GRP roofing, commonly known as fibreglass roofing, is one of the most popular flat roofing systems for residential garages, extensions, and outbuildings in the UK. Its seamless application, hard surface, and long lifespan make it an attractive alternative to traditional felt and a competitor to EPDM rubber membrane systems.

What GRP Is

GRP stands for glass reinforced plastic, a composite material made by combining glass fibre reinforcement with a polyester or vinyl ester resin matrix. When the resin cures, it produces a hard, rigid, waterproof material that is lightweight but very strong. GRP is used across industry for boats, vehicle body panels, and structural components, as well as for flat roofing where its seamless application is particularly valuable.

Installation Process

GRP flat roofing is applied wet onto a prepared deck of plywood or OSB boards. The process begins with the application of a catalysed polyester resin to the deck surface, followed by the laying of glass fibre chopped strand matting into the wet resin. Additional resin is applied and the matting is wetted out thoroughly. After curing, the surface is sanded and a gel coat topcoat is applied to provide the final weather-resistant surface and UV protection. The entire system cures to a single seamless membrane with no joints or seams that could fail.

Performance and Lifespan

A properly installed GRP roof has an expected lifespan of 25 to 50 years or more. The hard surface is resistant to foot traffic, debris impact, and the thermal cycling that causes felt roofs to crack and blister. It is impervious to UV degradation when a quality gel coat topcoat is used and maintained. The seamless application eliminates the risk of joint failure, which is the most common cause of flat roof leaks in membrane systems with laps or seams.

Advantages and Limitations

The main advantages of GRP are its seamless coverage, hard surface, long lifespan, and fire resistance. The main limitations are the skill required for installation, the sensitivity of the curing process to temperature and humidity, and the higher cost compared with EPDM rubber. GRP must not be applied in cold or damp conditions; below around 5 degrees Celsius or in humid weather, the resin may not cure correctly. For this reason, experienced GRP roofers are careful about timing installation and may require dry conditions before proceeding.

Summary

GRP fibreglass roofing is a seamless flat roof covering with a lifespan of 25 to 50 years. It is installed wet as a glass fibre and polyester resin laminate over a timber deck, then finished with a gel coat topcoat. Its seamless nature eliminates joint failure risks. Professional installation is recommended due to the skill required and the sensitivity of curing to weather conditions. Cost is higher than EPDM but the hard, walkable surface is an advantage in some applications.