How much resin do I need per m² for a GRP roof?

A GRP roof requires approximately 0.6 kg/m² of topcoat resin and 1.5 kg/m² of laminating resin. The topcoat provides UV protection and colour, while the laminating resin bonds with the CSM fibreglass mat.

What is the catalyst ratio for GRP resin?

Catalyst (MEKP) ratio depends on temperature: use 2% in cold weather (below 15°C), 1.5% at 15–20°C, and 1% above 20°C. Too much catalyst causes cracking; too little prevents proper curing.

What weight CSM mat should I use?

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.

Can I apply GRP in cold weather?

GRP should ideally be applied between 10°C and 25°C. Below 10°C, curing is unreliable even with extra catalyst. Never apply GRP in rain or on damp surfaces. The deck must be completely dry.

How long does a GRP flat roof last?

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.

TradeCalculator

GRP Flat Roof Calculator — Resin, CSM Mat & Trims

Calculate GRP fibreglass roofing materials: topcoat and laminating resin (kg/m\u00b2), CSM mat, trims and catalyst ratios by temperature.

Roof Length

Length of the flat roof deck

Roof Width

Width of the flat roof deck

Working Temperature

Affects catalyst ratio

Price Per kg Resin (optional)

How We Calculate This

This calculator works out the quantities of topcoat resin, laminating resin, CSM fibreglass mat, catalyst and edge trims needed for your GRP flat roof.

The formula

Topcoat resin (kg) = Roof area × 0.6 kg/m\u00b2

Laminating resin (kg) = Roof area × 1.5 kg/m\u00b2

CSM mat (m\u00b2) = Roof area × 1.15 (15% overlap)

Catalyst (ml) = Total resin (g) × Catalyst %

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.