Roofing kits are a convenient choice, but they’re not always the right fit . If you're working on an unusual shape, doing a repair, or you simply want full control over what you're buying, calculating your materials individually is straightforward, you just need to know the formula. In this guide we’ll take you through each material, the calculation method and a worked example using a 24m² roof so you can see exactly how it all comes together. Before You Start Calculating: What You'll Need to Know Before you can figure out how much mats you need, you need to know a few things: the length and width of your roof in metres, the weight of the matting - either 450gsm or 600gsm, how many layers you're going to need (1 if you just have the roof, 2 if you've got a balcony or a walkable roof) and the perimeter of your roof in metres. How to Begin: Measure Your Roof Size Measure the length of your roof in metres and then multiply that by the width in metres. Formula: Length (m) x Width (m) = Total Area (m²) Divide your roof into smaller parts and add together. This is crucial if there are walls, upstands or dormers (the vertical sections of the roof where it meets a wall). Include the upstands; these can be difficult, but you must consider them in your calculations. They need to be laminated just like anything else. Example Time: 4m wide x 6m long = 24m² Step 1: Calculate Your GRP Roofing Resin Resin is the binding agent that soaks into and hardens the matting. Coverage depends on the matting weight you choose. Formula: 450gsm matting: Area (m²) × 1kg, plus 10-15% wastage 600gsm matting: Area (m²) × 1.5kg, plus 10-15% wastage Worked example (24m², 450gsm): 24 × 1kg = 24kg + 10% wastage = ~27kg of resin Worked example (24m², 600gsm): 24 × 1.5kg = 36kg + 10% wastage = ~40kg of resin We stock standard GRP roofing resin as well as fire retardant options including Cure It, RES-TEC 1010, and Pro 25. Step 2: Calculate Your Chopped Strand Matting (CSM) Chopped strand matting is the reinforcement layer that gives GRP its strength. It's sold by weight. Formula: 450gsm matting: Area (m²) × 0.45kg, plus 10-15% wastage 600gsm matting: Area (m²) × 0.60kg, plus 10-15% wastage Double the figure for 2-layer applications (balconies, high foot traffic areas) Worked example (24m², single layer): 450gsm: 24 × 0.45 = 10.8kg + 10% = ~12kg 600gsm: 24 × 0.60 = 14.4kg + 10% = ~16kg We have 450gsm and 600gsm chopped strand matting available. Step 3: Calculate Your GRP Topcoat A Topcoat is the protective layer put on top of the dried laminate. It offers protection from UV rays and a strong seal against the weather. Formula: Area (m²) × 0.5-0.6kg Worked example (24m²): 24 × 0.55kg = ~13kg of topcoat We offer standard GRP topcoat in dark grey as well as coloured options including anthracite and jet black. Step 4: Calculate Your MEKP Catalyst The Catalyst starts to harden your resin and topcoat. It's important to use the right amount; too little and the laminate won't stick well, too much and it could break. To mix, use 2% of the total resin along with the weight of the topcoat. If it's chilly, go with 3% instead. Worked example (27kg resin + 13kg topcoat = 40kg total): 40kg × 2% = 800ml of catalyst Step 5: Trims and Edging Trims help to finish off the edge of your roof and are needed to help waterproof the edges. Calculate the entire perimeter of your roof and add 10% for corners and cuts. Formula: Perimeter (m) × 1.1 Worked example (4m + 6m + 4m + 6m = 20m perimeter): 20m × 1.1 = ~22 linear metres of trim Depending on your roof design, you'll need drip trims (for exposed edges that overhang a gutter) or raised edge trims (where the roof butts up against a wall or parapet). Browse our full range of fibreglass roofing trims, including the popular A200 drip trim and D260 wall flashing trim. Step 6: Tools You'll also need a toolkit to apply everything correctly. We offer three sizes depending on your project: Tool Kit 1 - Small projects (5-10m²) Tool Kit 2 - Medium projects (10-30m²) Tool Kit 3 - Large projects (30m²+) Each kit contains laminating rollers, consolidating rollers, resin brushes, mixing buckets and mixing sticks – everything you need to get started. Full Material Summary: 24m² Roof (450gsm, Single Layer) Material Calculation Amount Needed GRP Resin 24m² × 1kg + 10% ~27kg 450gsm CSM 24m² × 0.45kg + 10% ~12kg Topcoat 24m² × 0.55kg ~13kg MEKP Catalyst 2% of 40kg ~800ml Trims 20m perimeter + 10% ~22 linear metres Ready to Order? Once you have your figures, you can pick up exactly what you need from our full range, no bundled kit items, no overpaying. If you're not sure which products are right for your project, give us a call on 0800 612 7903 and we'll help you spec it out.
