Which one is better: One 600g layer or two 450g layers ? If you’ll only go onto the roof to clear the gutters or carry out repairs, you may not need the same matting as a balcony you use every day. Start with how you’ll use the roof, then check which matting the roofing system specifies. Two layers of 450g give you 900g of glass per square metre, compared with 600g in a single 600g layer. That means a heavier laminate, but the boards and joists still have to carry the weight of anyone using the roof. And if you’re adding a layer to an old roof, you need to know what condition it’s in first. One 600g layer or two 450g layers The weight on the roll tells you how much dry glass there is in each square metre. It doesn’t tell you the finished roof thickness. Chopped strand matting is worked into roofing resin to make the reinforced layer, or laminate, beneath the topcoat. Matting build-up Dry glass per m² What the difference means 1 × 450g 450g A common domestic roofing specification; check permitted access. 1 × 600g 600g More reinforcement in one layer, with higher resin demand. 450g + 225g 675g A mixed lay-up that needs system approval. 2 × 450g 900g A heavier build-up used in our balcony and walkway kits. Two 450g layers contain 50% more glass than a single 600g layer, so they need more resin and take longer to lay. That isn’t a promise of 50% more strength. Work the resin right through the mat and use a roller to remove any trapped air. If you’re laying two layers, make sure they bond properly to each other. If your roofing system calls for 600g matting, don’t assume 450g plus 225g will do the same job. There’s more glass in that combination, but you’ll need the supplier to confirm it’s suitable before using it. Can you walk on a 450g fibreglass roof A 450g roof may be suitable for the odd maintenance visit once it’s fully cured, depending on the roofing system. If you want to use it as a balcony or walkway, choose a system designed for people walking on it regularly. For balconies and walkways, we offer double-layer kits with two layers of 450g matting. They’re designed for regular foot traffic. The boards and joists underneath still need to carry the expected loads, and you’ll need a suitable slip-resistant finish. Extra matting cannot make weak joists or rotten roof boards safe to walk on. If the base is OSB OSB3 is commonly used under GRP roofing. Follow the roofing system’s instructions for the board thickness, supports and fixings, and make sure the boards are dry. If they’re loose or flex when you step on them, sort that out before laying the fibreglass. If the base is concrete For concrete, check that the roofing system can be used on it. The instructions should explain how dry the concrete needs to be, how to prepare it and which primer to use. A kit made for OSB won’t necessarily work on concrete, however heavy the matting is. Plan safe access and protection at roof edges before any work. The HSE’s roof-work guidance explains these precautions, including the risks around fragile rooflights. Can you add another layer to an existing 450g roof You may be able to, provided the old roof is sound and the new materials are compatible with it. The resin needs to stick to a surface that’s already cured and been topcoated, so there’s preparation to do before you lay any more matting. Before adding either 450g or 225g, check for: Soft or wet boards beneath the fibreglass. Loose laminate, blisters or cracks that keep returning. Movement at board joints, trims or wall junctions. Anything loose has to come off before you cover the roof again. The remaining surface will need preparing as the supplier directs, including sanding and a primer where required. Washing it won’t be enough to get a good bond. Another 450g layer would bring the total glass weight to 900g/m². With 225g, it would be 675g/m². Neither figure tells you whether the roof can be used as a balcony. And you may not need extra matting at all: if the fibreglass underneath is sound, recoating the fibreglass roof could be enough to replace a worn finish. How much resin do mixed layers need Check the coverage information for the resin you plan to use. If the supplier works out resin quantities by the weight of the glass, include both layers of matting. Keep the figures in kilograms throughout; litres measure volume, so they aren’t interchangeable. Say you’re covering 10m² with 450g plus 225g matting. If the supplier approves those layers and specifies 2.5kg of resin for each kilogram of glass, the calculation is: 450 + 225 = 675g/m², or 0.675kg/m². 10 × 0.675 = 6.75kg of dry glass. 6.75 × 2.5 = 16.875kg of resin, roughly 16.9kg. That’s the resin for the matting in this example. When estimating the full materials order, you’ll also need to allow for overlaps, corners, trims and waste. Add any resin needed to coat the boards, or primer, as the system requires. Calculate the topcoat and catalyst separately using the product instructions. The 2.5:1 ratio is an example, not a universal roofing rate or approval for a 450g + 225g build-up. Where lighter matting tape and tissue fit in 225g chopped strand mat 225g chopped strand mat can be useful for small patches and lighter work where the repair instructions specify it. You get about 4.44m² from a kilogram, before allowing for overlaps. But a single piece is much lighter than 450g or 600g mat, so it won’t replace a damaged layer of either on its own. 175g woven fibreglass tape Woven fibreglass tape is used with resin to reinforce joints, corners and other small areas where the system calls for it. The tape we stock is 175g/m². It isn’t a self-adhesive repair tape; you need compatible resin to bond it in place. Check which bandage your roofing system specifies before ordering. Woven tape and chopped strand bandage aren’t interchangeable just because they’re the same width. You need the correct type and weight too. 30g surface tissue To soften the coarse fibre pattern left by chopped strand mat, 30g surface tissue can be laid into the wet resin before the flowcoat. Its job is to improve the finish. It does not replace the reinforcement beneath it. Can tissue tape or 225g mat repair a roof crack Is the crack just in the topcoat, or has the fibreglass underneath split too? That’s the first thing to check. If the boards are damp or moving, deal with that before patching the roof. If the fibreglass itself has split, tissue won’t be enough to mend it. The repair needs reinforcement bonded to sound material around the crack, then a compatible weatherproof finish. Tape or 225g mat may be part of that repair, but what you need will depend on the damage and the roofing system. If you’re unsure what to order, call us on 0800 612 7903 or use the contact form. Let us know the roof size, what it’s laid on and whether you need maintenance access or regular foot traffic. For an existing roof, photos taken from a safe position will help us understand the repair.
