Felt vs EPDM vs GRP: Flat Roof Prices and Lifespan Compared

Supplied and fitted. A replacement flat roof costs £40 – £120 per m² depending on the system. Torch-on felt is cheapest at £40 – £75, EPDM rubber runs £55 – £100 and GRP fibreglass £65 – £120. A typical 25 m² extension roof lands between £2,000 and £4,000 including a new deck.

Every flat roof quote combines three different jobs: stripping the old covering, the deck and insulation underneath, and the membrane itself. Quotes that only price the membrane are not comparable — a felt figure with no decking line is a different job to an EPDM figure with a new warm deck. Our flat roof cost guide has the full per-m² breakdown, and the calculator prices your exact area.

Three-layer torch-on felt

The traditional budget option at £40 – £75 per m² fitted. Modern high-performance felt systems are far better than the pour-and-roll roofs of the 1980s, but the realistic lifespan is still 10 – 15 years before cracking, blistering or joint failure. Felt makes sense on sheds, tight budgets and roofs you expect to rebuild within a decade or so — not on a roof you want to forget about.

EPDM rubber membrane

EPDM is a single synthetic rubber sheet — often fitted with no seams at all on a domestic roof — at £55 – £100 per m² fitted. It goes down cold with adhesive (no naked flames, unlike torch-on felt), flexes with the building, and carries a proven 25 – 30 year service life with manufacturer guarantees of 20 – 25 years common through accredited installers. For most homes it is the best value per year of service.

GRP fibreglass

GRP is laid wet: resin and glass mat cure into a single seamless shell at £65 – £120 per m² fitted. It gives the toughest, best-looking finish and copes with complex shapes, parapets and occasional foot traffic better than sheet systems. The catch is installation — it cannot be laid on a damp deck or in rain, so weather windows matter and inexperienced installers cause most GRP failures. Choose an installer accredited by the system manufacturer.

The decking question

The membrane is only as good as the board it sits on. Probe the deck first: spongy, sagging or delaminated boards need replacing at £15 – £30 per m² for new OSB3. Some quotes price membrane over the existing deck to hit a lower figure — fine if the deck is genuinely sound, expensive if it is not, because the whole roof comes off again when it fails.

Cold deck vs warm deck

If the roof has had condensation problems, the fix is a warm-roof build-up — rigid insulation above the deck, membrane on top — at an extra £25 – £60 per m². It removes the cold surface where moisture condenses and rots timbers. Renovating 50% or more of a flat roof normally triggers a building regulations notification and an insulation upgrade to current standards anyway — NFRC CPS registered contractors can self-certify this.

Lifespan per pound

• Felt: ~10 – 15 years — cheapest now, dearest per year
• EPDM: ~25 – 30 years — best value for most homes
• GRP: ~25 – 30 years — best finish, needs a specialist fitter

Whichever system you choose, ask for the membrane spec and thickness, decking condition and replacement, edge trims, upstands and rainwater outlets, insulation if included, access, and the guarantee length — and whether it is insurance-backed if the installer ceases trading.