How Much Resin, Topcoat & Matting Do I Need for a GRP Roof?

The coverage rates that matter

Resin consumption depends on the weight of matting you're wetting out. A 600g/m² chopped strand mat laminate takes about 1.5 kg of resin per m². A lighter 450g laminate takes about 1.2 kg per m². These rates include wetting the deck and consolidating the mat with a paddle roller — they're real-world figures, not lab minimums.

Topcoat covers at about 0.72 kg per m² in a single flood coat. Matting weighs exactly what its name says: 600g mat is 0.6 kg per m², 450g mat is 0.45 kg per m². Add roughly 10% to the matting for cutting waste and overlaps — you always lose some to trimming around edges and details.

Two consumables scale with area too. Catalyst runs at 1–4% of the combined resin and topcoat weight depending on air temperature (2% is the standard dose at 18–25°C). Acetone for cleaning rollers and tools works out at about 0.1 litres per m² across a job.

Worked examples: 10, 15 and 25 m²

A 10 m² shed roof at 600g: resin 10 × 1.5 = 15 kg, topcoat 10 × 0.72 = 7.2 kg, matting 10 × 0.6 × 1.1 ≈ 6.6 kg, catalyst (15 + 7.2) × 2% ≈ 0.45 kg, acetone about 1 L. You'd buy one 20 kg tub of resin, one 20 kg tub of topcoat, 7 kg of matting, a 5 kg catalyst and a 5 L acetone.

A 15 m² garage at 600g: resin 22.5 kg (two 20 kg tubs), topcoat 10.8 kg (one tub), matting ≈ 10 kg, catalyst ≈ 0.67 kg, acetone 1.5 L. Note the resin tips just over one tub — this is where people get caught short mid-lay.

A 25 m² extension roof at 600g: resin 37.5 kg (two tubs), topcoat 18 kg (one tub, just), matting ≈ 16.5 kg, catalyst ≈ 1.1 kg, acetone 2.5 L. At this size a second tub of topcoat is a judgement call — a porous or textured deck will drink more, so most pros carry it.

Pack sizes and minimum-waste buying

Resin and topcoat come in 20 kg tubs. Catalyst comes in 5 kg or 25 kg. Acetone comes in 5 L or 25 L. Matting is the flexible one — we sell it per kilo, so you buy exactly what the roof needs rather than a fixed 30 m roll.

The buying rule is simple: round each material up to the smallest combination of packs that covers your need. 44 kg of resin is two 20s and… nothing else available in resin, so three 20s? No — resin is 20 kg tubs only, so 44 kg means 3 tubs unless you drop to a 450g laminate on part of the job. This is exactly the sort of trap the kit builder's minimum-waste solver exists for.

Don't be tempted to run resin thin to stretch a tub. Starved laminate — white, glassy patches where the mat isn't fully wetted out — is the single most common cause of early GRP failure. If the maths says 22.5 kg, buy 40 and keep the spare sealed; unopened resin stores for months somewhere cool.

What the numbers don't cover

These rates are for the flat field of the roof. Upstands, skylight kerbs and edge details add area you can miss on a plan measurement — measure the developed surface, not the footprint, and add 75 mm bandage laminate at every deck joint and trim junction.

Perimeter items are a separate calculation: trims in 3 m lengths per edge type, roughly one cartridge of trim adhesive per 6 m of perimeter, and jointing bandage along trim edges and board joints. See our trims guide for which profile goes where.

If arithmetic on a Sunday morning isn't your idea of fun, the kit builder does all of this from a drawing of your roof — and shows the working for every line, so you can see it's the same maths as this page.

Frequently asked questions