Every homeowner replacing a flat roof in 2026 gets two quotes: one for EPDM, one for GRP. Almost always the numbers are close, and almost never is it obvious which is the right call. This piece cuts through it. Real 2026 UK cost per m² for both systems, what is included at each price point, and which one actually wins over 30 years.
Headline answer: EPDM sits at £100 to £160 per m² supply and fit in 2026. GRP sits at £90 to £140 per m². On a typical 25m² rear extension that's a £250 to £500 gap. Over 30 years, EPDM is almost always the cheaper of the two once you account for lifespan, warranty and repair.
What the per-m² price actually includes
The number a roofer quotes per square metre is not the number you pay. Both EPDM and GRP prices should include the covering, but different roofers include different things. When you are comparing, make sure both quotes cover:
- Strip and dispose of the old covering, down to the deck.
- Deck check and repair (OSB3 or 18mm plywood, replaced where rotten).
- Insulation upgrade to meet Part L Building Regs (typically 100 to 140mm PIR).
- Vapour barrier under the insulation on warm-deck builds.
- Edge trims and drip flashings in coated aluminium.
- Upstands against any wall or abutment.
- Outlet and rainwater goods connections.
- VAT + warranty in writing (10-year workmanship as standard from us).
If either quote excludes any of these, add £15 to £40 per m² to normalise the comparison.
EPDM: £100 to £160 per m² supply and fit
EPDM (ethylene propylene diene monomer) is single-ply synthetic rubber. It arrives on site as a single sheet cut to the roof size, glued down with contact adhesive, and dressed into all the edge and upstand details. No naked flame required. In 2026 the typical UK supply-and-fit ranges are:
- Budget EPDM (Firestone RubberCover, ClassicBond): £100 to £120 per m².
- Standard mid-range EPDM (Firestone RubberGard): £120 to £145 per m².
- Premium mechanically-fixed EPDM (single ply on larger roofs): £140 to £160 per m².
Warranty terms: manufacturer 20 years, installer 10 to 15 years. Real-world lifespan: 30 to 50 years with normal maintenance. Best on: simple rectangles, single-fall roofs, extensions with no complex upstands. Worst on: heavily-detailed roofs with lots of pipe penetrations (GRP is often better there).
GRP: £90 to £140 per m² supply and fit
GRP (glass reinforced polyester, also called fibreglass) is a wet-laid liquid system. The roofer bonds a base coat, lays fibreglass matting, saturates it, then applies a topcoat. Once cured it becomes a seamless hard shell over the entire roof and all its details. Ranges in 2026:
- Budget generic GRP (unbranded resin, hand-laid): £90 to £110 per m².
- Standard branded GRP (Cure-It, Kemper GRP): £105 to £125 per m².
- Premium GRP with flow coat + third layer (roof terraces, balconies): £120 to £140 per m².
Warranty terms: manufacturer 20 to 25 years, installer 10 years. Real-world lifespan: 20 to 30 years, sometimes less if the resin was laid in the wrong weather. Best on: complex shapes, pipe penetrations, foot-traffic terraces. Worst on: temperature-sensitive UK conditions (must be laid dry, above 5°C, or the resin does not cure properly).
A GRP roof laid in the wrong weather is the single biggest complaint we see.
If a roofer promises to lay GRP in November on a wet week, walk away. Damp resin cures unevenly, hairline cracks appear within two years, and the full re-lay costs the same as the first job. EPDM is far more forgiving of British weather at install time.
The 30-year cost comparison, side by side
On a 25m² extension roof, here is what both systems cost from day one to year 30. This is where the honest comparison shows up.
- GRP initial install (year 0): £2,500 (£100/m² average).
- GRP re-coat (year 12): £600 to £900. Necessary on most GRP roofs to keep the warranty valid past year 10.
- GRP full replacement (year 25): £3,500 at 2051 prices (assumes 3% annual inflation, £150/m² supply and fit).
- GRP 30-year total: approximately £6,750.
- EPDM initial install (year 0): £3,000 (£120/m² average).
- EPDM maintenance (years 5, 15, 25): £150 each for edge trim check and re-seal.
- EPDM full replacement (year 40+): not required inside a 30-year window on a properly-installed EPDM roof.
- EPDM 30-year total: approximately £3,450.
Even after paying more up front, EPDM wins the 30-year cost comparison by roughly £3,300 on a typical rear extension. That gap widens on larger roofs and narrows on smaller ones, but for any residential extension in Buckinghamshire, EPDM is our default recommendation for lifetime value.
When to choose GRP anyway
All that said, GRP still wins in a few specific situations:
- Foot-traffic terraces and balconies. GRP with a flow coat is walkable and cleanable. EPDM should not be walked on regularly.
- Complex roofs with 5+ pipe penetrations. GRP flashes any detail seamlessly. EPDM needs individual boots.
- Very tight budgets with short-term ownership. If you are selling inside 5 to 10 years, GRP saves £500 that you will not lose on the survey.
- Existing GRP where the deck is sound. Repairing GRP with GRP is cleaner than overlaying with a different system.
Red flags on either quote
Regardless of which system you go with, watch for these three phrases on the quote:
"Overlay of existing covering." Never a good idea on a flat roof. If the existing covering has failed, the deck below has almost certainly taken damage. Strip and rebuild is the only honest answer.
"Standard insulation." Ask for the exact spec (thickness and material). Anything less than 100mm PIR on a modern Part L compliant roof is a red flag.
"Warranty subject to annual inspection at customer's cost." Legitimate manufacturers do not require annual inspections. Some installer warranties do. Read the terms before you sign.
How we quote flat roofs at Clarkson Roofing
Every EPDM and GRP quote we send is fixed-price, itemised, and includes every line above. VAT is shown separately, the deck condition is confirmed by physical inspection before we quote (never after), and the manufacturer and installer warranty terms are attached to the quote in full. If you would like a comparison against your own roof, our flat roofing service page covers the process, or head straight to the contact form. First reply lands within one working day.
For a fuller comparison of every common flat roof material, our EPDM vs GRP vs felt guide takes you through torch-on felt, cold-apply liquid systems and green roofs too.
