What Is GRP Roofing

Glass reinforced plastic, usually shortened to GRP and often called fibreglass, is a hard wearing liquid roofing system that cures to form a seamless, waterproof skin. It is widely used across the UK on flat and low pitched roofs, dormers, bay windows, terraces, porches, and complex junctions where a single piece membrane or traditional felt struggles to achieve neat detailing. If you are weighing up options for a new flat roof or a refurbishment, understanding what GRP roofing is, how it is built, what it costs, and what rules apply will help you make a confident and compliant choice.

What GRP roofing actually is

GRP roofing is a site applied laminate. A rigid roof deck, usually OSB3 tongue and groove boards, is primed and then layered with fibreglass matting that is saturated with polyester resin. Once the resin cures, a coloured topcoat is applied to seal the surface and provide UV resistance. Because the liquid resin bonds the mat into a single sheet, the finished roof has no joints in the field area and very few mechanical seams. Factory made trims in matching GRP are fixed at edges and upstands so water is directed into gutters and away from the building. The system can be formed into curves, steps, internal corners and complex shapes, which is why it appears so often on dormers and parapet roofs.

Where GRP roofing works best

GRP excels on small to medium sized flat roofs that need crisp detailing and a long, low maintenance life. It is a good choice for bay windows and canopies that are highly visible from street level, because the finish is clean and smart. It is also popular on balconies and terraces, since anti slip topcoats can be added. For larger flat roofs, the system can still be used but design considerations such as expansion joints and staged application become more important. On roofs that move a great deal, perhaps due to lightweight structures with significant thermal expansion, a single ply membrane with more elasticity may suit better. For green roofs or heavily trafficked terraces, alternative systems designed for those loads should be considered unless the GRP specification is tailored for that use.

The anatomy of a typical GRP warm roof

Most modern flat roofs are built as warm roofs so that insulation sits above the structure and the deck stays closer to indoor temperature. A GRP warm roof build up usually starts with the existing structural joists and a vapour control layer that reduces moist air moving up from the heated interior. Above this sits rigid insulation to the required thickness for current energy targets. A new OSB3 deck is fixed on top of the insulation using long fixings and suitable washers. GRP trims are fixed at the edges, the deck is primed, and the fibreglass laminate is laid, consolidated, and topcoated. The result is a continuous waterproof layer, sloped to falls, that meets the gutter or outlet cleanly. Cold roofs, where insulation is below the deck, still exist in older buildings, but they require generous cross ventilation to prevent condensation. When refurbishing, most homeowners take the opportunity to convert to a warm roof for better energy performance and simpler moisture control.

Falls, drainage, and outlets

Flat roofs are not truly flat. To move water reliably the deck should be built to a design fall, often 1 in 40, so that a finished fall of at least 1 in 80 is achieved after deflection. GRP can be laid over firring strips or tapered insulation to create these falls. Formed outlets and scuppers in GRP trim lead water into gutters or downpipes. Good drainage is essential. Standing water shortens the life of any flat roof, encourages algae, and increases thermal stress. A well detailed GRP roof will shed water freely and will include overflow routes for heavy downpours so that water never backs up under the trims.

How GRP roofing is installed

The process is methodical. The deck must be dry, sound, and dust free. Trims are fitted first so edges and upstands are defined. The deck is primed where required, then fibreglass mat is cut to size and laid into wet resin. Air is rolled out so the fibres are fully saturated. End laps and corners are staggered and reinforced to maintain thickness. Once the laminate has cured, a pigmented topcoat is applied. In warm weather the roof may be complete in a day on a small area. Larger roofs are tackled in sections so that each area is weather tight at the end of the shift. Weather matters. GRP should be applied in dry, mild conditions, typically above 5 degrees Celsius and below the threshold at which resin cures too quickly. Moisture, frost, and heavy rain during application can spoil the finish or trap defects, so a professional installer will plan around the forecast.

Fire, structure, and safety during installation

Fibreglass resins are catalysed with MEKP, a hazardous substance that must be handled with care, and vapours require good ventilation. Installers use appropriate gloves, masks, and eye protection, and they manage waste and spill control carefully. Although GRP systems can be specified with tested fire performance, the installation stage itself avoids hot works, which many homeowners prefer when the roof is close to timber walls, fences, or planting. The roof structure must be checked for load capacity, because adding insulation and new decking changes the weight the joists carry. Fixings through insulation should be sized so the deck is firm and does not deflect as people move across it.

Expected lifespan and warranties

A correctly installed GRP roof with a quality topcoat can achieve 25 to 30 years of service life and often more with light maintenance. Many branded systems offer warranties in the 20 to 25 year range, sometimes longer when installed by approved contractors. The topcoat is the sacrificial weathering surface. After a decade or more it can be refreshed to extend life still further. Mechanical damage from ladders or dropped tools is the most common cause of localised repairs, and these are straightforward because the same resin chemistry allows patches to bond to the existing laminate.

Costs you can expect in the UK

Prices vary with access, size, insulation thickness, number of outlets, and detailing around rooflights and parapets. As a broad guide, supply and fit GRP warm roofs typically sit between £90 and £140 per square metre on straightforward domestic projects. Small, simple overlays without insulation may start around £70 per square metre, though many refurbishments trigger insulation upgrades under energy rules, which increases cost but lowers bills. Complex terraces with anti slip finishes, steps, and perimeter upstands can rise to £150 to £200 per square metre. A modest single garage roof often totals £1,200 to £2,500 depending on specification and location. Always ask for a written quote that itemises insulation thickness, deck type, trims, outlet details, and the warranty offered so you can compare like for like.

GRP compared with other flat roof options

Homeowners commonly compare GRP with felt and EPDM rubber. Felt, whether torch applied or cold applied, remains cost effective and can perform well when detailed correctly, but it relies on multiple laps. EPDM is supplied as large sheets so there are few seams, and it is very flexible, which helps on larger roofs or those with significant movement. GRP offers a hard, seamless finish with sharp detailing and a wide colour choice through pigmented topcoats. It is particularly strong where you need formed upstands, neat internal corners, or integrated trims. The right choice depends on size, movement, detailing needs, and how you plan to use the surface. If a balcony is to be regularly walked on, specify an anti slip GRP finish or consider a system designed specifically for heavy foot traffic.

Planning permission and building regulations

Replacing a flat roof covering on a like for like basis generally falls under maintenance and does not need planning permission. If you raise parapet heights, add rooflights, or create a terrace with guarding, planning rules may apply. Listed buildings and homes in conservation areas require care, and consent may be needed where appearance changes.

Building regulations apply when more than a quarter of the roof area is replaced. In practice this means most refurbishments of older flat roofs must meet current thermal targets, which often points to a warm roof with adequate insulation. The design must also address structure, drainage falls, ventilation strategy if a cold roof is retained, and fire performance. A contractor who is part of a recognised self certification scheme can notify building control on your behalf, or you can apply directly to the local authority. It is sensible to agree the compliance route in writing before work begins.

Common mistakes and how to avoid them

Most problems arise from poor preparation or rushing in the wrong weather. Laying GRP onto a damp deck traps moisture that later tries to escape, causing blisters. Skipping falls leads to standing water, which accelerates ageing. Thin laminates at corners and upstands are vulnerable to cracking. Trims fixed without adequate mechanical fixings can lift in storms. The cure is straightforward. Begin with a dry, rigid deck, plan your falls, check the forecast, reinforce corners and edge details, and fix trims carefully into sound timber. On larger roofs incorporate movement joints if required by the manufacturer’s guidance.

Maintenance for a long service life

GRP is low maintenance, but not maintenance free. Keep gutters and outlets clear so water can escape. Remove leaves and debris that trap moisture. Avoid dragging sharp ladders across the surface. If the finish becomes dull after many years in direct sun, a fresh topcoat can be applied after cleaning and light abrasion. Inspect after severe weather for signs of impact damage from branches or debris. Small chips can be repaired neatly with a colour matched gel repair kit.

Practical examples where GRP shines

Dormer cheeks and flat roofs at the back of terraced homes benefit from GRP because limited access makes a quick, joint free system attractive. Bay windows with decorative lead look trims can be replicated in GRP for a lighter, maintenance friendly alternative. Complex parapet roofs with internal gutters and multiple outlets can be shaped cleanly with GRP trims that would be difficult in sheet materials. Homeowners who want a colour to match fascia boards can choose pigmented topcoats so the roof complements the rest of the exterior.

Choosing an installer

Look for installers who can show training on the branded system they intend to use and who provide a written specification rather than a one-line quote. Ask how they will achieve falls, what insulation thickness they propose, how they will detail outlets and rooflight kerbs, and how they will protect neighbouring surfaces during works. Insurance, references, and a clear warranty document are worth as much as the headline price. A professional will also explain how building control will be notified and will include waste removal in the proposal.

Is GRP roofing right for your project

If your project is a small to medium flat roof where appearance and sharp detailing matter, if you want a seamless, joint free finish, and if you are keen on a long service life with light maintenance, GRP roofing is an excellent candidate. If the roof is very large, subject to significant movement, or will be used heavily by foot traffic, compare specifications carefully with EPDM and felt systems, and consider balcony specific build ups. With a good survey, a compliant warm roof design, skilled application in the right weather, and simple seasonal care, a GRP roof will protect your home cleanly and reliably for many years.