Fibreglass Roofing questions answered in plain English

A GRP flat roof on a garage or extension costs €2,800 to €6,500, and €3,800 to €6,500 on a 30 to 40 m² kitchen extension. Laid, GRP runs €75 to €140 a m². The cost guide breaks it down.

Yes. GRP is glass-reinforced polyester: glass fibre mat set in resin and finished with a coloured topcoat. Roofers use both names for the same roof.

25 to 35 years when it's laid on a sound deck in dry weather. See how long it lasts.

Sometimes, if the deck underneath is dry and sound: €1,800 to €5,000. See GRP over an existing roof.

Nothing goes down in the wet: GRP laid on a damp deck fails. A good roofer keeps a sheet on site, stops, covers the roof and comes back on the next dry day.

Yes, while the resin and topcoat cure, for a day or so. Keep windows near the roof shut while it's laid; it goes once the roof has cured.

Ask for two in writing: the roofer's workmanship guarantee (10 years is common for a good one) and the material guarantee on the GRP system (often 20 to 25 years).

One flat roofing company that covers your county. One company only, not a list.

OSB3 tongue-and-groove sterling board has an open wood-strand surface that absorbs catalysed polyester resin deeply, locking the GRP laminate into the timber. Plywood has dense glue lines and smooth face veneers that can delaminate from rigid resin under summer heat.

A properly consolidated GRP roof laid on 18mm T&G OSB3 with a 25mm perimeter expansion gap at abutment walls moves quietly as a single unit. Clicking noises usually happen only when thin boards were under-fixed with smooth nails instead of ring-shank nails or decking screws.

Yes. Standard 450g/m2 GRP handles normal maintenance foot traffic for window cleaning or chimney access, while daily-use balconies and terraces are upgraded to 600g/m2 or double-layer 450g/m2 laminate with a quartz non-slip gritted topcoat.

The OSB3 deck must be bone dry and air temperature must sit between 5C and 30C with no rain until the resin and topcoat cure (typically 45 to 90 minutes per coat). On damp Irish days, the roofer strips and prepares in stages under temporary weather cover.

Very straightforward: the existing grey topcoat is sanded back to raw laminate with 40-grit abrasive paper, wiped clean with acetone, and new resin, chopped strand mat and topcoat are chemically bonded directly onto the original layer.

Over 90% of Irish domestic GRP roofs are finished in dark slate grey (RAL 7015) or lead grey (RAL 7043) to match natural slate and uPVC guttering, though any BS or RAL pigment can be mixed into the topcoat.