GRP fibreglass flat roofs
Glass-reinforced polyester — hand-laminated on site, finished with a pigmented topcoat. Hard-wearing, walkable, takes paint or anti-slip aggregate. Right answer for balconies, dormers and any flat roof people walk on. Twenty-five year manufacturer warranty.
What GRP actually is
Glass-Reinforced Polyester — the same material technology used for boat hulls, swimming pools and bus bodies. On a flat roof it's a three-stage application:
- An 18mm OSB3 or marine-ply deck is fitted across the rafters with stainless screws.
- A catalysed polyester resin is rolled onto the deck, into which chopped-strand glass mat is bedded. The resin penetrates the mat and cures over several hours into a single rigid laminate.
- A pigmented topcoat is applied — also catalysed polyester resin, in light grey, dark grey, or any standard RAL colour. This is the UV-stable layer that you actually see and walk on.
The result is essentially a fibreglass shell that's bonded to your roof deck. There are no seams, no laps, no joints. The whole roof is a single piece of laminate — properly applied, it's the most durable flat roof material available.
Where GRP wins over EPDM
EPDM rubber is excellent and we install plenty of it. But GRP wins in three specific scenarios:
- Balconies and roof terraces. Anywhere people walk on the flat roof regularly. GRP is harder underfoot than EPDM, doesn't dent under furniture, takes anti-slip aggregate naturally.
- Dormer roofs. Particularly on the front of a property where the roof is visible. GRP can be coloured to coordinate with the building, looks more "considered" than rubber, and detailing into the cheeks of the dormer is cleaner.
- Where rigidity matters. GRP is essentially solid once cured — it doesn't have the slight rubbery give of EPDM. For a porch roof you might lean a ladder against, or a small flat over a bay window, GRP is more forgiving of point loads.
Where EPDM wins over GRP
EPDM is the better choice for:
- Older extensions on Victorian houses. Some movement is expected in older buildings; EPDM tolerates movement better than rigid GRP.
- Cold-weather installation. GRP needs ambient temperatures above 5°C and dry conditions to cure properly. EPDM can be fitted in colder weather.
- Large simple rectangles. EPDM comes in big sheets and seamless installation on a simple 30m² roof is quick. GRP on the same job takes longer.
The brands we fit
Cromar Pro GRP. Our default — UK-manufactured (Heckmondwike, West Yorkshire), excellent resin quality, full range of pre-formed trims, 25-year manufacturer warranty.
Restec GRP. The alternative we'll fit on request — slightly higher topcoat solids percentage, particularly good for walkable roofs that need maximum hardness.
Fitting timescale
A typical GRP install on a 25m² extension roof runs over three days:
- Day 1: Strip existing membrane, check and replace deck as needed, fit new 18mm OSB3 deck if total replacement is required.
- Day 2: Apply resin and chopped-strand mat in sections, allow to cure. Fit pre-formed corner trims and edge trims. Form upstands.
- Day 3: Apply pigmented topcoat in your chosen colour. Allow to cure. Final tidy and removal of scaffold.
Cure times depend on ambient temperature. In summer we can sometimes compress this to two days; in cold weather it may stretch to four.
Colours and finishes
For walkable surfaces (balconies, roof terraces) we recommend either an anti-slip aggregate broadcast into the wet topcoat — gives a textured finish that's hard-wearing and safe in wet weather — or pre-formed anti-slip walkway strips bonded to the surface.
Warranty
Our workmanship guarantee on a full GRP install is 10 years. The Cromar Pro manufacturer warranty on materials is 25 years. Both kick in from completion date. Routine maintenance (rinse off moss, keep outlets clear) is required but minimal.
Other things we install & repair
Leak Diagnosis & Repair
Tracking the leak path properly, then patching it permanently.
Read more →Felt Roofing
Three-layer SBS torch-on felt for awkward shapes and tight detail.
Read more →Solar Panel Mounts
Ballasted tilt frames that don't pierce the membrane.
Read more →Skylights & Rooflights
Kerb-mounted Velux, Fakro and Keylite flat-glass rooflights.
Read more →Commercial Roofs
Single-ply for industrial units, schools and retail.
Read more →Concrete Flat Roofs
Refurbishing post-war concrete decks with overlays.
Read more →Get a free written quote — no charge, no obligation
Tell us about your flat roof. We'll arrange a site visit, give you a fixed-price written quote, and explain the options in plain English.