GRP (Fibreglass) Flat Roofing

GRP (glass-reinforced plastic) roofing, widely called fibreglass roofing in the UK, is a liquid-applied waterproofing made by laminating glass-fibre matting with resin on the roof. It is especially common on domestic flat roofs in Britain and Ireland, such as extensions, garages and dormers.

Construction

A typical system is built in stages:

1. a smooth, rigid timber deck, commonly OSB or plywood boards laid tight and fixed
2. edge trims and drip details fixed and laminated in
3. a laminate of catalysed polyester resin worked into chopped strand glass-fibre mat with rollers to remove air
4. after curing, a pigmented topcoat that protects the laminate from UV and gives the finished colour

Some systems use other resins, such as acrylic or polyurethane-modified chemistries, and are effectively reinforced liquid-applied membranes.

Characteristics

• Seamless: the whole roof, including upstands and trims, is one laminate.
• Light and hard: suitable for occasional foot traffic and easy to clean.
• Rigid: GRP has limited ability to accommodate deck movement, so it needs a stable, well-fixed deck; cracking at board joints and trims is a common defect when decks move.
• Weather-sensitive: the resin must cure in dry conditions within suitable temperature ranges.

Suitability

Because of its rigidity and dependence on a stable timber deck, GRP is considered less suited to large roofs, where single-ply, RBM or other liquid systems are generally preferred. Like other liquids, quality depends heavily on installer skill and correct resin and catalyst ratios.

Comparison with other liquids

GRP is one of several resin-based flat-roof options. Modern cold-applied reinforced liquid systems using polyurethane or PMMA resins with polyester fleece are more flexible and are often chosen for larger roofs, balconies and detailing.