A drip on the garage floor. A damp patch on the ceiling. You might spot them after heavy rain, but the roof may have had a small fault for months. A well-built GRP roof can handle British rain, frost and sun. But it needs a firm base, well-fitted edges and a clear path for rain to drain away. The weather on the day it’s fitted matters too. So what should you look out for? Here’s how the weather affects your roof, which signs need a closer look and what to check before fitting a new one. Is GRP roofing suited to the UK climate? GRP, or glass reinforced plastic, combines glass fibre reinforcement with resin to form a continuous waterproof covering. In a conventional system, a roofing topcoat provides the exposed finish. Our guide to GRP roofing explains the layers in more detail. Weather What Matters on the Roof What to Check Heavy rain Drainage and watertight junctions Outlets, gutters and wall flashings Frost and snow Existing defects and snow loading Cracks, blocked drainage and structural concerns Summer sun Surface heating and UV exposure Topcoat condition and movement details Strong winds Secure fixing and impact damage Edges, trims and fallen debris Coastal exposure Salt spray alongside wind and rain Exposed metalwork and system suitability A roof that withstands winter weather cannot necessarily be installed in it. Fresh resin, topcoat and exposed decking need the conditions specified for that particular system. How GRP roofs cope with heavy rain A sound GRP covering keeps rain out, but the roof still needs a fall towards suitable drainage. Leaves piled over an outlet can undo the benefit of an otherwise well-built roof. A puddle after a shower doesn't, by itself, prove the covering has failed. Water that repeatedly remains in the same place deserves a closer look, especially if that area is growing or the ceiling below becomes stained. The cause could be a blockage, inadequate falls or a sagging deck. What if it rains while the roof is being fitted? Moisture can interfere with curing and the bond to the deck. Don't laminate onto damp boards just because the surface looks dry. If rain arrives mid-job, stop, protect the unfinished work safely and get advice on any affected material before continuing. Can frost and snow damage a GRP roof? Cold weather doesn’t mean your fibreglass roof is going to split. The concern is water getting into an existing crack or a joint that isn’t properly sealed. If that water freezes, it can make the damage worse. A thick layer of snow adds weight to the roof. It’s the joists and boards underneath that carry it, so adding more topcoat won’t help. Sagging or unusual creaking needs checking. If there’s a lot of snow up there and you’re worried, ask a qualified roofer or structural professional for advice. Don’t climb up to clear it while the roof is icy. You could slip, and scraping at the snow with a shovel can damage the fibreglass. Clear the gutters and outlets before winter so melting snow can drain away. How summer heat and sunshine affect GRP Even on a fairly mild day, sunshine can heat the roof well above the air temperature. The roof then cools again once the sun goes down. That warming and cooling causes movement, which needs to be allowed for when a conventional rigid GRP roof is fitted, especially around walls, corners and larger areas. We cover this in more detail in our article on GRP roof cracks and blisters. The topcoat is the part left facing the weather, so it needs to be a UV-resistant finish made for your roofing system. Follow the product’s instructions for how much to use and how thick to apply it. More isn’t necessarily better. If the finish is looking worn, check the roof underneath before buying anything to cover it. Our GRP topcoat refurbishment kits may suit an existing roof where the laminate and boards are still sound and the surface has been properly prepared. A fresh topcoat won't fix a moving deck or an unresolved leak. Find the cause before covering the symptoms. What to check after a storm Strong winds can lift loose roof edges. Falling branches can damage the GRP too. The boards, trims and joins all need to be firmly in place to help keep the rain out. A garage by the sea gets more wind than an extension tucked behind the house. Thicker fibreglass matting alone won’t make a roof ready for an exposed spot. The whole roof needs to suit the site. Before a storm, put away loose garden items and have any roof faults checked while it’s safe. The Met Office advises staying inside during a storm, so leave repairs until it has passed. Once it’s safe, look at the roof from the ground or a window. Check for loose trims, fallen branches and fresh damp patches indoors. If something has moved or the roof has started to leak, arrange for a roofer to take a look. Is GRP roofing suitable for coastal homes? The fibreglass covering doesn't rust like steel. That is useful beside the sea, but it doesn't make every part of the roof immune to coastal exposure. Exposed metal fixings, flashings and other fittings need to suit the location. Wind-driven rain and debris still matter, and any salt contamination must be removed as specified before repairs or recoating. Tell your supplier if the property is on an exposed coast. Ask whether the complete roofing system and its accessories are suitable. Don't choose a resin simply because it says “marine grade”: suitability for one fibreglass application doesn't establish suitability for a roof. EPDM rubber doesn't rust either. The same need to check the complete system applies; salt exposure alone isn't enough to choose between the two. Choosing resin and topcoat for the weather Start with a complete roofing system suitable for the deck and roof use. Then check the conditions needed to install it. Fitting a GRP roof in cold weather Ask whether the system has approved winter materials, and check the limits for both resin and topcoat. Cold weather can slow curing, and a sunny lunchtime doesn't guarantee suitable conditions later in the afternoon. Use the specified catalyst and mixing chart. Adding an extra splash won't make damp or frozen decking acceptable. Working with GRP on warm days Higher temperatures can shorten working time. Plan manageable batches, check the roof surface temperature and follow the instructions for applying topcoat over the cured laminate. Our roofing resins and topcoats include different systems. Match the products to each other before ordering, rather than assuming every tin is interchangeable. There is no single minimum installation temperature for all GRP products. Follow the current instructions for the exact resin, catalyst and topcoat, including the conditions needed while they cure. Looking after your roof through the year Spring and autumn are useful reminders, with another look after severe weather. From a safe position, check for: Leaves or silt collecting around drainage points. Loose trims or gaps where the roof meets a wall. New cracks, blisters or exposed glass fibres. A change in where water collects. Damp patches inside, especially after wind-driven rain. Arrange safe access for cleaning or repairs. Photograph changes so you can show a roofer what has happened. For suitable existing GRP surfaces, our refurbishment guidance recommends washing rather than jet washing. Need help choosing your roofing materials? Before you order, have your measurements, deck details and a few photos ready. Mention any leaks, awkward corners and whether the building is exposed to coastal winds. Not sure what you need for your roof? Give us a call on 0800 612 7903, or send us a few photos and your measurements. We’ll talk you through the options and help you choose the right materials.
