A GRP roof that was watertight in spring can start splitting along its edges by mid-summer. Roof is not damaged, walked-on or hit. It was merely basking in the sunshine, warming up during the day and chilling at night, and this motion has exploited the weakest link in the chain. This is thermal expansion, and on UK flat roofs it is one of the most common reasons a fibreglass roofing kit cracks or blisters before its time. Almost all of it traces back to how the roof was built, not to any flaw in the material itself. Get the detailing right and a GRP roof, along with the resin and topcoat protecting it, will outlast most of the things bolted to it. What Does Thermal Expansion Do to a GRP Roof? It's what cracks the surface, normally at the corners and edges. The roof grows a little in the heat and shrinks back as it cools, and if it's been laid with nowhere to take up that movement, something has to give. Glass reinforced plastic is hard and stiff. It is this rigidity that makes GRP so hard-wearing and gives it a smooth surface, but it also means that GRP does not take to flexing. The timber deck underneath behaves differently. On a hot day a dark roof surface can climb well past air temperature, and the boards and the laminate both expand. After sunset they cool and contract again. Over one summer day, the surface expands and contracts complete cycles, and over each day of the year when the sun is visible it does so again. Let the roof bounce and no damage occurs. Remove that room and the stress must be transferred to some other place. It finally reaches the joints, the trims, the corners and the upstands, where the cracks are likely to appear first. Why Does UK Weather Make Cracking Worse? UK weather is harder on a flat roof than a steady warm climate, because the damage comes from the size and speed of the temperature swing, not from heat alone. A bright day in April or September can heat a roof surface well above air temperature, then let it drop close to freezing overnight. That daily expansion and contraction is the real culprit, and the UK serves it up for most of the year. Frost makes it worse. Water sits in a hairline crack, freezes, and forces it wider. And since a UK roof is damp most of the year, there's always water around to get in and do that. What Actually Causes the Cracking and Blistering? The movement is only the trigger. The real problem is a roof that wasn't built to take that movement. A handful of faults cause most of it: No gap at the sides: When the boards are slammed shut tightly without being able to move, the surface has NO place to expand, and it will crack open. This fault leads to more thermal cracking than all the other faults put together. Deck movement: If the boards give when you walk on them, the laminate on top gives too, and it'll crack. Usually that's loose fixings or water in the timber. Stress at the details: Corners, rooflight kerbs and upstands take more movement than the open field of the roof. The flat-roof standard BS 6229 calls for upstands to be detailed separately from the wall so they can move independently. If those points aren't reinforced and detailed properly, they crack long before the flat areas do. Improperly Laid: Resin and topcoat will not cure well in cold or damp conditions, and will not cure if laid too quickly. Bad cure will result in a soft surface which is the breeding ground for blisters and pinholes. The latter two are typically the cause of blistering and bubbling symptoms. Moisture and/or air bubbles get trapped under the topcoat, or the topcoat does not adhere well to the laminate surface, raising it off. After that water sits in or under that blister and freeze-thaw does the rest. How Do You Prevent Thermal Cracking? Most of the prevention happens before the resin goes down. None of it takes special skill, but it is the first thing to go on a rushed job. Get the Deck and Expansion Gap Right Use the correct grade of timber boarding and fix it down so there is no flex. Then leave a movement gap at the perimeter and around any penetrations. That gap gives the surface somewhere to expand into instead of cracking, which makes it the most important detail on the whole roof. Reinforce the Detail Points Corners, upstands and rooflight kerbs should be laminated to take more stress than the open field, because that is the load they actually carry. Skip this and the roof fails at the edges first, every time. Match the Kit to the Roof A small garage roof and a big flat roof don't behave the same. The larger the roof, the more it moves as it heats and cools, and the more the material and the edges have to cope with. A kit that's fine on a porch won't hold up on a 50 square metre flat roof. Our GRP roofing kits come in budget, standard and heavy-duty tiers for that reason. For bigger or more demanding roofs, go with the heavy-duty kit. Trims matter as much as the laminate. They hold the edge down and let it move without splitting, and the wrong one will let water creep back under a roof you've otherwise laid well. Not sure what your roof needs? Get in touch and we'll sort you out. Lay It in a Sensible Weather Window A dry day with workable temperatures gives the resin and topcoat the cure they need. The UK does not offer many of those, so it pays to wait for one instead of forcing the job through poor conditions. How to Spot and Fix It Early Cracks are easiest to deal with while they're still in the topcoat. Once water gets into the deck, a small patch becomes a full recovery. The trouble with a hairline crack is it looks like nothing, while water runs underneath and travels. The damp patch on your ceiling is often nowhere near the actual leak. Check it a couple of times a year, and after any big storm. Cracks tend to start at the edges and corners. Look for lifted trims, blisters, pinholes in the topcoat. And keep the gutters clear so water isn't sitting on it. Caught early, most of this is a simple patch, grind it back, lay in fresh matting and resin, recoat. If the roof's just tired rather than failing, a GRP topcoat refurb kit redoes the whole surface without stripping it back. A well-built GRP roof shouldn't crack from heat at all. When one does, the heat has just found a fault that was already there. Patch the crack and ignore the cause, and the same split's back next summer. Browse the full range of GRP kits and trims and get everything you need in one delivery. FAQs Why Does a Fibreglass Roof Crack in Summer? It's almost always thermal movement. GRP expands in the heat and shrinks back when it cools, and if there's no expansion gap round the edges it's got nowhere to go. That's why the cracks turn up at the corners first. Can a Cracked GRP Roof Be Repaired, or Does It Need Replacing? A large number of cracks can be fixed. If the damage is confined to the surface and is small in size, it can be ground back, re-laminated and recoated. Replacement is only necessary when the bottom deck has shifted, has become rotten, or has cracked all over the roof. Why Does My GRP Roof Make Cracking or Banging Noises? Yes, larger ones, particularly. The sounds are created by the surface increasing and decreasing in size due to temperature changes. If the movement occurs at the mid span of a large roof (no expansion allowance) then it is loud enough to be heard. It's a symptom that the roof doesn't have anywhere to relieve the stress. What Causes Blistering on a GRP Roof, and How Do I Stop It? Blisters are typically caused by moisture or air that has been entrapped, or by a poor cure. To prevent it, lay it in appropriate weather conditions on a dry, sound deck, allow the resin and topcoat to cure correctly and avoid water from building up on the deck's surface.
