GRP stands for Glass Reinforced Plastic — also known as fibreglass roofing. It’s a flat roofing system built by layering fibreglass matting and saturating it with liquid resin, which then cures into a single, rigid, waterproof membrane. Unlike felt roofing, which relies on overlapping sheets bonded or torched together, GRP forms one continuous surface with no seams or joints.
For London homeowners, this matters more than it might seem. Seams are almost always where flat roofs fail first — water finds its way in through joints, laps, and edge details that degrade over time. GRP removes that weak point entirely, which is a major reason it’s become the go-to choice for extensions, garages, dormers, and balconies across boroughs like Ealing, Harrow, and Barnet, where many Victorian and Edwardian properties have rear additions with flat roof sections.
How GRP Roofing Is Installed
GRP isn’t a material you can simply lay like felt or slate. It’s applied as a system, and the process matters as much as the material itself:
- Deck preparation — The existing roof deck (usually plywood) is inspected and, if needed, replaced or reinforced. Any damp or rotten timber must be addressed first, as GRP is only as good as what’s beneath it.
- Trims and edge details — Timber trims are fixed around the roof perimeter to define the roof’s edges and give the resin something to key into.
- Fibreglass matting — Matting is rolled out across the deck and edges.
- Resin application — Liquid resin is applied by roller, saturating the matting and bonding it to the deck. This is typically done in two coats: a base coat and a top coat containing UV-stable pigment.
- Curing — The resin cures into a hard, seamless membrane, usually within a few hours depending on temperature and humidity.
This is a skilled, weather-sensitive process. Resin needs the right temperature range to cure properly, and installers need experience to avoid trapped air bubbles or uneven thickness — both of which can shorten a roof’s lifespan. This is one reason it’s worth having a professional flat roof installation carried out by an experienced team rather than treated as a DIY project.
Key Benefits of GRP Roofing
Seamless and leak-resistant
Because the entire roof cures as one piece, there are no joints for water to exploit. This is the single biggest advantage over felt.
Long lifespan
A properly installed GRP roof typically lasts 20–30 years with minimal intervention, outperforming standard felt roofing considerably.
Low maintenance
GRP doesn’t require the periodic recoating or patching that ageing felt roofs often need. Occasional inspection and gutter clearance are usually sufficient.
Weather resistant
London’s flat roofs face a demanding mix of rain, ponding water, and UV exposure. GRP handles all three well — it’s inherently waterproof and, with UV-stable top coats, resistant to sun degradation.
Load-bearing and walkable
GRP roofs can support foot traffic, planters, or lightweight furniture, making them a practical choice for roof terraces and balcony conversions — a growing trend in flats across Camden and Kilburn.
Aesthetic flexibility
GRP is available in a range of colours (grey and black are most common), giving homeowners more visual control than traditional felt.
GRP vs Felt: A Quick Comparison
| Feature | GRP Fibreglass | Felt Roofing |
|---|---|---|
| Seams | None (seamless) | Multiple overlapping joints |
| Typical lifespan | 20–30 years | 10–15 years |
| Leak risk | Low | Higher, especially at joints |
| Installation skill required | High | Moderate |
| Upfront cost | Higher | Lower |
| Maintenance | Minimal | Periodic recoating/patching |
| Walkable | Yes | Limited |
If you’re weighing these two systems specifically for cost and lifespan trade-offs, our dedicated felt vs GRP roof comparison goes deeper into pricing scenarios for London properties.
Where GRP Works Best on London Homes
GRP is particularly well suited to:
- Rear extension roofs — common on Victorian terraces in Acton, Willesden, and Finchley
- Garage roofs — where low pitch and easy access make GRP straightforward to install
- Dormer roofs — where seamless waterproofing around a complex junction is critical
- Balconies and roof terraces — where the roof also functions as a walkable surface
- Commercial flat roofs — where long-term reliability reduces maintenance callouts
It’s less commonly used as a full replacement for large pitched roof areas, where tiles or slates remain the standard. For guidance on which system suits your property type, our flat roofing services page covers felt and GRP options side by side.
Is GRP Roofing Worth the Extra Cost?
GRP typically costs more upfront than felt, largely due to material costs and the skilled labour required for correct resin application. However, when you factor in the extended lifespan — often double that of felt — and the reduced likelihood of leak-related repairs, many homeowners find the lifetime cost works out favourably.
This is especially relevant for landlords and property managers, where unplanned leak repairs and tenant disruption carry real costs beyond the roofing work itself. A seamless, long-lasting system reduces both.
Before deciding, it’s worth getting a proper assessment of your existing roof deck and structure, since GRP suitability depends on what’s underneath. A full roof inspection will confirm whether your current deck can be reused or needs replacing as part of the works — a factor that significantly affects overall cost.
Signs Your Current Flat Roof Might Need Replacing
If you’re considering GRP as an upgrade, it’s often because an existing felt or older GRP roof is showing wear. Common warning signs include:
- Ponding water that doesn’t drain within 48 hours
- Visible cracking, bubbling, or blistering on the surface
- Soft or spongy areas underfoot
- Staining or damp patches on ceilings below
- Daylight visible through the roof deck from inside
If any of these sound familiar, it’s worth arranging an inspection sooner rather than later — small issues on flat roofs tend to escalate quickly once water finds a way in.
Maintaining a GRP Roof
One of GRP’s biggest selling points is how little upkeep it needs, but “low maintenance” doesn’t mean “no maintenance.” To get the full 20–30 year lifespan:
- Clear debris and leaves from the surface periodically, especially near drainage points
- Check for any physical damage after storms
- Avoid dragging heavy or sharp objects across the surface
- Have the roof inspected every few years, particularly around edge trims and upstands
Getting a GRP Roof Installed in London
Because correct application is so dependent on installer skill, it’s worth choosing a contractor with proven GRP experience rather than a general roofer who occasionally does flat roofs. Ask about the resin system used, expected cure times, and warranty terms before committing.
If you’re ready to explore GRP for your property, our team can assess your existing roof and talk you through whether GRP, felt, or another system is the right fit. Request a free quote and we’ll arrange a site visit.
❓ FAQ
What does GRP stand for in roofing?
GRP stands for Glass Reinforced Plastic, a seamless flat roofing system made from fibreglass matting saturated with liquid resin.
How long does a GRP roof last?
A properly installed GRP roof typically lasts 20–30 years, significantly longer than standard felt roofing.
Is GRP roofing more expensive than felt?
Yes, GRP usually has a higher upfront cost due to materials and installation skill, but its longer lifespan and lower maintenance needs often make it more cost-effective over time.
Can GRP roofing be walked on?
Yes, GRP is load-bearing and suitable for foot traffic, making it a good choice for roof terraces and balconies.
Is GRP suitable for period properties in London?
Yes, GRP is commonly used on rear extensions, garages, and dormers of Victorian and Edwardian homes, provided the roof deck is properly prepared.
How do I know if my flat roof needs replacing rather than repairing?
Ponding water, cracking, soft spots, or ceiling staining below the roof are all signs a replacement may be more cost-effective than ongoing repairs.
