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EPDM vs GRP Flat Roof — Which Should You Choose?
Both are good systems and both are sold badly. The right answer depends on the shape of your roof, how much foot traffic it gets and how the edges and upstands need to be detailed — not on which material a salesman prefers.
- EPDM: one-piece membrane, no seams on most domestic roofs
- GRP: rigid, walkable, trims bonded in
- EPDM copes better with movement
- GRP takes a cleaner edge on balconies
- Both need a sound, dry deck first
- We fit and quote for both — no bias
Where EPDM wins
On a straightforward garage, extension or dormer roof, EPDM is usually the better buy. The membrane comes as a single sheet, so on most domestic roofs there is no field seam at all — and a seam is where flat roofs fail. Rubber stays flexible in cold weather and moves with the deck rather than cracking against it, which matters on timber decks that shrink and swell through the year.
- ▸Best for simple rectangles with few penetrations
- ▸Very forgiving of deck movement
- ▸Cold-applied — no hot works on your property
- ▸Repairs are simple and cheap
Where GRP wins
GRP is laid wet and cures into one rigid, seamless shell that includes the trims and upstands. That makes it the stronger choice for balconies, walk-on terraces, roofs with lots of pipe penetrations, or anywhere you want a crisp architectural edge. It also takes a non-slip finish, which EPDM does not do as neatly.
- ▸Walk-on and balcony roofs
- ▸Complex shapes and multiple upstands
- ▸Hard-wearing surface you can maintain from
- ▸Needs dry weather and correct temperature to cure
What actually decides the lifespan
Neither material typically fails on its own. Flat roofs fail at the perimeter, at upstands and around penetrations — and almost always because the deck underneath was wet or unsupported when the new covering went on. We strip back and check the deck on every flat roof we quote. If the boards are soft, they get replaced; if the falls are wrong, we correct them with firrings before anything is bonded down.
FAQs
Which is cheaper, EPDM or GRP?+
On a like-for-like garage roof EPDM is usually slightly cheaper to install, mainly because it needs less labour and no cure time. On complex roofs the gap closes because GRP's trims are formed in place rather than bought in.
Can you lay either over my existing felt?+
We don't recommend it. Overlaying hides a wet deck and voids most manufacturer guarantees. We strip back to the deck, inspect it and replace what's failed.
How long will each one last?+
Both are realistically 20–30 year systems when the deck, falls and detailing are right. We give the same workmanship guarantee on both.
Can I walk on them?+
GRP with a non-slip finish is designed for foot traffic. EPDM will take occasional maintenance access but is not a terrace surface without a walkway layer.
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