GRP is glass-reinforced polyester: chopped strand matting laid over the deck, wetted out with resin and cured in place, then finished with a pigmented topcoat. Unlike a membrane, it becomes a rigid shell bonded to the boards. That rigidity is simultaneously its greatest strength and the source of nearly every GRP failure we are called to.
Why people choose it
It produces a genuinely seamless surface including upstands and trims, it is hard enough to walk on without a separate protection layer, and it can be finished in a colour. On balconies, walkways, dormer cheeks and roofs that get used, that combination is difficult to beat.
It also bonds to a well-prepared deck completely, so there is no route for water to travel underneath if the surface is ever breached.
Installation conditions matter
GRP is a wet-applied chemical system. It needs a dry deck, appropriate temperatures and a weather window, laying it in the wrong conditions produces poor cure, pinholes and adhesion problems that only show up months later. A contractor willing to lay it in drizzle to keep to a schedule is doing you no favours.
- Deck must be OSB3 or ply, dry and correctly fixed
- Trims bonded in before the laminate, not after
- Full-width matting properly consolidated with no dry patches
- Topcoat applied within the resin's overlaminate window
How it fails
Because it cannot flex, it cracks wherever the structure moves and it cannot follow, commonly over deck joints, at the junction with a wall upstand, or across a roof where joists deflect under load. Fine surface crazing is usually just weathered topcoat and can be recoated; a linear crack means movement and needs cutting out and laminating in, with the movement itself addressed.
Repairs and recoating
GRP repairs well when done properly: grind back, key the surface, laminate in new matting and re-topcoat. Because the material is chemically the same, a good repair is genuinely integral rather than a patch stuck on top. Recoating a sound but faded roof with fresh topcoat is a cheap way to extend its life by years.
Common questions
Is fibreglass better than rubber?
For a walkable, hard-wearing, colour-finished roof, yes. For a moving timber deck or a roof with imperfect falls, EPDM is more forgiving. We recommend based on the specific roof rather than a house preference.
Can GRP go over an existing flat roof?
No, it needs to be laminated onto a clean, dry, correctly fixed board deck. Anything else compromises the bond, which is the whole point of the system.
Can a cracked GRP roof be repaired?
Yes, and repairs integrate well. The important step is diagnosing why it cracked. Laminating over a moving joint just moves the crack a few centimetres.
Does the colour fade?
Topcoats weather over time, particularly darker colours. A recoat restores appearance and adds protection without touching the laminate underneath.