What Is GRP?: Glass Reinforced Plastic Explained

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What is GRP? Glass Reinforced Polymer Uses and Properties: GRP stands for Glass Reinforced Polymer, a composite material made by combining glass fibres with a polymer resin. It's also commonly known as fibreglass, and sometimes referred to as GFRP (glass fibre reinforced polymer) or FRP (fibre reinforced polymer). GRP is prized for its high strength-to-weight ratio, corrosion resistance, and electrical non-conductivity, which is why it's used everywhere from handrails and stair treads to wind turbine blades and helicopter rotors.

GRP explained

GRP gets its strength from a division of labour between its two components. The glass fibres provide tensile strength, resisting being stretched or pulled apart, while the resin matrix binds the fibres together, transfers load between them, and protects them from moisture and impact. Neither material alone performs anywhere near as well; it's the combination that gives GRP its high strength-to-weight ratio.

The resin itself is a thermoset plastic, meaning it undergoes a one-way chemical reaction as it cures, forming permanent molecular bonds that can't be melted and reshaped afterwards. This is a key difference from conventional thermoplastics, which soften when heated and can be reformed repeatedly. It's also why GRP holds its shape and properties reliably over decades outdoors, once cured, it isn't going to soften, warp, or deform under heat the way a thermoplastic product might.

What is GRP made of?

Not all GRP is made the same way. Manufacturers choose between different resin types depending on what the finished product needs to withstand, GRP profiles, for instance, are typically produced from one of these resin systems depending on the environment they're specified for.

Polyester resin is the more affordable, widely-used option, common in general-purpose products like tanks, panels, and grating, though it's more prone to absorbing water over time and offers weaker resistance to acids and alkalis. Vinyl ester resin costs more but delivers greater strength, corrosion resistance, and resistance to fatigue and impact, which is why it's the more common choice for GRP products facing harsher conditions, including boats and wind turbine blades.

The glass fibre itself matters too, most standard GRP uses E-glass fibre, though products destined for particularly aggressive chemical environments sometimes use C-glass instead for its extra chemical resistance

How are GRP products manufactured?

GRP manufacturing methods fall into two broad categories: open moulding, where the laminate is exposed to open air during fabrication, and closed moulding, where the material cures inside a sealed, two-sided mould or vacuum bag. Pultrusion sits apart from both, as a continuous process rather than a single-piece mould-and-cure method. The method chosen affects everything from surface finish to production volume, whether you're looking at GRP nose covers or structural profiles.

Open-mould methods

GRP hand lay-up moulding: The hand lay-up GRP process sees a glass fibre reinforcement (a woven or chopped strand mat) and occasionally inner core mats form a sandwich between layers of glass for extra strength. This ‘sandwich’ is cut to shape before being laid on the surface of a waxed open mould. After this, resin is applied to fully saturate the glass/core laminate. This is left to cure before being removed, trimmed, painted, and polished.

GRP spray lay-up moulding: The spray lay-up process is semi-automated. A pressurised spray gun with a glass fibre ‘chopper’ head unit is used to apply the laminate to the mould in a continuous process. The fibre is chopped directly into the resin stream being directed into the mould.

Closed-mould and automated methods

GRP resin transfer moulding: GRP resin transfer moulding sees a mixed resin injected at pressure into a mould. This process usually is used to produce high volumes of smaller parts of consistent quality.

GRP compression moulding: In the GRP compression moulding process, a preheated polymer is placed in an open mould cavity before it is closed in with a top plug to ensure the material is in contact with all the mould’s areas.

GRP long fibre injection moulding: The highly automated GRP long fibre injection moulding process is used to manufacture precision-made parts. The process requires a GRP moulding system which uses a polyurethane matrix and long glass fibres which are applied robotically to a mould, where the matrix and fibre mix is compressed, cured, and hardened to the desired shape.

Pultrusion

GRP pultrusion: Used to make structural profile sections with a constant cross-section, such as GRP profiles, this continuous process pulls glass fibres through a resin bath and then a heated die, shaping and curing the material in one ongoing pass rather than a single batch. Colour pigment is mixed through the resin before curing rather than applied afterwards, so it runs through the material's full thickness and won't wear off the way a surface coating could.

Properties and uses of GRP products

GRP products such as GRP sheets have many desirable properties and uses. Properties include:

Property	Why It Matters
Strength-to-weight ratio	Matches or exceeds steel's tensile strength at a fraction of the weight, making it far easier to lift, transport, and install than a metal equivalent.
Lightweight	A direct consequence of that strength-to-weight ratio; GRP structures weigh significantly less than steel or aluminium alternatives of comparable strength.
Durability	Resists impact, wear, and weathering over decades of outdoor use, without the ongoing maintenance metal or timber typically demand.
Corrosion and chemical resistance	With no metal content, GRP can't rust, and its resin matrix (polyester or vinyl ester, depending on the grade) resists a wide range of chemicals rather than reacting with them.
Electrical non-conductivity	Unlike steel or aluminium, GRP doesn't conduct electricity, making it a safer choice around live electrical equipment.
Radar and radio wave transparency	Lets radio frequency signals pass through largely undisturbed, rather than reflecting or absorbing them the way metal does, a property put to use in radomes and antenna housings.
Warm to the touch	Low thermal conductivity keeps GRP closer to ambient temperature than metal, so it doesn't get scalding hot or freezing cold, a useful safety property for handrails and walkways.
Fire performance	Standard GRP isn't inherently fire-resistant, but flame-retardant additives can be built into the resin during manufacturing where this is specified.

Uses of GRP include:

Use	Why GRP Suits This
Anti-slip flooring for wet and dry environments	Textured, gritted surfaces provide grip in areas prone to moisture, oil, or heavy foot traffic, helping meet HSE's requirements for suitable, slip-resistant workplace flooring.
Electronic enclosures	Electrical non-conductivity protects both the equipment and anyone working near it.
Handrails	Corrosion resistance and low maintenance suit exposed outdoor and industrial settings.
Helicopter rotor blades	High strength-to-weight ratio keeps blades light without sacrificing structural performance.
Wind turbine blades	Combines the strength needed to withstand constant flexing with a fraction of the weight of metal, which is why GRP dominates composite material use in this sector.
Safety grating in public and industrial areas	Anti-slip surface and corrosion resistance hold up under constant footfall and weather exposure.
Sporting equipment such as crossbows and kayaks	Lightweight strength improves performance without compromising durability.
Drain coverings	Corrosion resistance withstands constant water exposure without rusting or degrading.
Water pipes	Chemical resistance and non-corroding properties suit long-term water and wastewater use.
Automotive, electronic, transportation, and leisure applications	Broad chemical resistance, low weight, and durability suit a wide range of further industries.

Benefits Of GRP

In addition to the beneficial properties mentioned above, GRP offers the following benefits:

Long lifespan

Requires little to no maintenance

Sustainable, low-energy manufacturing process

Easy fabrication and machining

Insect infestation resistant

Wide operating temperature range

Now that you know more about GRP, it’s uses and benefits, you can see why glass reinforced polymer remains a popular choice across diverse industries around the world. Choose the best GRP products for your needs with BM Steel.

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Gritted Solid Top Plate GRP Panel Black RAL 9005 - 4mm

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6m Pultruded GRP Equal Angle 25 x 25 x 3mm

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6m GRP Handrail Tube Top Rail - 50mm Diameter - Yellow

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FAQs about Glass Reinforced Polymer

Is GRP stronger than steel?

It's a common assumption that plastic automatically means weaker, but that's not quite right. The real answer depends on the type of load involved. Steel keeps a clear advantage in compressive strength, resisting being crushed, and in stiffness, resisting bending under load. GRP's edge shows up specifically in tension, resisting being pulled apart, where it can match or outperform steel once the weight difference is factored in (see the properties table above for the full comparison). In practice, this is why GRP tends to replace steel where tension and weight matter most, and why steel remains the better choice wherever a structure needs to resist crushing or hold its shape under heavy compressive load.

What is the lifespan of GRP material?

Properly installed GRP roofing commonly lasts 25 to 50 years, well beyond felt (10 to 15 years) or EPDM rubber (20 to 30 years). UV-stabilised topcoats can extend this further still, since sunlight exposure is one of the main factors limiting how long GRP lasts outdoors. Exact lifespan depends on the specific product, its environment, and installation quality, but a 25 to 50 year expectation is a reasonable benchmark across most GRP applications, not just roofing.

What is the difference between GRP and PVC?

The two aren't really competing products, they're built differently from the ground up. PVC is a single, uncomplicated plastic (polyvinyl chloride) with no reinforcement, valued for being cheap, quick to install, and easy to work with, which is why it's so common in pipework and window frames. GRP is a composite, glass fibres embedded in a resin matrix, giving it substantially greater strength and rigidity than PVC on its own, along with better performance in corrosive or high-pressure environments. PVC generally wins on upfront cost and ease of installation. In short: choose PVC for straightforward, lower-stress applications, and GRP where strength, chemical resistance, or structural performance matter more than initial price.

Can you walk on a GRP roof?

Generally, yes, provided it's been built for that purpose. A walkable GRP roof needs a thicker laminate than a standard one, with 600g per square metre of chopped strand mat generally treated as the minimum for safe foot traffic; a standard roof uses a lighter laminate not designed to take regular weight. If you're unsure which specification your roof was built to, it's worth getting it checked before assuming it's safe to walk on, since a surveyed, confirmed walkable specification is a very different thing from a lighter covering that simply happens to be made of the same material.

Posted by Killian Ward
7th September 2023

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