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.

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.

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.