Why Is My GRP Roof Cracking?

GRP stands for glass-reinforced plastic: layers of glass matting soaked in resin, finished with a coloured topcoat called the gel coat. Once cured it is hard and stiff, which is its strength on a walkable, jointless roof. The same stiffness is why it cracks when something beneath it moves.

Three movements do most of the damage. The first is thermal: a dark roof in summer sun swells and then contracts through a cold night, and over many cycles the stress concentrates at corners, step-downs and the edge of the roof. The second is the deck. Plywood or boards expand with damp and shrink as they dry, and a laminate laid over a joint between two sheets can be pulled apart along that line. The third is the building itself. A settling extension or a flexing joist shows up first as a straight crack above the line of the support.

A stiff roof with a long unbroken run, no movement joint and a deck that was not fixed down tightly will crack sooner than a small, well-supported one. That is a fault in the build, and the repair has to deal with the cause as well as the line.

The thickness of the laminate matters as much as the movement. A properly laid roof has glass matting built up in layers so the shell can take foot traffic and flex slightly without breaking. Where the laminate was laid thin, perhaps with a single layer of matting to save time, there is little strength in reserve. You may see white marks where the glass fibres show through, or a pattern of fine lines called crazing across the surface.

The edges are the next weak point. Beside a house wall the roofer forms an upstand, a short turned-up edge, and at the eaves there is a trim that carries the water off. If these are poorly fixed they can move in wind, and the stress tears the laminate beside them. A crack that follows the line of a trim is often a sign of loose timber below it, which is a different problem from the laminate itself. Our answer on a flat roof edge trim leaking covers what to look for there.

Pooling water makes everything worse. A roof that holds a puddle loads the shell unevenly, and a cracked area under standing water will leak far sooner.

Some of those marks are harmless. Fine hairline crazing that sits only in the topcoat, does not catch a fingernail and has no dark stain beneath it can often be left, or sealed with a fresh topcoat when the roof is next tidied up. You are really judging three things: whether the crack goes through to the matting, whether it is open enough to see a gap, and whether the ceiling inside is stained or damp.

Walk the roof only if it is safe, and walk lightly, because a cracked shell over soft decking can give way. Look from the ground first, then from a window if there is one. A crack along a deck joint, a crack across a corner or a split where the roof meets a wall deserves a roofer's eye soon, as water can soak into the deck and rot it from underneath.

If you are weighing this up against a bigger job, our page on whether to repair or replace a flat roof walks through the choice.

A repair that lasts starts by finding out why the crack opened. A roofer cuts or grinds the crack out in a V shape, wider than the visible line, then dries and cleans it and lays new strips of glass matting in resin across the repair, finishing with a matched topcoat. Smearing sealant or brushing resin on top of the crack only hides it, and it will reopen with the next cold snap. If the deck beneath has moved, the loose boards are fixed down or the damaged section is cut out and replaced first.

For a national guide, Checkatrade's June 2026 repair table puts GRP repair at £130 to £180 per m², excluding VAT, for jobs up to 3 m². Treat that as a yardstick only. Your own price depends on the length of crack, the access and what lies underneath, so the single written price waits until the survey has shown how far the cracks run.

Where cracks are spread across the whole roof, the laminate is thin and the deck is soft, patching is a poor buy. Less lists GRP at 25 to 40 years, but a badly laid roof will not reach that. A new deck and a thicker, properly trimmed laminate can then be the sounder spend. See how long a GRP roof lasts and our flat roofing page for more.