A GRP flat roof is a flat roofing system built from glass reinforced plastic, the same fibreglass laminate used on boat hulls. Layers of catalysed resin and chopped-strand glass matting are laid over a conditioned timber deck and cure into one solid, jointless waterproof shell. Because there are no joints or laps to fail, a well-installed fibreglass flat roof commonly lasts 25 to 30 years or more, which is why it has largely replaced traditional felt on domestic extensions, dormers, porches and garages.
What is a GRP flat roof made of?
GRP stands for glass reinforced plastic. On a flat roof it is built up in stages rather than rolled out like a sheet material. The result is a rigid, jointless surface bonded to the deck below.
- The deck. A GRP roof needs a stable, dry base, usually 18mm exterior-grade OSB3 boarding fixed to the joists. Movement in the deck is the enemy of any fibreglass laminate, so the boarding and fixings matter as much as the resin.
- Trims. GRP edge, drip and gutter trims are bonded around the perimeter to form clean edges and shed water where you want it.
- The laminate. Catalysed resin is applied and glass matting is worked into it to remove air, then a coloured topcoat resin seals the surface.
- The topcoat. This is the visible, UV-stable finish, typically dark or light grey, that gives the roof its durability and appearance.
We stock complete GRP systems and the individual components on our flat roofing products range, so roofers can pick up resin, matting, topcoat and matching trims from one counter.
GRP vs felt vs EPDM: how they compare
Felt (torch-on built-up felt) and EPDM rubber are the two systems most people weigh a fibreglass flat roof against. Each has a place, and the right choice depends on the deck, the shape of the roof and how long you want it to last.
| Feature | GRP fibreglass | Torch-on felt | EPDM rubber |
|---|---|---|---|
| Typical lifespan | 25 to 30-plus years | 10 to 20 years | 30 to 40 years |
| Finish | Jointless, rigid, walk-on | Layered sheets with laps | Single flexible membrane |
| Joints and seams | None once cured | Multiple lapped joints | Few, but glued/taped seams |
| Install conditions | Dry weather only, temperature-sensitive | Naked flame (heat/fire risk) | Adhesive-based, dry preferred |
| Best suited to | Complex shapes, upstands, foot traffic | Budget jobs, simple decks | Large simple areas |
The headline difference in the GRP vs felt flat roof question is the seams. Felt is laid in overlapping layers that are bonded with heat, and those laps are the first thing to lift, split or let water in as the roof ages. A GRP roof cures into a single continuous skin with nothing to peel back, so it copes well with the detailing around parapets, pipe penetrations and rooflights where felt tends to fail first. EPDM rubber is a strong, long-lived alternative, but it is a flexible membrane rather than a hard shell, so GRP is usually the pick where the surface needs to take regular foot traffic or a walk-on finish.
How long does a GRP flat roof last?
GRP roof lifespan is one of its biggest selling points. A correctly specified and installed fibreglass roof will typically give 25 to 30 years of service, and many go well beyond 30 with little measurable deterioration. Most GRP systems carry a manufacturer guarantee of around 20 years or more when fitted to the system specification. The topcoat is UV-stable, so unlike felt it does not dry out and crack in the sun, and there are no laps to work loose. Basic upkeep is light: keep gutters and outlets clear, and check the surface every few years.
Why installation temperature and dry weather matter
The one real constraint with fibreglass roofing is that it is a wet-lay chemical process, so the weather has to cooperate. The resin cures through a reaction started by a catalyst, and that reaction is temperature-dependent.
- It must be dry. The deck has to be completely dry before work starts, and no rain can fall while the resin is going off. Moisture trapped under the laminate causes blistering and poor adhesion.
- Temperature range. As a rule GRP is laid above about 5°C and out of very hot sun. Too cold and the resin will not cure properly; too hot and it goes off before it can be worked. Installers adjust the catalyst ratio to suit the day’s temperature.
- Curing times. A topcoat is usually touch-dry within roughly an hour, but full cure takes longer, up to a couple of days, and foot traffic should wait around 24 hours in mild conditions.
In practice this means GRP is a spring-to-autumn job in most of Kent, Surrey and Sussex, or a dry, mild spell in winter. It is worth planning around, because rushing a fibreglass roof in the wrong conditions is the most common cause of early failure.
What affects the cost of a GRP flat roof?
We supply materials rather than quote fixed installed prices, and any honest answer on a GRP flat roof cost depends on the specifics of the job. The main factors are straightforward:
- Roof area. More square metres means more boarding, resin, matting and topcoat, plus more labour time.
- Deck condition. A tired deck that needs stripping and re-boarding adds material and labour. GRP needs a sound, dry base to bond to.
- Trims and detailing. The number of edges, upstands, corners, outlets and pipe penetrations drives how much trim and hand-detailing is involved.
- Access and labour. Height, scaffolding and how easily the deck can be reached all feed into the day rate.
For a homeowner trying to sanity-check quotes, it helps to look at a national reference for typical UK flat roof replacement costs before comparing installers. If you are pricing the materials for a job, our team can put a supply list together against your measurements. You can get a quote on GRP systems and trims, and we will help you specify the right resin quantities for the area and the weather.
Is a fibreglass roof the right choice for you?
GRP tends to be the strongest option for extensions, dormers, bay roofs, balconies and any flat area that needs a hard, walk-on, joint-free finish and a long life. Felt still makes sense on tight budgets or simple sheds, and EPDM suits large, plain areas. Where a GRP roof earns its keep is on the fiddly roofs with lots of detail, the ones where a seam would eventually be the weak point.
Brian Gow Roofing Supplies has been supplying flat roofing materials to trade and homeowners across Sussex, Kent and Surrey since 1996, so if you are unsure which system fits your roof, ask. You can read more about our flat roofing service and support, or call in and see the systems in person at our main Haywards Heath roofing supplies depot at Scaynes Hill. Bring your measurements and we will help you get the specification and the quantities right first time.
Frequently asked questions
What is a GRP flat roof?
A GRP flat roof is a flat roofing system made from glass reinforced plastic, or fibreglass. Catalysed resin and glass matting are laid over a dry timber deck and cure into one solid, jointless waterproof shell. Because it has no joints or laps to fail, it is a durable, walk-on finish suited to extensions, dormers, garages and balconies.
Is GRP better than felt for a flat roof?
For most jobs, yes. A GRP roof cures into a single continuous skin, while felt is laid in overlapping layers whose laps tend to lift and split as they age. GRP typically lasts 25 to 30 years against felt’s 10 to 20, and copes better with detailing around upstands and penetrations. Felt still suits simple, budget-led roofs.
How long does a GRP flat roof last?
A correctly installed GRP flat roof usually lasts 25 to 30 years, and many go beyond 30 with little deterioration. Most systems carry a manufacturer guarantee of around 20 years or more when fitted to specification. The UV-stable topcoat resists cracking in the sun, and with no seams to work loose, upkeep is limited to clearing gutters and outlets.
Can you install a GRP roof in winter?
You can, but only in the right conditions. GRP must be laid on a completely dry deck with no rain during curing, and generally above about 5°C. In cold weather the resin will not cure properly, so winter work needs a dry, mild spell and a catalyst ratio matched to the temperature. In practice most GRP roofs go on between spring and autumn.
What affects the cost of a GRP flat roof?
The main factors are roof area, the condition of the existing deck, and the amount of trim and detailing around edges, upstands and penetrations. Access, height and scaffolding affect labour. A larger roof or one needing a fresh deck uses more material and time. We supply the systems and can price materials against your measurements.
Is a fibreglass roof any good?
A fibreglass roof is one of the most durable domestic flat roofing options available. Its jointless, rigid finish handles foot traffic and complex shapes well, resists UV without cracking, and commonly lasts 25 to 30 years or more. The main trade-off is installation: it needs dry weather and the right temperature to cure, so it is not a wet-day job.