Most GRP roofing problems don't begin with anything dramatic. It's usually a deck that was nearly dry, a batch mixed too large, or an awkward corner left until later. The roof may look all right at first. The trouble shows once the weather turns and movement, trapped air or a poorly finished edge starts to matter. GRP isn't especially forgiving, but it isn't a guessing game either. Get the deck, materials and order of work right before the catalyst comes out and the job is much easier to control. The full process of fibreglassing a flat roof is fairly straightforward when those stages are followed in order. Here, we're looking at what turns that straightforward roof into wasted material or a repair job. Before you start, check the things that can stop the job Safe access is part of the job Roof work carries a serious fall risk. Plan access and edge protection before the materials go up; a ladder shouldn't quietly become the place you spend the day working from. Read the HSE roof-work guidance and its working-at-height checklist before you begin. Resin, catalyst and cleaning products also need to be handled in line with their labels and Safety Data Sheets. If the structure, access or detailing is beyond your experience, bring in a competent roofer. Check whether Building Regulations apply Planning permission and Building Regulations are two separate things, and it’s easy to get them mixed up. A small roof repair won’t normally require a building control application. However, replacing a larger area of the roof covering, altering the structure or changing the insulation may bring Building Regulations into play. If you’re unsure, the Planning Portal explains the rules for work on an existing roof, while GOV.UK provides guidance on Building Regulations approval. Rules differ across the UK, so check with the relevant building control body if the scope is unclear. 1. Treating the roof deck as “good enough” A new GRP laminate can't strengthen a weak deck, dry wet timber, stop movement or send standing water towards an outlet. What a GRP-ready deck actually looks like For a standard timber-decked system, that normally means clean, dry and securely fixed 18mm OSB3 tongue-and-groove roofing board. It isn't universal, though. The board, fixings and preparation must match the product being used. Keep its instructions beside you; the official Cure It installation guides show how detailed system requirements can be. Walk the deck before starting. Any spring, damaged edge or proud fixing needs attention. Check the falls and drainage too. Covering a problem because “the laminate will hold it” generally means covering it twice. 2. Trusting the weather forecast a bit too much A dry icon on a weather app isn't the whole plan. Conditions must stay suitable through preparation, laminating, topcoating and the initial cure. Overnight dew, a cold deck or early rain can stop the work before it has properly begun. The cure window counts as part of the forecast Follow your system's limits, remembering that material and deck temperature matter too. Don't stretch the job into a doubtful evening because everything is already on the roof. Curing GRP resin in cold conditions takes more planning, especially later in the year when the usable working window gets shorter. Leave some margin too. An exact weather window disappears quickly when preparation takes longer than expected. 3. Ordering for the main area and forgetting the rest of the roof Length by width gives you the main deck area. It doesn't allow for upstands, trim joints, corners, outlets, overlaps or normal wastage. Those smaller details still use matting and resin. Measure what will actually be covered Sketch the roof and mark each drainage edge, raised edge, wall junction, outlet and penetration. Measure the upstands too. A proper fibreglass roof material estimate should include those details and an allowance for waste, while the GRP kit calculator can help you turn the measurements into a sensible kit size. Running short halfway through the laminate is more than an inconvenience. By the time more material arrives, the first section may be outside its normal overcoating window and need additional preparation. 4. Guessing the catalyst or making one enormous batch Catalyst isn't something to add by eye. The amount depends on the product, catalyst grade and conditions. Its mixing chart matters more than a percentage remembered from another job. Smaller batches keep the job under control Measure properly and mix around the sides and base of the bucket. Only prepare what you can use within the stated working time. A large batch may feel quicker, but the reaction can accelerate in a deep bucket, leaving you racing hotter, thickening material. Set the roof out in manageable sections and use clean, repeatable batches. You'll have more control and less cured resin left in a bucket. Most importantly, don't mix ratios or components from different systems simply because they look similar. 5. Assuming the matting will sort itself out Once the resin is down, it's tempting to roll quickly and move on. This is where dry fibres, wrinkles and air pockets get left behind. What proper wet-out looks like Pre-cut the matting before mixing. Wet it out at the stated coverage rate until it changes from dry white to evenly translucent, then use a clean paddle roller. Pay attention to overlaps, corners and changes of level; the roller is removing trapped air, not just smoothing the top. White patches and bubbles are unfinished work. Topcoat may colour over them, but it won't turn them into a sound laminate. If the resin starts to gel before the section is consolidated, stop rather than dragging half-cured material around the roof. 6. Leaving trims and awkward details until later The open area of a roof is usually the simple bit. Edges, wall junctions, outlets, corners and penetrations are where the planning earns its keep. Plan where the water is meant to go Dry-fit the trims and pre-cut each reinforcement before the main mix. Use the correct profile and follow the system's method for fixing, joining and reinforcing it. Drip and raised edges do different jobs; swapping them because one is left over isn't harmless. Choosing the right GRP roof trims and flashings matters because each profile controls a different edge or junction. On a complicated roof, send us a sketch before ordering rather than inventing the detail on the day. 7. Treating topcoat like ordinary paint Topcoat gives the roof its weathering and UV-protective surface. It isn't ordinary paint, and it can't rescue dry matting, trapped air or a badly formed corner underneath. Coverage matters in both directions Wait until the laminate has reached the cure stage stated by the manufacturer. If it has been left beyond the system's recoat window, become wet or picked up dirt, follow the specified preparation before continuing. Then catalyse the topcoat using its own chart and apply it at the stated coverage rate. Spreading it thin to make the tin reach the last few metres is false economy. Piling it on isn't better; the GRPROOF application manual, for example, warns against applying topcoat either too thinly or too thickly. Keep a wet edge, cover the details properly and leave the finished roof alone for the full cure period. A bit of planning is cheaper than doing the roof twice Before opening the first tin, make sure the deck is sound and dry, the weather window is long enough, the quantities add up and every detail has a plan. Cut the matting, lay out the tools and keep the mixing chart visible. Preparation feels slow when you're keen to start. It is still quicker than repairing a failed corner or stripping back a badly cured roof. Our complete GRP roofing kits cover a range of roof sizes, but don't guess if the measurement, trim layout or system choice is unusual. You can contact our team or call us on 0800 612 7903 before you begin. A quick question at that stage can save a fairly unpleasant one halfway through the job.
