What is the difference between 450gsm and 600gsm GRP?

GSM stands for Grams per Square Metre of the glass matting. **450gsm** is the standard for most domestic roofs with no foot traffic. **600gsm** is heavyweight and recommended for balconies or areas where people will walk (e.g., for window cleaning). 600gsm requires 33% more resin, adding roughly 20-25% to the material cost.

Why can't GRP be installed when it's cold or wet?

GRP is a chemical reaction. If there is any moisture on the boards, the resin will not bond, leading to 'delamination' (peeling). In cold weather (below 5°C), the catalyst cannot start the curing process. Professional roofers use 'Winter Catalyst' for temperatures down to 0°C, but 10°C+ is the ideal for a perfect finish.

Does GRP roof crack over time?

A properly installed GRP roof on OSB3 boards with expansion gaps should not crack. However, 'Alligatoring' (surface cracks) occurs if the topcoat is applied too thickly. Structural cracks usually happen when the roof is larger than 50m² and hasn't been fitted with expansion joints.

The Seamless Waterproofing Solution

GRP Fibreglass Costs 2026

GRP (Glass Reinforced Plastic) is the ultimate choice for flat roofs. It creates a single, monolithic, jointless membrane that is incredibly tough and easy to repair. In 2026, it remains a premium alternative to traditional felt and EPDM rubber.

The "Layered" Strength

Unlike rubber or felt, GRP is built in layers on-site. This allows it to be molded perfectly around pipes, vents, and complex chimney junctions with zero joints.

1
Structural OSB3 Decking
2
Base Resin & Catalyst
3
Emulsion Bound Matting
4
Fire Retardant Topcoat

30+ Years

Anticipated Lifespan

Price per m² (2026)

Full Insulated System

£210 – £270 per m²

Standard GRP Replacement

£95 – £150 per m²

*Full system includes stripping the old roof, new OSB3 decking, and PIR insulation boards to meet 2026 Building Regs.

Specialist Labor

GRP requires chemistry. Mixing the catalyst at the wrong ratio or applying in the wrong humidity will cause total roof failure.

Specialist Team (Daily)£400 – £500

600gsm Heavyweight Premium+25%

Winter Catalyst Surcharge+£15 / m²

Why it's expensive

GRP involves multiple trips to the site for base coat and topcoat, plus the cost of expensive GRP trims (A200/B200).

The "Pinholing" Disaster

If a roofer applies the topcoat too quickly or on a windy day, tiny "pinholes" can form. These are invisible to the naked eye but allow water to slowly seep into the glass matting, rotting the roof from the inside. Always check for a Cure-It or similar manufacturer warranty to ensure the install was done correctly.

Vandal Proof

GRP is extremely hard. It is resistant to foot traffic and cannot be cut with a knife like rubber.

Fire Rated

Professional GRP systems are BROOF(t4) fire rated, making them safe for any UK property.

Solar Ready

The high structural strength of GRP makes it an ideal base for solar panel mounting systems.

GRP Roofing FAQ

Get a GRP Estimate

Use our 2026 calculator to see how GRP fibreglass compares to EPDM and Felt for your specific flat roof area.

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