GRP fibreglass roofs: lifespan, defects and maintenance

What is a GRP roof?

GRP is a composite material made of polyester resin reinforced with fine strands of glass. GRP stands for Glass Reinforced Polyester, also given as Glass Reinforced Plastic, and GRP roofs are commonly called fibreglass roofs.

The resin and glass fibre form a strong, waterproof membrane. That makes GRP a favourite for flat roofs and other applications, and it has become increasingly popular in the UK for its durability, versatility and appearance.

How does a GRP roof perform?

GRP performs well on flat roofs because it forms a seamless, watertight membrane with no joints for water to find. Its main performance characteristics are:

• Waterproofing: the seamless finish prevents water ingress, which is particularly useful on flat roofs, as they are more prone to water pooling and leaks.
• Durability: GRP resists impact, abrasion, chemical exposure and everyday wear and tear, and it can withstand foot traffic for occasional maintenance access.
• Flexibility: it can be moulded to fit various roof shapes and sizes, including complex designs and details.
• Low maintenance: regular inspections and cleaning are usually enough to keep it in good condition.
• UV resistance: it resists UV radiation, which helps prevent degradation and discolouration over time.

Buying a home with an older roof? Get a survey price for the home you are buying.

How long does a GRP roof last?

A GRP roof that is installed correctly and maintained properly can last between 20 and 30 years, and in some cases even longer. Four factors decide where in that range a roof falls:

1. Quality of materials: the quality of the resin and glass fibres has a significant effect on durability and resistance to the weather and wear.
2. Installation: a smooth, clean and dry substrate (the surface the GRP is laid on), correct application of the resin and fibreglass layers, and adequate curing time.
3. Maintenance: clearing debris, inspecting for damage and dealing with minor issues promptly extend the roof's life. Neglect can lead to premature deterioration and costly repairs.
4. Environmental factors: heavy rain, snow and UV radiation can affect longevity, although GRP is known for its resistance to these elements in the UK climate.

What defects affect GRP roofs?

The common defects are cracking, delamination, pinholes, blistering, discolouration, leaks and ponding water. Each has a typical cause:

What risks come with installing a GRP roof?

Most GRP risks arise during installation, when poor preparation or curing can compromise the roof for its whole life.

• Health and safety: the work involves chemicals that can be harmful, so gloves, masks and eye protection should be used, and working at height needs measures to prevent falls.
• Curing time: premature exposure to rain or other conditions while the resin cures can compromise the roof's integrity. The manufacturer's guidelines should be followed and the roof protected during this period.
• Surface preparation: dust, oil or moisture on the substrate can affect adhesion of the resin and fibreglass layers, leading to leaks and failures.
• Thermal movement: GRP expands and contracts with temperature changes, which can cause stress and cracking over time. Expansion joints and allowing for movement help mitigate this.
• UV degradation: prolonged intense sunlight can still cause some degradation, which a UV-resistant topcoat helps to resist.
• Chemical exposure: solvents and strong acids can damage GRP, so spills should be cleaned promptly.

How is a GRP roof installed?

A GRP roof is built up in layers on a sound, solid deck, typically of OSB3 boards. The steps are:

1. Preparation: check the existing roof structure is sound and can support the covering, remove debris and contaminants, and repair cracks or holes so the substrate is smooth, even and dry. Edge trims are fixed in place to guide water run-off.
2. Primer: a primer compatible with the resin and substrate is applied evenly by roller or brush and left to dry as the manufacturer instructs.
3. Fibreglass mat: the mat is cut to size with overlaps at the edges and laid smooth, free from wrinkles or air bubbles.
4. Resin: the resin is mixed with the appropriate catalyst and rolled or brushed over the mat until it is fully saturated, working in sections to keep a wet edge and avoid dry patches.
5. Curing: the resin cures according to the manufacturer's guidelines while the roof is protected from rain.
6. Topcoat: once cured, a UV-resistant topcoat is applied evenly, giving the waterproof, finished look.

Various systems are on the market for different needs and budgets, including Cure It, Topseal and Scott Bader. Warranty, finish and overall cost are the factors to weigh when choosing one.

How should a GRP roof be maintained?

A GRP roof should be inspected by a roofer at least twice a year so problems are caught early. Routine care includes:

• Clearing leaves and other debris, which can trap moisture.
• Checking for cracks or other damage, especially after extreme weather.
• Re-coating when the top layer wears, to maintain the roof's integrity.
• Cleaning the surface periodically and dealing with any issues promptly.

What affects the cost of a GRP roof?

The cost depends on the size and complexity of the roof, the quality of materials and labour rates.

• Materials: resin, fibreglass mat, primer and topcoat vary in price by quality and brand. Higher-quality materials may cost more at first but can offer better durability.
• Labour: this varies with the roof's complexity, the installers' experience and regional rates.
• Maintenance: regular inspections and cleaning should be budgeted for.
• Repairs: costs depend on the extent of damage. Prompt attention helps keep them down.

How does GRP roofing affect the environment?

GRP's long lifespan means fewer replacements, which reduces waste and resource use. Other points made for it are:

• Minimal maintenance reduces the need for cleaning chemicals.
• GRP is not biodegradable.
• Sustainably sourced materials and installers who follow environmentally friendly practices reduce the impact further.

What to look for when viewing a property with a GRP roof

• Cracks, particularly where the roof takes foot traffic.
• Blisters, pinholes or signs of the layers separating.
• Discoloured or worn areas of topcoat.
• Standing water after rain, and blocked outlets or debris.
• Edge trims in place to guide water run-off.
• Signs of leaks inside, below the roof.

What a survey can — and cannot — tell you

A building survey provides an informed assessment of the visible condition of a property. Inspections are non-intrusive and limited to areas that are accessible at the time of inspection.

For a GRP roof, a survey records visible defects early, before cracking, blistering or ponding lead to leaks and costly repairs.

• Visible cracking, blistering, pinholes and discolouration of the topcoat.
• Signs of ponding water and the condition of drainage.
• Edge trims and the general finish of the covering.
• Debris build-up and evidence of leaks below the roof.

However, there are limitations to what a survey can determine without intrusive investigation.

• The condition of the decking beneath the covering.
• The quality of the resin and glass fibres used.
• Whether the substrate was clean and dry, and the resin properly cured, at installation.
• Delamination that is not yet visible at the surface.

Where higher-risk indicators are identified, further specialist or intrusive investigation may be recommended.