What is GRP Fibreglass Roofing?

GRP fibreglass roofing is a seamless, jointless flat roof system made by laminating fibreglass matting with polyester resin that cures to a rigid waterproof shell. It lasts 30-50 years with a 20-year guarantee — far longer than felt. It suits complex Reading flat roofs with penetrations and upstands.

GRP in Plain English

GRP stands for Glass Reinforced Polyester — a composite material made by laminating fibreglass matting with a liquid polyester resin that cures to a hard, rigid shell. It is the same material used for boat hulls, car body panels and water storage tanks. On a flat roof, it is applied wet to the deck and cures in place to form a seamless, waterproof surface with no joints, seams or overlaps where water can enter.

This is the key advantage over both traditional felt and EPDM rubber: a correctly installed GRP flat roof has no joints. Every other flat roofing system has joints somewhere — between sheets, at edges, around penetrations. Every joint is a potential failure point. GRP eliminates them.

How GRP Is Installed

GRP fibreglass roofing systems installed to manufacturer specification carry guarantees of up to 25 years, reflecting the seamless single-shell construction that eliminates the lap joints where built-up felt systems typically fail first.

Installation requires a sound, clean deck — typically OSB (oriented strand board) or plywood. The deck is primed, then a layer of fibreglass matting is laid over the entire surface. The installer applies a catalysed polyester resin across the matting, working quickly before it begins to cure. A second layer of matting and resin follows. Once fully cured — typically a few hours in warm dry conditions — a topcoat of pigmented gel coat is applied, which provides UV protection and the finished colour.

Edge details, upstands and drainage outlets are all formed in the same material, with the laminate running continuously from the deck up the upstand and over the edge. This seamless integration of all the details is what makes a well-done GRP job so durable — there is genuinely nowhere for water to find a path through.

💡 GRP installation is weather-dependent. The resin requires dry conditions and temperatures above approximately 5°C to cure correctly. We do not lay GRP in rain or when frost is forecast within the curing period. An installation carried out in poor conditions can delaminate — this is the most common source of GRP failures we see on properties in Reading.

GRP vs Felt: The Comparison Reading Homeowners Need

The thousands of flat-roofed garages and extensions across Woodley, Earley, Whitley and Southcote were built with 3-layer built-up felt. Here is how GRP compares:

	3-Layer Felt	GRP Fibreglass
Service life	10–20 years	30–50 years
Joints	Multiple (sheet overlaps, edge details)	None (seamless)
Thermal movement	Cracks at low temps	Rigid, minimal movement
UV resistance	Degrades over time	Gel coat provides long-term protection
Maintenance	Periodic inspection, patching	Minimal — wipe clean if needed
Typical cost (20m²)	£800–£1,400	£1,800–£3,000
Manufacturer guarantee	None typically	20–25 years

GRP vs EPDM: When to Choose Which

EPDM rubber is our other main flat roofing recommendation. For most standard rectangular garage or extension roofs in Reading, EPDM is slightly cheaper and performs comparably in service life. GRP is the better choice when:

The roof has complex geometry — irregular shapes, multiple angles, internal corners
There are multiple penetrations — soil pipes, cables, rooflights — where seamless integration around the penetration is important
There are multiple upstands and abutments where edge details need to be continuous with the deck
The deck has variable falls that need to be managed without creating pockets

For a simple rectangular garage roof in Woodley or Earley with one drainage outlet, EPDM and GRP are both appropriate and the cost difference makes EPDM the pragmatic choice. For a complex kitchen extension roof in Caversham with two dormers, a soil stack penetration and a valley, GRP's seamless nature makes it the right specification.

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Last reviewed: 25 April 2026 by Reading Roofing Experts.

GRP in Reading's Flat Roof Context

Reading has tens of thousands of flat-roofed garages and kitchen extensions built between 1960 and 1990. Most were built on 3-layer built-up felt. That felt is now 35 to 65 years old — two to four times its design life. When these roofs start to fail — and they are failing across Woodley, Earley, Whitley and Southcote in increasing numbers — the replacement decision is between felt, EPDM and GRP.

For the standard garage flat roof in Reading — a simple rectangle with one drainage outlet, a straight parapet on one or two sides — EPDM and GRP are both appropriate. The cost difference is modest. We typically recommend EPDM for this configuration because it is marginally simpler and faster to install and performs identically in service. For the kitchen extension with a soil pipe penetration, a bay window roof with complex drainage, or any roof with more than two upstand details, GRP is the better specification — the seamless laminate handles complex geometry more reliably than any sheeted system.

One Reading-specific factor is the garage conversion trend. Many of the integral garages on Woodley, Earley and Lower Earley estate properties are being converted to habitable rooms. A converted garage requires the flat roof above to perform to a higher standard — temperature regulation, airtightness, and longevity all become more important when people are living in the space below. GRP with rigid insulation above or below the deck is the correct specification for converted garage flat roofs. EPDM can be used but requires careful insulation design to avoid condensation at the deck interface.

Common Questions

A correctly installed GRP roof carries a 20-year manufacturer guarantee and has a realistic service life of 30 to 50 years. The main factor in longevity is installation quality — GRP laid in poor conditions or by an inexperienced installer can delaminate within a few years. The material itself is extremely durable when correctly applied and protected by a good gel coat.

It depends on the roof. For complex shapes with multiple penetrations and upstands, GRP is the best option — its seamless nature eliminates every joint that could fail. For simple rectangular roofs, EPDM rubber is slightly cheaper and performs comparably. We specify based on your specific roof geometry rather than a blanket preference.

Sometimes, if the existing deck is structurally sound and dry. Laying GRP over a wet or degraded deck will trap moisture and cause problems. We inspect the deck before specifying the approach — sometimes replacement of the deck boards is needed before a new GRP system can be laid. We advise honestly on which approach applies to your property.

The most common cause of GRP cracking in Reading is deck movement — if the deck boards beneath the GRP are moving significantly with temperature or load changes, the rigid GRP laminate cracks at the points of movement. This is usually a sign that the GRP was laid without adequate allowance for expansion joints, or over a deck that has deteriorated. A cracked GRP system requires assessment to determine whether patching is viable or full replacement is needed.

We assess at survey and recommend based on your roof's geometry. The deciding factors are roof shape complexity, number of penetrations and upstands, deck condition, and budget. Call us on 0118 327 2383 for a free survey and we will specify the right system for your property.

Minimal. A GRP roof in good condition requires little more than keeping it clean and clear of debris. Inspect it annually — look for any cracking, delamination at edges or upstands, or blocked drainage outlets. Most GRP roofs in Reading require no active maintenance for 10 to 15 years after a correctly performed installation.

On a standard 20m² garage roof, 3-layer felt replacement costs approximately £800 to £1,400. GRP fibreglass costs approximately £1,800 to £3,000. The difference is £800 to £1,600 upfront. GRP lasts 30 to 50 years against felt's 10 to 20 years. Over a 40-year period, the felt roof requires at least one — probably two — replacements while the GRP needs none. The total cost of the felt option over 40 years typically exceeds GRP.