How a GRP Roof Is Laid

1. DeckOSB3 18 mm tongue and groove boards, screwed or ring-shank nailed to the joists, every edge supported
2. ResinRoofing resin with hardener, about 1.5 kg a m², brushed and rolled onto the dry deck
3. Glass fibre matChopped strand mat rolled into the wet resin until it goes clear: 450 g/m² for most roofs, 600 g/m² for walkways and balconies
4. TrimsPre-formed GRP edge trims, bonded round the edges and laminated in
5. TopcoatA pigmented topcoat, about 0.4 kg a m², after a light sanding: grey as standard, with anti-slip aggregate where it's walked on

Each GRP system has its own manual, and the roofer should follow it to the letter: that's what the material guarantee depends on.

• GRP is never laid in wet or damp conditions, or on a damp deck: it doesn't bond, and it fails. A good roofer keeps a sheet on site and stops and covers the roof if rain comes.
• Hardener is mixed at 1 to 4%, adjusted for the temperature, and resin and topcoat aren't put on surfaces above 35°C.
• The laminate cures in about 4 to 6 hours at 15°C or above before the topcoat goes on. Below 10°C the cure is unreliable.

In an Irish winter that means a dry spell, and sometimes a wait. A GRP roofer who presses on in drizzle is a red flag.

The risk with GRP is the workmanship: it needs the right preparation, weather and curing, and a bad job cracks. Most cracks start at the trims, at joints in the deck under the laminate, or where the roof meets a wall.

• Trims bonded with trim adhesive and laminated in, not just nailed.
• Upstands formed up the wall, with a flashing over them.
• Deck boards with every edge supported, so they don't move under the laminate.
• A fall to the outlet, so water doesn't sit.

Five questions before you agree

1. Which GRP system, and are you trained on it?
2. A new OSB3 deck, or the old one?
3. What mat weight?
4. What happens if it rains on the day?
5. What does the guarantee cover: materials, workmanship, or both, and for how long?