GRP Roof Calculator: Resin, Mat and Topcoat to Order

A GRP roof is bought by weight, not by area. The mat is sold in grams per square metre, the resin and topcoat in kilo tubs, and the catalyst as a percentage of the resin. Most shortfalls come from forgetting that the laminate also runs over every trim and up every wall, and from working out resin on the deck area instead of the weight of mat it has to wet out. Enter your roof and edges and the calculator turns it into a shopping list.

How the calculator works

It starts with the area the laminate has to cover. That is the deck, plus a strip along every edge: the width of mat that runs over each drip or raised edge trim, and the height of the upstand on any edge that meets a wall. Then it works through the build-up in the order a roofer buys it:

• Mat. Laminate area, times the number of layers, plus your wastage allowance, gives the square metres of chopped strand mat. Times the mat weight gives the kilos.
• Resin. Resin is worked out from the weight of mat, not the area of roof, because heavier mat soaks up more resin. Kilos of mat times your resin to mat ratio gives kilos of resin, rounded up to whole tubs.
• Topcoat. One coat over the whole laminate area, at the coverage you enter, plus the same wastage allowance.
• Catalyst. A percentage of the weight of resin and of topcoat, worked out separately because they are mixed separately.

With the defaults, a 4 m by 3 m lean-to roof with a drip edge at the front, a wall at the back and raised edges on both sides has 13.6 m² of laminate. In one layer of 600 g mat with 10% for laps, that is about 15 m² and 9 kg of mat, 22.5 kg of resin (two 20 kg tubs), 7.5 kg of topcoat (one 10 kg tub) and about 0.6 kg of catalyst in total.

Setting the inputs

Mat weight and layers. Domestic GRP roofing systems mostly specify a single layer of 450 g or 600 g chopped strand mat on the deck. Some ask for a second layer or an extra strip of mat over board joints and trims. Follow the data sheet for the system you are buying rather than the default here, because the mat weight drives every other figure.

Resin to mat ratio. Hand-laid chopped strand mat usually takes between two and three times its own weight in resin to wet out fully. Too little leaves dry, white fibres that let water in; too much makes a brittle, resin-rich laminate. Your resin supplier will state the ratio they expect, so put that figure in.

Catalyst. The catalyst rate is not a fixed number. Manufacturers give a range and expect you to use less in warm weather, when the resin goes off quickly, and more in cool weather. Mix it by weight or with a proper dispenser, and never go outside the range on the tin. The calculator shows the figure for the percentage you enter so you can buy enough, not as a mixing instruction.

Edges. The defaults assume 100 mm of laminate over each trim and a 150 mm wall upstand. The flat roof fall calculator works to the same 150 mm upstand. Measure the face of the trims you are buying: a deep raised edge trim takes more mat than a slim drip.

Before you mix anything

GRP has to be laid on a sound, dry deck, in dry weather, above the minimum curing temperature for the resin. Our GRP fibreglass roofing guide explains why laminate laid in damp or cold conditions cracks and delaminates later. If you are still deciding between materials, the flat roof cost comparison calculator sets GRP against felt and EPDM over the life of the roof, and the EPDM calculator does the same sum for a rubber roof.

Common questions

How much resin do I need per square metre of GRP roof?

It depends on the mat. Multiply the mat weight in kilos per square metre by the resin to mat ratio. One layer of 600 g mat at a ratio of 2.5 needs 1.5 kg of resin per square metre; one layer of 450 g mat at the same ratio needs about 1.1 kg. Then add your allowance for laps and waste.

Is 450 g or 600 g mat better for a roof?

Neither is better in general; use what the roofing system specifies. Heavier mat gives a thicker laminate per layer but needs more resin and takes longer to wet out properly.

Why is my resin figure higher than a kit suggests?

If a kit is sized on the deck area alone, it will come up short. Once the laminate is taken over trims and up walls, a small roof needs noticeably more material than its plan area: the 4 m by 3 m example above has 13% more laminate than deck, before any wastage. The calculator shows the deck and laminate areas side by side so you can see the difference.

Does the calculator include deck boards and trim adhesive?

No. It gives trim lengths but not the deck boards, nails or trim adhesive, which depend on your deck layout and the trim system you choose.