Why do GRP roofs crack?

GRP stands for glass-reinforced plastic, which is fibreglass matting set in resin. Once cured it is hard and unbroken, with no joints to leak, and that is its strength. It is also the reason it cracks: it does not stretch the way a rubber membrane or felt does. Timber boards swell with moisture and shrink as they dry, and the whole roof flexes slightly with temperature. Over the years, that movement concentrates at any weak line. On an older roof, the first signs are often hairline marks that open a little more each winter and close up in summer.

The weak lines are where the boards meet, so cracks often follow the grid of the sheet joints beneath. If the deck was fitted tight with no allowance, or the boards were not fixed firmly, the movement is bigger. This is the most common cause of long straight cracks that appear in a regular pattern across the roof, and it explains why a crack that is patched often comes back along the same line.

Quality of laying matters more with GRP than with most coverings. The laminate needs enough layers of matting and enough resin to reach the thickness the system is designed for. It is fair to ask any installer what thickness they laid and how many layers of matting went in; they should be able to answer for their own work. A thin laminate flexes more, is easier to puncture and cracks sooner. Resin that was mixed badly or laid in cold or damp conditions can cure unevenly, and a surface that was not clean can stop the layers joining up.

The topcoat is the visible finish and the first line of defence against sunlight. If it was skimped, the surface weathers and crazes into a network of fine cracks like dry mud. Those are often cosmetic, but they warn that the laminate below is now exposed. Our guide to GRP fibreglass roofing explains how a correctly laid roof is built up, layer by layer, from deck to topcoat.

Long straight cracks along the board lines point to deck movement. Short cracks at the corners or at the foot of a wall point to stress at the angles, where the laminate has to turn sharply upwards. A spider web around one point usually means something hit it, whether a ladder foot, a dropped tool or a branch. Impact cracks are local, and they are the easiest to repair neatly. A fine web across the whole surface is the topcoat ageing.

Cracks along the edge are worth a close look, since any water entering at the perimeter runs straight into the end grain of the deck. The trim and the drip there matter as much as the main surface, and we cover them in the answer on flat roof edge trims and drip details. Photographs of each crack, taken close up and from a few steps back, give a clear record to work from before any repair starts. We also check the upstands and corners, because that is where the laminate works hardest and a repair needs extra detail to last.

Smearing sealant over a crack seldom lasts, because the crack moves. A proper GRP repair grinds back the damaged area to clean, sound laminate, lays new matting and resin across it, and finishes with a topcoat that matches the rest. If the crack runs along a board joint and the deck is moving, the cause also needs dealing with, or the new patch cracks in the same line within a year or two.

Water that has been getting in may have damaged the boards below. We check for that before covering anything, because laying new laminate over a wet or rotten deck sends the problem into hiding. Where the deck is sound, you receive a written quote, one total after the survey, shaped by the area to be repaired and how easily we can reach it. A short run of cracked laminate at one corner is a very different job from cracks across the whole field of the roof.

Less (last updated March 2026) gives a typical life of 25 to 40 years for GRP. A roof within that range, with a few localised cracks and a sound deck, is a candidate for repair. A roof with widespread cracking, a weak laminate and soft boards is a candidate for replacement, whatever its age, and repeated patching adds up to more than the work that would cure it.

The decision is the same whichever covering you have, and we set out the signs in our answer on repairing or replacing a flat roof. If a new covering is the answer, our comparison of EPDM, GRP and felt shows how they differ when a deck moves. The flat roofing hub lists everything we lay.