Flat Roof Cost

A replacement flat roof costs £40 – £110 per m² supplied and fitted. A typical single garage (18 m²) runs £900 – £2,800, a house extension roof (25 m²) £1,200 – £3,800 and a large dormer or outbuilding proportionally more. Felt is cheapest; EPDM and GRP cost more but last twice as long.

System-by-system prices

• 3-layer torch-on felt: £40 – £75 per m² fitted, 10 – 15 year life
• EPDM rubber membrane: £55 – £100 per m² fitted, 25 – 30 year life
• GRP fibreglass: £65 – £120 per m² fitted, 25 – 30 year life
• New OSB decking: £15 – £40 per m² if the old boards have failed
• Warm-roof insulation upgrade: £25 – £60 per m²

Felt, EPDM or GRP?

Felt suits sheds and tight budgets but expect to replace it within 15 years. EPDM arrives as a near-single sheet with minimal seams and is the best value on most homes. GRP gives the toughest, best-looking finish and handles complex shapes and balconies, but needs dry fitting conditions.

Cold deck vs warm deck

If your flat roof has condensation problems, a warm-roof build-up — insulation above the deck — is the proper fix rather than another membrane over a cold deck. It costs more up front but meets current building regulations and stops rot in the timbers.

What the quote should itemise

Decking condition and replacement, membrane spec and thickness, edge trims and upstands, rainwater outlets, insulation if included, access and guarantee length. A 20-year insurance-backed guarantee from an accredited installer is worth paying a modest premium for.