²) | £4,000 – £5,500 | £3,000 – £4,500 |
| Large flat roof (40m²+) | £6,000 – £8,000+ | £4,500 – £6,500 |
All prices include materials, labour, and waste removal. Scaffolding may be additional depending on access. View our full flat roofing service page for more details.
What About the Old Felt Roof?
If your existing flat roof is traditional felt, it's likely reached or passed its useful life (10–15 years). Both GRP and EPDM can be installed over existing felt in some cases — if the decking beneath is sound. However, if there's any sign of rot, damp, or structural movement, the old covering needs stripping back to the deck first.
We assess this during the free inspection and will always recommend the approach that gives the longest-lasting result.
Which Should You Choose?
There's no single "best" system — it depends on your property and priorities:
- Choose GRP if you want a seamless finish, plan to walk on the roof, have a complex shape, or want a specific colour.
- Choose EPDM if you have a large, simple flat area, want to keep costs down, or need installation flexibility around weather.
Both systems come with manufacturer-backed guarantees of 20+ years when installed correctly. The key word is "correctly" — a poorly installed flat roof of either type will fail prematurely. Our flat roofing specialists are experienced with both GRP and EPDM and can recommend the right system for your specific project.
Can I Combine Flat Roofing With Other Work?
Yes — and it's often cost-effective to do so. If your flat roof connects to a pitched roof section, addressing both at the same time saves on scaffolding and ensures the junction between the two is properly waterproofed with quality leadwork. Similarly, replacing gutters and fascias alongside a flat roof replacement avoids paying for access twice.