GRP vs Felt vs EPDM: Which Flat Roof Is Best?

The quick verdict

GRP: the most durable and rigid of the three, seamless, walkable, 25–30+ year lifespan, and the best-looking finish — but it's weather-dependent to install and needs a sound, dry timber deck. Best for garages, extensions, balconies and any roof people will stand on.

EPDM rubber: a single sheet glued down, very quick to install, tolerant of building movement, 20–25 year lifespan. Weakest on looks and on complicated shapes where sheets must be joined. Best for simple rectangles wanted done in a day.

Felt (torch-on): the cheapest upfront and the traditional choice, 10–20 years depending on grade. It involves open flame, joints and seams throughout, and is the least DIY-appropriate of the three. Best where budget rules everything or on a roof due for replacement anyway within a decade.

Cost, honestly

In materials, felt is cheapest at roughly £10–15 per m² for a decent two-layer system. GRP runs around £17–20 per m² in core laminate materials plus trims. EPDM lands in a similar band to GRP once you've bought the membrane, adhesives and edge trims.

Fitted prices flip the picture less than you'd think: contractors typically charge £50–70/m² for felt, £70–90 for EPDM and £80–110+ for GRP. The GRP premium is mostly labour and skill, which is exactly the part a competent DIYer keeps in their pocket.

Per year of expected service, GRP is usually the cheapest of the three: £600–650 in materials over 25–30 years on a 15 m² garage is around £22 a year. A cheap felt roof redone every 12 years rarely beats that.

Lifespan and how each one fails

GRP fails at badly-made details, not in the field: dry laminate from too little resin, missed bandage over board joints, or laying on a damp deck. Done right it's a single seamless shell with no joints to open — which is why properly-laid GRP roofs from the 1990s are still out there.

EPDM fails at seams and edges — a simple one-sheet roof has few of both, which is why it does well on plain rectangles and worse around skylights, pipes and complicated perimeters. UV resistance is excellent; punctures from dropped tools or ladders are its practical weakness.

Felt fails by ageing: UV embrittles it, joints open, blisters form and water tracks under the layers. Modern SBS felts are far better than the 1970s stuff, but it remains the shortest-lived system, and the one where 'patch it again' becomes an annual event.

Installation and DIY difficulty

GRP is genuinely DIY-achievable but demands a dry day between 5 and 30°C, methodical catalyst dosing, and a full dry run of trims before any resin is mixed. Physically it's the most forgiving — no flames, no wrestling a heavy sheet — but chemically it's the least forgiving of shortcuts.

EPDM is the easiest DIY install on a simple roof: unroll, relax, glue, roll out the air. Complications appear at internal corners, outlets and joins, where the folding and patching technique matters and mistakes hide until winter.

Torch-on felt means open flame on a timber building and an insurance conversation — for DIY we'd simply say don't. Cold-applied felt systems exist but sit at the budget end on lifespan.

Looks, foot traffic and details

If the roof is visible from bedroom windows, GRP wins — a smooth, pigmented, glossy or matt finish in your choice of colour, with crisp pre-formed edges. EPDM is a black rubber sheet and looks like one. Felt is felt.

For foot traffic — balconies, terraces, roofs used for window cleaning access — GRP is the only one of the three genuinely made for it, especially with an anti-slip finish (our Premier tier). EPDM tolerates light traffic; felt dislikes it.

Complicated roofs — multiple levels, skylights, pipe penetrations, odd shapes — favour GRP because the laminate is made in place and simply follows the shape. Sheet systems fight geometry; liquid ones flow around it.

Frequently asked questions