GRP stands for Glass Reinforced Plastic — a composite material made by laminating fibreglass matting with a polyester resin, creating a hard, seamless, waterproof shell. In roofing, GRP is applied directly onto a plywood deck as a liquid resin that cures in situ, bonding permanently to the deck surface and forming a completely jointless membrane. It's now the most popular flat roof system on domestic extensions in Tonbridge and across Kent.
How GRP Roofing is Installed
Installation on a typical Tonbridge kitchen extension follows this sequence:
- Deck preparation — existing covering is stripped. Deck boards are inspected and any wet or rotten sections are replaced. Joints are filled and the surface is sanded smooth.
- Drip trims fitted — aluminium drip edge and upstand trims are screwed to the deck perimeter to define the finished edge and provide a bonding surface for the GRP.
- Resin and matting application — catalysed polyester resin is applied to the deck. Fibreglass chopped strand mat is laid into the wet resin and stippled through to eliminate air bubbles. A second coat of resin bonds everything into a single laminate.
- Top coat — once the laminate has cured, a pigmented GRP topcoat is applied. This provides UV resistance, colour and a hard-wearing surface.
- Trim sealing — all upstand trims and drip edges are glassed in to the main membrane, eliminating any joints.
The critical step is eliminating air bubbles during the matting stage — bubbles create weaknesses in the laminate that can delaminate under thermal stress. A correctly installed GRP roof should produce a single void-free laminate across the entire deck surface.
Industry standard: GRP flat roofing installed to BS EN 14509:2013 (Self-supporting double skin metal faced insulating panels) standards carries a manufacturer design life of 20–25 years. The British Standard requires void-free laminate with no delamination and a minimum topcoat DFT (dry film thickness) of 400 microns. Source: BS EN 14509:2013 / GRP manufacturer technical data
Why GRP is Well-Suited to Tonbridge Properties
The Medway Valley microclimate in lower Tonbridge creates specific conditions that favour GRP over felt and EPDM:
- High ambient humidity — felt membranes in high-humidity environments experience accelerated lap joint adhesion failure. GRP has no lap joints.
- Heavy autumn leaf debris — debris accumulation on flat roofs blocks outlets and creates sustained ponding. GRP handles ponding water better than felt, which can soften adhesion bonds under sustained saturation.
- Freeze-thaw cycling — GRP is dimensionally stable through temperature changes, unlike EPDM which can shrink in cold conditions if incorrectly specified.
GRP vs EPDM vs Felt — Comparison
| GRP | EPDM | 3-Layer Felt | |
|---|---|---|---|
| Typical lifespan | 20–25 years | 20 years | 15–20 years |
| Joints | Seamless | Lap joints (bonded) | Lap joints (torched) |
| Deck bond | Chemically bonded | Adhesive bonded | Torch bonded |
| Repair if damaged | Re-glass + topcoat | EPDM patch | Torch-on patch |
| Best for | Extensions up to ~40m² | Large/complex shapes | Budget applications |
| Installed cost (per m²) | £80–£140 | £70–£120 | £55–£90 |
| Temperature sensitivity | Low | Moderate | High (frost can't torch) |
Free flat roof survey — we'll specify the right system for your property. Call 0173 224 1138
📞 0173 224 1138What GRP Cannot Do
GRP is not appropriate for every flat roof application:
- Very large areas (over ~60m²) — at large areas, GRP becomes difficult to install correctly in single-coat lamination without joints. EPDM is better suited for large garage complexes or commercial flat roofs.
- Highly complex shapes — multiple internal angles, many outlets and penetrations make GRP more difficult to install without weak points. EPDM's flexibility is an advantage on complex plans.
- Cold weather installation — GRP resin won't cure correctly below 5°C. We don't install GRP in Tonbridge during frost conditions. This is a genuine constraint: Tonbridge's valley-floor frost pocket means there are more days per year when GRP installation is not possible than on elevated nearby properties.
Maintenance Requirements
A correctly installed GRP roof requires minimal maintenance. The annual checks are:
- Clear debris from the surface and outlets — Tonbridge leaf fall means autumn clearing is essential
- Check upstand and trim junctions for any signs of lifting or movement
- Inspect for surface crazing or surface chalking of the topcoat — this is cosmetic and repairable with a topcoat refresh, not a membrane failure
- Check drainage falls are still positive — deck settlement over years can reverse falls and create ponding where there was none initially