GRP flat roof cost by project type
New extension (new build flat roof on a single-storey rear extension): GRP roof covering over warm deck insulation, including OSB3 deck, vapour barrier, PIR insulation and GRP laminate — £90–£130/m² supply and install. Replacement (re-roofing over existing structure): strip existing covering, inspect deck, repair or overlay, apply new GRP — £80–£120/m². Garage flat roof replacement: 20–30m² garage roofs are the most common domestic GRP project — expect £2,000–£4,500 for a standard garage. Bay window flat roof: 3–6m² GRP to bay top — £600–£1,500. Dormer cheeks and dormer roof: GRP commonly applied to dormer box cheeks and dormer flat roofs — £600–£1,200 for a standard dormer. Call-out flat roof repair (blister, split, ponding area): £300–£800 for a small repair to an existing GRP roof.
How GRP flat roofing works
GRP (glass reinforced polyester) roofing is a cold-applied, seamless flat roofing system. The process: (1) Structural deck preparation — OSB3 or plywood deck laid over joists, sealed with resin. (2) Fibreglass matting — woven glass fibre mat saturated in polyester resin, laid seamlessly over the entire deck including all upstands, edges and penetrations. (3) Top coat — polyester gelcoat applied to provide UV protection and colour (grey, slate or custom). The result is a monolithic, joint-free surface with no lap seams — the primary failure mode of traditional felt roofing. GRP can be walked on for maintenance access, is resistant to ponding water, and does not rot or support mould growth. Temperature range: -40°C to +80°C. GRP is thermally stable in all UK weather conditions. Standard systems are certified to BS EN 14695.
GRP vs EPDM vs built-up felt: which should you choose?
All three are viable flat roofing options for London domestic extensions. GRP fibreglass: pros — seamless, hard surface, walked-on easily, established 25+ year track record, available in any colour, repairs are easy and clean. Cons — requires skilled installation (temperature-sensitive resin cure), not suitable for very large areas (thermal expansion risk over 200m²). Cost: £80–£130/m². EPDM rubber: pros — single-ply, lightweight, very long lifespan (30–50 years), cold-applied (no fire risk), suitable for large areas. Cons — dark grey appearance (no colour options), seams at edges must be carefully detailed. Cost: £60–£100/m². Built-up felt (3-ply): pros — lowest upfront cost. Cons — shortest lifespan (15–20 years), visible laps and seams, prone to blistering and cracking, increasingly difficult to source skilled fitters for quality work. Cost: £50–£80/m². For London domestic extensions and replacement roofs, Builderr recommends GRP or EPDM over traditional felt in all cases. The marginal cost difference is small; the lifespan and maintenance saving is significant.
Guarantees and warranties for GRP flat roofs
GRP flat roof warranties vary significantly between manufacturers and installers. Manufacturer product warranty: most GRP systems (Fibreglass Roofing Supplies, Owens Corning, Cromar) offer 25–30 year product warranties when installed by accredited contractors. Installer workmanship warranty: a reputable installer should provide a minimum 10-year workmanship warranty. Combined system warranty: some manufacturers offer 25-year joint guarantees (product + installation) when work is carried out by approved installers. Building Regulations: GRP flat roofs are accepted as a compliant flat roof covering under Part C (moisture resistance) when specified and installed correctly with adequate upstand heights (typically 150mm minimum at all junctions). When commissioning flat roof work in London, always ask for evidence of manufacturer accreditation, the warranty certificate in your name, and check the installer's liability insurance. Builderr uses FRS (Fibreglass Roofing Supplies) systems with full 25-year guarantees on all new-build extensions.
