Ask ten UK roofers which flat roof system to use and you'll get a fairly even split. GRP and EPDM have both been on British houses long enough to prove themselves, and either one will outlast the felt it's replacing by decades. They just get there in completely different ways. GRP is a liquid. You roll resin over glass matting and it cures into one solid shell bonded to the deck. EPDM turns up as a rubber sheet, glued down cold, usually in one piece with no joins anywhere. Which one suits you has less to do with the spec sheets than with what you'll actually do with the roof, and when you're planning to fit it. What you're choosing between GRP (glass reinforced plastic, better known as fibreglass) is built in place. The resin soaks into the matting, hardens, and bonds to the deck. Trims and upstands become part of that same shell, so there's nothing to peel back or split open. EPDM is a synthetic rubber membrane, usually 1.2mm thick, cut to size and bonded down. Most garages and extensions take a single piece. It stays flexible for its whole life and moves with the building underneath it. What each one costs For a typical DIY job the gap is real. Our 450g GRP roofing kits start at £138.60, and the heavy-duty 600g kits start at £179.50, covering anything from 5m² up to 100m². A ClassicBond EPDM flat roof kit starts at £431.75, and the garage-specific kits from £520.10. Both come with membrane, adhesives, trims and tools. GRP usually wins on smaller roofs. EPDM closes the gap as the area grows, because you're buying one sheet rather than steadily more resin. We've broken the maths down properly in our cost per m² guide. Fitting it yourself Honestly, this is where most people should make the call. EPDM is more forgiving. It's cold-applied, so there's no curing chemistry to get wrong. You unroll the sheet, let it relax, fold it back and glue it. Get the deck flat and clean and a first-timer can do a shed or garage in a day. GRP asks more of you. You're mixing catalyst at a ratio that shifts with the temperature, consolidating the matting so no air gets trapped, and working against a pot life. Rush it and you'll get bubbles. Plenty of homeowners do it well, but read the instructions twice and pick a settled dry spell. Will anyone walk on it? If the answer is yes, GRP wins. Our 600g kits are rated for foot traffic, balconies and walkways, and the topcoat takes a beating. The 450g kits are for roofs nobody stands on. EPDM is fine to walk on for maintenance, but rubber is soft. A dropped chisel or a dragged ladder foot can nick it. On a garage roof you visit twice a year, that's a non-issue. On a roof terrace, it rules EPDM out. Lifespan and repairs We quote 25+ years on standard GRP, 50+ years on heavy-duty, and up to 50 years on EPDM. Fitted properly, both leave felt behind by a wide margin. Repairs differ more than lifespans do. A GRP repair is resin over resin, so it fuses into the original and vanishes under a fresh topcoat. An EPDM repair is a patch bonded over the damage. Both hold fine. If you're patching rather than starting fresh, we've covered GRP repair versus replacement and the same question for EPDM. Movement and weather Rubber flexes, and that's EPDM's real edge on an older building or a deck you're not confident about. It absorbs movement instead of fighting it. GRP is rigid, which is exactly why the deck matters so much. On sound 18mm OSB3 with proper expansion trims it's bombproof. On a tired, springy deck you're asking for trouble. It's worth understanding how thermal expansion causes cracking before you commit either way. Building rules worth checking Re-roofing rarely needs planning permission if the appearance doesn't materially change, according to the Planning Portal. Building Regulations are the bigger consideration. Strip a flat roof back to the joists and you may be required to upgrade the insulation while you're there, under Approved Document L. Our guide to warm roof build-ups explains how that works in practice. So which one? Neither wins outright, which is why we stock both. GRP is the right call when the roof gets walked on, when you want a coloured finish you can recoat years later, or when you're laying a fresh deck anyway and there are awkward upstands that benefit from a seamless shell. Go rubber for a shed, garage or dormer nobody stands on. EPDM also makes sense if you're fitting in October rather than July, if it's your first roof, or if the building is old enough that movement is a genuine concern. Still torn? Run your measurements through the GRP Kit Calculator, have a look at the full GRP kit range and EPDM range, or read our flat roofing FAQ. Get in touch or call 0800 612 7903, and we'll tell you which system fits, even when it's the cheaper one.
