The short answer
A GRP roof cracks because the surface is rigid and the structure under it is not. GRP is glass-reinforced plastic, a cured resin laminate. When the deck below shifts with damp, heat or foot traffic, or the laminate is too thin or badly bonded, the stiff skin splits, usually along board joints, at edges and in corners.
GRP is a hard, jointless surface that lasts well when laid on a firm base. Cracks tell you the base is moving or the laminate was thin.
Movement in the deck below
GRP is cured resin reinforced with glass fibre. Once set it is rigid, with very little give. The boards under it, plywood or similar, are timber, and timber swells when it takes up moisture and shrinks as it dries.
Why do cracks follow the board joints?
Because that is where the movement happens. Each sheet of board shifts a little against its neighbour, and the rigid skin above has to span the joint. A joint that opens by a fraction of a millimetre can open the surface too, leaving a straight line crack running across the roof in the pattern of the boards. That is a classic sign that the deck has moved, or that the boards were laid with no gap and are now pushing against each other.
Poor fixing makes it worse. If the boards were not screwed down tightly, or the joists beneath are bowed or too far apart, the deck flexes underfoot and each step stresses the surface. Damp from a poorly vented roof, or a leak that has found its way under an edge, will swell the timber from below and break the skin from the inside.
Edge trims, corners and thin laminate
The edges are where GRP is tested most. The laminate has to be finished at an edge trim, turned up at a wall and wrapped around corners. All of those are places where one stiff layer meets another.
What causes cracks at the edges and corners?
Trims fixed too loosely move in the wind and crack the resin along their length. Corners that were laid with sharp folds, not a gentle radius, concentrate stress, so a fine star-shaped crack tends to appear at the bend. At a house wall, the building and the roof expand by different amounts, and a rigid joint between them will eventually open.
Thin laminate is the other common cause. The glass-fibre mat has to be fully saturated and built up to the right thickness. Lay it too thin, or leave dry patches, or work in cold or damp weather so the resin does not cure, and the surface is brittle from day one. Crazing, a network of tiny hairline cracks, can mean the topcoat was applied too thick, or the resin mix was wrong.
Our page on flat roof trims and upstands explains the edge details that GRP depends on.
Repair, or relay the roof?
A single crack along a board joint on a sound roof can be repaired. The crack is opened out with a grinder so the resin can reach the full depth, the area is cleaned and dried, and new matting and resin are laid across it, then topcoated. The repair lasts only as long as the cause stays fixed: if the board joint is still moving, the new patch can crack again.
When is a GRP roof beyond repair?
Long runs of cracking across many boards, soft or spongy areas underfoot, water stains on the ceiling below, or a deck that visibly bounces all point to a failed base, not a failed surface. Re-coating that roof only covers the problem. Less (Oct 2026) puts GRP at 25 to 40 years, so a roof that cracks within its first few seasons points to the way it was built.
Relaying means stripping the old laminate, replacing damaged boards, fixing the deck down properly with a small gap at each joint, and laying new GRP with trims that suit the edge. We show you photos of the cracks and the boards beneath, then the price is one total for the work. See how long a flat roof lasts for the ranges by material.
Looking after a GRP roof
A well-laid GRP roof asks for little attention. A few habits help it stay that way.
How do I stop a GRP roof cracking?
Limit how often anyone walks on it, and lay boards or a walkway if you must reach a rooflight or a gutter. Clear leaves from the outlets so the roof drains, because standing water adds weight and keeps the deck damp. Check trims and corners once a year, from a landing window or a ladder held by someone else, and watch for hairline cracks that open after frost.
Treat cracks early. A fine line sealed in dry weather is a small job, whereas water that has got below the laminate rots the boards and turns a small repair into a replacement deck.
If you are having a new flat roof laid, ask how the deck will be fixed, whether the boards will be spaced and whether the trims will be screwed or bonded. Those answers say a lot about how the roof will behave in a few years. For a wider view of the options, the flat roofing page sets out the choices, and flat roof patch repair covers when a small fix is enough.
More on GRP cracks
Is a hairline crack in GRP a leak?
Not always. A crack in the topcoat only can stay dry underneath, but one that reaches the glass-fibre layer will let water in. A damp ceiling stain below the crack, or darker resin around it, shows water has got through. Seal it soon, in dry weather.
Can GRP be laid over an old felt roof?
Not directly. The old felt is normally stripped, because GRP needs a clean, firm, dry deck to bond to and cannot be fixed to a soft surface. Sound boards underneath stay in place, and new board goes in wherever they are not.
The full picture: Flat roofing in Taunton ยท More short answers