EPDM rubber and GRP fibreglass are the two most durable flat roofing systems for Devon's climate. Both outperform felt by a significant margin. The choice between them depends on roof size, shape, access, and the homeowner's priorities. This guide sets out the honest differences.
EPDM rubber and GRP fibreglass both outlast felt significantly, providing 25-30 years in Devon. GRP suits standard extensions under 50 square metres with its seamless rigid surface; EPDM suits complex shapes and larger areas with its flexibility. GRP costs 10-15% more than EPDM.
GRP (Glass Reinforced Plastic) is a fibreglass composite system applied as a liquid that cures to form a seamless rigid sheet. It bonds directly to the roof deck and is finished with a UV-resistant topcoat. GRP is the system most commonly associated with high-quality flat roofing in Devon.
EPDM (Ethylene Propylene Diene Monomer) is a synthetic rubber membrane manufactured in large sheets and applied with bonding adhesive. It is flexible, highly elastic, and tolerant of thermal cycling. EPDM has been the dominant flat roofing system in North America for decades and is now widely used in the UK.
Both systems provide excellent performance in Devon's wet climate and both significantly outperform felt. The specific performance differences are minor but worth understanding.
BS 6229 governs the design of flat roofs including single-ply and liquid systems, requiring correct falls and deck compatibility for both EPDM and GRP.
Source: BS 6229
GRP’s rigid surface is more resistant to point-impact damage (dropped tools, hail, foot traffic) and provides a harder surface that debris does not penetrate. Its seamless construction provides no joints for Devon's rainfall to exploit.
EPDM’s elasticity handles thermal cycling slightly better than GRP on very large roof areas where thermal movement is significant. On typical Devon residential extension sizes (under 50m²), this advantage is not practically relevant. EPDM’s flexible membrane also tolerates minor deck movement better than GRP, which makes it preferable on older decks where some flex is expected.
GRP is typically 10–15% more expensive than EPDM for the same roof area. The cost difference reflects the higher material cost of the GRP laminate system and the slightly longer installation time (GRP requires cure time between laminate and topcoat layers).
For a typical Devon rear extension of 25–35m², the difference between EPDM and GRP is approximately £200–£400 on the total job cost. Given that both systems provide 25–30 year service life in Devon's climate, the cost-per-year difference is negligible.
Felt is significantly cheaper than either system upfront but requires re-covering every 10–15 years in Devon. Over a 30-year period, two felt re-covers plus the original installation typically costs more than a single GRP or EPDM installation.
Not sure whether GRP or EPDM is right for your Devon flat roof? We assess and recommend at the survey stage with no pressure toward either system. Call 01392 759 295 for a free assessment.
📞 Call 01392 759 295GRP is our first recommendation for most Devon flat roofs where access for application is straightforward and the roof area is under 50m². The reasons: its rigid surface is easy to inspect for damage; topcoat re-coating at 10–15 years is straightforward and extends the roof life without full replacement; and the seamless surface gives Devon's rainfall nowhere to exploit.
GRP is particularly well suited to Devon's Wonford, St Thomas and Whipton extension stock where standard rear extension sizes and good access make installation efficient.
EPDM is our recommendation in four situations: irregular roof shapes where the flexible membrane handles complex geometry better than rigid GRP; large areas over 50m² where thermal movement is significant; older decks where minor flex is present and likely to cause GRP crack propagation; and where immediate installation is required and the GRP cure time would create a weather window risk.
For Crediton’s cob building extensions and other irregularly shaped Devon flat roofs, EPDM’s flexibility makes it the correct specification.
“GRP on our rear extension. Seamless, hard surface, easy to inspect. They explained the difference between GRP and EPDM clearly.”
“EPDM recommended for our irregular-shaped conservatory roof. Correct choice for the geometry. Professional installation.”
“They compared both options honestly and let us choose. No pressure. We went GRP and it’s excellent.”
Both perform well in Devon's wet climate. GRP is our first recommendation for standard extension sizes under 50m² with straightforward access. EPDM for complex shapes and larger areas.
25–30 years correctly installed, with topcoat re-coating at 10–15 years.
25–30 years. Seam and perimeter maintenance is the key requirement over the system's life.
GRP is typically 10–15% more expensive. Over a 30-year lifespan, the cost-per-year difference is negligible.
Yes, but the deck condition must be assessed first. Compromised decks should be repaired or replaced before either system is installed.
Both perform equally well in coastal conditions. GRP’s seamless surface gives it a marginal advantage in salt-air environments where there are no seams for salt aerosol to penetrate.
Free survey and honest recommendation based on your specific roof shape, size and deck condition. Call 01392 759 295.
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01392 759 295Last reviewed: 21 July 2026
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