Why a rigid roof cracks
GRP stands for glass-reinforced plastic. Layers of glass matting are soaked in resin on the roof, and it sets hard into one sheet with no joins. That is the strength of a GRP roof, with no laps to open, and also its weakness: it cannot stretch much.
When the timber deck under it swells, shrinks or flexes, the skin above has to follow, and a hard skin follows by breaking. Cracks that form along the lines of the boards point to this. A roof that bounces underfoot, or one with boards of mixed thickness, is a candidate.
Temperature plays a part too. The sun heats the surface through the day and the night cools it. Over many seasons that cycle stresses a laminate, especially around corners and along a wall junction. A crack might appear first at an outlet, a rooflight corner or a step.
A hairline crack that opens and closes with the weather is a sign of movement, not of a one-off knock. None of this makes GRP a poor choice. It means the roof needs the right deck, the right thickness and well-fixed edges.
Three usual causes
When a roofer looks at a cracked GRP roof, three questions come up first.
- Is the deck moving? Boards that were poorly fixed, or that have got wet, shift and take the laminate with them. Cracks that run in straight lines suggest board joints below.
- Is the laminate thin? A coat that is too light resists flexing and wear less well, and it shows pinholes and surface crazing before it cracks.
- Are the edge trims loose? Trims are the strips that finish the roof edge and carry the water into the gutter. A trim that has lifted or been knocked lets the edge of the laminate flex, and cracks begin there. Our page on flat roof edge trims explains what they do.
It helps to know which of the three you are dealing with, because a repair that ignores the cause will crack again in the same place. A crack that returns along the same line is a sign that the deck needs fixing, not the surface. Photograph each one and compare it with the board layout if you can see the underside.
Repair or re-laminate
A local repair is sensible when cracks are few, small and in one area, and the deck feels solid. The roofer cuts back to sound laminate, cleans and dries the area and lays new glass and resin over it, overlapping the edges. This suits impact damage or a crack at one corner.
Re-laminating means laying a fresh layer of glass and resin across the whole roof, or stripping back to the deck and starting again. It is the answer when cracks run widely, when the laminate is thin throughout, or when repairs keep failing. If the deck is damaged, boards are replaced first, otherwise the new surface will move just as the old one did.
Neither route should be chosen from the ground. The roof needs to be looked at, and often tapped and walked with care, to find loose or hollow areas. A hollow sound under a tap can mean the laminate has lifted from the deck even where no crack shows.
Our guide to repair or replace a flat roof sets out the usual decision points for any covering.
How long GRP lasts, and what to do now
Age is another factor. HomeBuilding gives GRP a typical life of 25 to 30 years, while Less gives 25 to 40 years. Neither figure is a promise, and a roof laid thin or on a poor deck can fall short of them. Look at how long yours has been down and what has happened to it since. Our page on how long a GRP roof lasts goes further.
In the meantime, avoid walking on a cracked area and keep outlets clear, since standing rain seeps into every crack it meets. Photograph the cracks with a coin or a tape in the frame to show their size, and note whether they appear after a cold spell or in line with the boards.
Do not seal a crack with a quick tube from a shop and call it done. A sealant bridging a moving crack tears again within weeks. Send us the photos and we say what we think is moving. After the survey you receive one written total. The rest of this work sits under flat roofing.
