The short answer
A GRP roof cracks when the deck beneath it moves and the rigid fibreglass cannot follow. The usual places are board joints, perimeters and wall junctions. Fine crazing in the topcoat is a surface effect and often cosmetic, but a crack through the laminate lets water in. Local cracks can be ground out and re-laminated. Widespread cracking points to a failing deck or an ageing roof.
Movement at joints and edges
GRP stands for glass-reinforced plastic, a roof covering made by laying glass fibre matting and resin over a deck. When it cures it forms one rigid, continuous sheet, with no joins across the main area. That is a strength, because the main area has no joins to open. It is also a weakness, because the sheet cannot stretch.
Timber moves. Boards swell with damp and shrink in dry weather, joists settle, and a deck flexes under weight. Where two boards meet, even a small shift pulls at the laminate above, and over time a line of cracking can appear along the joint, mirroring the boards.
Perimeters and wall junctions are the other hot spots. At the perimeter, the fibreglass is dressed over a trim or up a wall, and differential movement between roof and wall puts it under strain. A crack that follows a wall line is usually movement.
Poor preparation makes it worse. If boards were not fixed firmly, or the laminate was laid too thin or in cold, damp weather, it is more prone to fail. Typical lives are 25 to 30 years according to HomeBuilding, and 25 to 40 years according to Less, and cracking early in that span points to a cause worth finding.
Topcoat crazing and impact
The visible surface of a GRP roof is a gel coat or topcoat, a pigmented layer over the laminate. It protects the glass fibre from sun and water. Over the years it weathers, and a network of fine cracks called crazing can develop across the surface.
Crazing alone is often cosmetic, a sign of ageing and ultraviolet exposure. It matters when the cracks reach through to the glass fibre, because then water can wick along the strands. A roofer checks whether the cracking is only in the coat or runs through the laminate, by looking closely and sometimes by gently probing.
Impact is the third cause. A dropped tool, a heavy foot, a fallen branch or a ladder placed on the roof can crack the shell. Cold weather makes fibreglass less tolerant of knocks. Our answer on walking on a flat roof covers what each covering tolerates.
Sun and heat also cycle the surface. Repeated expansion and contraction fatigues weak areas, particularly dark ones, and can open hairline cracks over time. A well-maintained topcoat slows that.
Repair options
A single crack in a sound roof is a straightforward repair. The area is cleaned and dried, the crack is ground out to a V so new resin can key in, and fresh glass matting and resin are laminated over it. Once cured, a new topcoat is applied and blended in.
Surface crazing can be treated by sanding and recoating, which renews the protective layer. That is a maintenance job and not a structural repair, and it should follow checking the laminate beneath.
Cracks along board joints are harder. If the cause is movement, the repair may crack again, and a more lasting fix addresses the movement. That could mean refixing boards, adding a flexible break over the joint, or, on an older roof, re-laying the covering.
Our flat roof leaking answer shows how to trace a leak back from a stain, and our flat roof paint answer explains why coatings alone do not stop a crack that moves.
Costs depend on the length of crack and the access. HomeBuilding gives GRP at £110 per square metre supply-only (September 2025), and the written quote sets the repair out as one total once the survey is done.
When cracking means a new roof
The pattern tells the story. A few cracks in one corner of an otherwise good roof can be mended. Cracks along many joints, across the main area or in several places, together with soft spots or a sagging deck, suggest the substrate is failing.
Age matters too. If the roof is at the far end of its typical range, repairing it repeatedly stops being good value. We look at the laminate thickness, the state of the deck and whether water has got in, then tell you plainly what we found.
If the decking is wet or rotten, no repair to the laminate will last. Our answer on rotten flat roof decking explains how that is handled. A strip-back is also the moment to improve the build-up, because Approved Document L brings insulation in whenever the waterproof layer is renewed.
Replacement may still be GRP, or it may be EPDM or felt, depending on the roof and what you want. The flat roofing page sets out the choices. Our survey photographs show the cracks and what lies beneath, and the quote that follows is a single written total.
