The short answer
GRP, or fibreglass, is a rigid shell, so it cracks when the structure under it moves or when it was laid too thin or poorly bonded. Hairline crazing in the surface coat is cosmetic, while cracks that run through the laminate let water in. Local cracks can be ground out and relaminated, but widespread cracking usually means the cause lies in the deck.
Movement beneath the roof
GRP stands for glass-reinforced plastic. A GRP roof is made in place: glass-fibre matting goes onto a timber deck and is soaked in resin, which sets into a rigid, joint-free shell finished with a coloured gel coat. Unlike felt or rubber, it does not stretch.
That rigidity is its strength and its weakness. If the deck moves, the shell has to move with it or crack. Timber expands and contracts with moisture and temperature, boards flex under foot, and joists deflect. A shell bonded to boards that move at different rates cracks along the board joints.
The pattern gives clues. Straight cracks following the lines of the boards point to deck movement. Cracks across the corners and at the junction with walls suggest movement between the roof and the building. A crack that opens and closes with the seasons points to thermal movement.
Good GRP roofs allow for movement. Boards are laid with their edges sealed, expansion joints are formed at long runs, and the deck is stiff enough not to flex. Where these details were skipped, cracks are likely to recur, however often they are repaired.
HomeBuilding gives GRP a typical life of 25-30 years, and Less gives 25 to 40 years, but cracking can shorten that if the deck is unstable.
Thin laminate and poor workmanship
Not all cracks come from outside. A laminate laid too thin, with too few layers of glass matting or too little resin, is weak from the start. It may crack under foot traffic, from hail or a dropped tool, or simply from the daily expansion and contraction of the surface.
Poor bonding is another cause. If resin was applied to a damp deck, or the matting was not fully wetted out, there are voids and weak bonds that fail early. Resin that was mixed with the wrong ratio of catalyst may not cure properly and stays brittle or tacky.
Weather during laying matters. GRP cures by a chemical reaction that is temperature-sensitive, and laying in cold or damp conditions can produce a poor cure. Roofers choose dry days within the resin maker's working range.
The gel coat is the visible skin. Fine crazing in it, like a network of hairlines, is often cosmetic and arises from ageing, ultraviolet light and thermal cycling. Deeper cracks that show white glass fibres or open when pressed go through the laminate and let water in.
Do not leave an open crack. Water entering a GRP roof can soak the deck, which then swells and moves, causing further cracks. Our page on GRP fibreglass roofs covers how a sound roof is built.
Repairing a cracked GRP roof
A local crack can be repaired well. The area is cleaned and dried, the crack is ground out into a shallow V so the new material has something to key into, and the surrounding gel coat is feathered back. New glass matting is laid over the groove with fresh resin, built up in layers, and finished with a matching topcoat.
A repair like this lasts only if the cause is addressed. If a crack follows a board joint because the deck moves, relaminating over it will simply crack again. The fix may need the deck to be stiffened, the board joints to be covered with a flexible strip before the new laminate goes on, or a movement joint to be formed.
Weather and temperature affect repair as well as installation. Resin will not cure properly on a wet surface or in the cold, so repairs are scheduled for dry spells, and temporary sealing covers an urgent leak until then.
Colour matching is tricky. A repaired patch can show against an older gel coat, and a full topcoat across the roof gives a uniform finish.
Our page on whether a flat roof can be patch repaired explains how to judge when a patch is sensible. HomeBuilding puts supply-only GRP at £110 per square metre (HomeBuilding, Sep 2025), but repair prices are set after a survey.
When cracking means replacement
Repair makes sense where damage is confined and the deck holds firm. Replacement wins when cracks are widespread or keep returning, when the boards beneath are soaked or soft, or when the laminate is thin and brittle across the whole roof.
A full replacement lets the cause be corrected. The old laminate is removed, the deck is repaired or replaced with stiff, well-fixed boards, and a new shell is laid with proper detailing at joints and edges. If the build-up lacks insulation, the Building Regulations come into play, because Approved Document L counts the renewal of a flat roof's waterproof layer as renovation, which brings its Table 4.3 into play.
A different covering may suit the roof better. EPDM, felt and lead all respond differently to movement, and a roof that moves a lot may do better with a flexible membrane. We lay out the trade-offs and typical lives, and recommend whichever the evidence favours.
On a survey we examine the cracks, check the deck from beneath where we can, test the fall and photograph everything. You see the evidence and a clear recommendation, followed by a single total in writing.
The flat roofing hub sets out the coverings and the repair options, and we advise repair when repair will do.
