GRP Flat Roof Calculator — Resin, CSM Mat & Trims

A GRP roof requires approximately 0.5 kg/m² of topcoat resin (manufacturer range 0.4–0.5 kg/m²) and 1.5 kg/m² of laminating resin for 450g CSM. The topcoat provides UV protection and colour, while the laminating resin wets out and bonds the chopped strand mat. Those are rates per m² of laminate, not per m² of roof plan, and they are bare coverage figures with no allowance in them. Add the turn-up at any abutting wall and a wastage allowance for detailing and roller losses before you multiply: Cure It’s own materials schedule orders 35 kg of resin for a 20 m² roof, where the bare rate gives 30 kg.

Catalyst (MEKP) ratio increases as it gets colder, per the manufacturer addition chart: roughly 4% at 6–12°C, 3% at 13–20°C, 2% at 21–28°C and 1% at 29–35°C. Never use less than 1% (even in summer it must fully cure) or more than 4% (the gel time will not reduce further). Too much catalyst can cause cracking; too little leaves a tacky, uncured laminate. Do not apply GRP below about 5°C.

Yes. BS 6229:2018 requires the waterproof layer to be turned up to not less than 150mm above the finished roof surface at all abutments, and on a protected roof (paving, gravel, green) that 150mm is measured above the top of the protective layer. The only relaxation is at door and balcony thresholds, where level access allows 75mm with a projecting sill, voids and falls away from the threshold. A free edge is different: it is closed with a moulded edge or drip trim, and the laminate only laps that trim by about 50mm on the deck, so nothing is added there. Tell the calculator how many sides abut and it adds the turn-up on those runs only.

Standard GRP flat roofs use 450g/m² chopped strand mat (CSM). For heavy-duty applications or walkways, you may use 600g/m² or a double layer. Allow 15% extra for overlaps.

Standard GRP roofing resin and topcoat can be applied down to about 5°C (and rising), which is why the catalyst chart runs to 4% for the coldest 6–12°C band — more catalyst compensates for the slower cure as it gets colder. Curing simply takes longer in the cold, so allow extra time before recoating. Below about 5°C, switch to a manufacturer cold-cure (winter-grade) resin and topcoat, which are formulated to work down to 0°C. Never apply GRP in rain or on a damp deck — the surface must be completely dry whatever the temperature.

A properly installed GRP flat roof lasts 25-30 years or more. It's seamless, waterproof and UV stable. It can be walked on and is low maintenance. Periodic cleaning and checking of trims is recommended. For comparing GRP with other membrane options, see our Flat Roof Membrane Calculator.