You’ve measured the roof, but choosing a kit brings up a few more questions. Do you add extra for waste? Which edges need trims? And does the kit include the tools you’ll need? Our GRP kit calculator and EPDM kit builder help you work through those choices. Have your measurements and a rough sketch handy. Here’s how to use them, starting with a simple 5 m by 3 m roof. Measure your roof before opening the calculator Measure the finished roof deck’s length and width in metres. The building’s dimensions alone may miss an overhang, so check the surface you’re actually covering. Only measure from a safe position; ask your roofer if access is awkward. For a rectangular roof, multiply the two measurements: 5 m × 3 m = 15 m² Keep both measurements written down. A total of 15 m² doesn’t tell the calculator how long each edge is. On your sketch, mark the gutter, walls, raised sides, rooflights and pipes. Note any upstands separately. For an L-shaped roof, split the plan into rectangles and measure each section without counting the same area twice. If the calculator’s shape doesn’t match your roof, ask us to check the layout before ordering. Using the GRP kit calculator Choose the matting weight and tools The first screen asks for “GRM”, whether you need a tool kit, and your roof dimensions. GRM refers here to the fibreglass matting weight, commonly written as gsm: grams per square metre. You’ll see 450 and 600 options. Use the weight specified for your project. If the roof will be used as a balcony or terrace, check the complete roof specification with us; choosing heavier matting alone doesn’t make a roof suitable for regular use. Select “Yes” for a tool kit if you need one. If you already have roofing tools, check what’s required before choosing “No”. Enter the actual length and width For our example, enter 5 under Length and 3 under Width, then select “NEXT STEP”. Do not add extra for waste to the GRP calculator’s dimensions. It asks for your actual roof size and rounds up to the nearest kit it offers. That instruction applies to these inputs. If you’re buying materials separately, our guide to estimating fibreglass flat roof materials covers a different calculation. Which edge option should you choose? The next screen shows your area and four edges labelled A–D. Match each one to your sketch and select the appropriate trim. Drip edge: where water leaves the roof and enters the gutter. Raised edge: a side where water should stay on the roof. Wall detail: where the roof meets a wall. Check the trim’s dimensions and matching parts, too. Our GRP roof trims and flashings guide explains the profiles. There’s also an “Own Trims” option if you already have suitable trims. Continue to “Your Kit” and check the listed edges, trim quantities and price before adding anything to your cart. Using the EPDM kit builder The EPDM calculator opens our EPDM kit builder. Its steps are Roof Shape, Edges, Surface and Trims. Start with “Create Kit”. Select the shape and enter each edge Choose “Square/Rectangle” for this flat-roof example. The builder also shows an “Apex” shape option. Click an edge marker on the drawing. Enter its exact length, check whether the units are metres or feet, and choose its edge type. “Kerb” prevents water running over that edge; “Gutter Drip” is where water runs into a gutter. The builder says it adds an allowance for overhang. Follow that instruction rather than adding your own allowance to the entered lengths. For a 5 m by 3 m rectangle, check that opposite edges read 5 m and 3 m respectively. This example uses exposed kerb and gutter edges. If your roof meets a wall or needs details the builder doesn’t show, speak to us before choosing a substitute. Choose the surface and trims Next, choose the type of board on your roof: OSB3 with square edges, tongue-and-groove OSB3, or plywood. This describes the roof deck; selecting it does not add decking boards to the kit. Choose black plastic trims, black metal trims, or “I don’t need trims”. Review your roof details, then use “Create kit” to see the proposed contents. Check the items and any optional extras before ordering. Check the suggested kit before ordering Take a minute to compare the result with your sketch: Are the measurements and units correct? Does every edge match the actual roof? Have wall junctions, upstands, outlets and pipes been accounted for? Does the selected surface match the deck you’ll use? Are the tools and any additional materials covered? A calculator works from the details you enter. It can’t inspect the deck or confirm the roof structure is sound. Damaged boards and unusual details need attention separately. Ready to size your roof kit? Use the GRP kit calculator or build an EPDM kit with your measurements beside you. If a shape or edge has you stuck, contact our team with a sketch or photo. We can help you check the details before you order.
