10 Common GRP Roofing Mistakes (and How to Avoid Them)

Catalyst mistakes

Guessing the dose. Catalyst is 1–4% by weight of resin depending on air temperature (2% at 18–25°C, 3% at 11–17°C, 4% at 5–10°C, 1% at 26–30°C) — not 'a glug'. Under-dosed resin never fully cures and stays cheesy; over-dosed resin gels in the bucket or cures brittle. Weigh the resin, measure the catalyst.

Not adjusting through the day. The dose that suited a 9°C morning will flash-set at 2pm on a sun-warmed deck. Recheck the temperature every couple of hours and adjust the next mix, not the current one.

Poor mixing. A full minute of stirring, scraping sides and bottom. Streaks of unmixed catalyst mean brittle lines; pockets of uncatalysed resin mean soft spots that never cure. Both are invisible until the roof is down.

Weather mistakes

Laying below 5°C. There is no catalyst dose that fixes cold — the cure stalls, the laminate stays soft, and the usual outcome is stripping the roof. If the thermometer says 4°C, the answer is no.

Ignoring dew and damp. Moisture on or in the deck is the classic cause of blistering and bond failure — the resin cures over the water, then the water vapour pushes it off. The deck must be visibly and physically dry, and the overnight forecast matters because the topcoat needs to go tack-free before dew falls.

Racing incoming rain. Rain on uncured laminate turns the surface milky and ruins the bond for the next layer. If the radar says two hours and the roof says three, laminate half the roof properly rather than all of it badly — a clean overlap joint onto yesterday's work is easy; a rain-struck laminate is not.

Laminating mistakes

Starving the laminate. White, dry-looking patches in the cured laminate are unwetted glass — a sponge for water and the number-one early failure. The mat should go fully translucent; at 600g that takes about 1.5 kg of resin per m², and 'making the tub stretch' is a false economy of around £100 against a failed roof.

Skipping consolidation. The paddle roller isn't optional — it's what drives out the air. Trapped air bubbles become blisters the first hot day and punctures the first frost after that.

Skipping the bandage. Every board joint and trim edge needs a 75 mm bandage strip laminated over it before the main lay-up. Roofs that crack along neat straight lines a year in are almost always missing this step — the board joint moves, the unreinforced laminate above it can't.

Trim and detail mistakes

Wrong trim on the edge. A drip trim (A200) on an edge that should keep water on the roof sheets water down your walls; a raised edge (B230) where the gutter should be gives you a swimming pool. Name each edge's job with water first — the trim then picks itself.

Site-mitred corners instead of corner units. Two trims meeting at a corner want the pre-formed corner moulding; a mitred, bandaged bodge is the single most common leak point we're asked to diagnose.

No fall. GRP tolerates ponding far better than felt, but a dead-flat roof still concentrates every future problem in the puddle. 1:80 minimum to the gutter edge — fixed with firring strips at deck stage for pennies, unfixable afterwards.

Topcoat mistakes

Laminating with topcoat, or topcoating too thin. Topcoat contains wax so it cures tack-free — used as a laminating resin it stops layers bonding. And it's a flood coat at about 0.72 kg/m², not a paint: scrubbed-out thin topcoat weathers chalky and lets UV at the laminate underneath.

Leaving the laminate bare for weeks. The laminate needs its topcoat within a few days — after that the surface needs abrading for the topcoat to bond, and an unprotected laminate soaks up UV and moisture in the meantime.

Walking the roof too soon. Off the roof for 24 hours after topcoat, gentle for a week. Footprints in a soft topcoat are permanent, and flexing an under-cured laminate can crack it before it ever sees weather.

Frequently asked questions