GRP fibreglass is worth the extra cost over felt in coastal Worthing because it lasts 40–50 years versus 8–12 for felt. It has no joints or seams — the primary felt failure point — and is impervious to salt air and UV. Over 30 years, GRP installed once costs less than felt replaced repeatedly. Call 01903 905 128.
If you have a flat roof on a garage, extension or bungalow in Worthing, the choice of material will determine whether you're back spending money in 10 years or largely free of concern for 40. The answer in a coastal environment is not ambiguous — but the full picture is worth understanding.
The Two Main Flat Roofing Options in 2025
What Makes GRP Different
For most domestic flat roofs in Worthing, the choice comes down to two systems: GRP fibreglass or felt (also called built-up felt or torch-on felt). EPDM rubber is a third option worth understanding. Here's an honest assessment of each.
British Standard BS 6229, the code of practice for flat roofing, sets out the drainage and falls requirements — a minimum fall that ponding-prone felt roofs frequently fail to maintain over time.
Source: British Standard BS 6229
GRP Fibreglass Flat Roofing
Glass Reinforced Plastic (GRP) is a composite material — fibreglass matting saturated with polyester resin, finished with a gel coat topcoat. When correctly installed, the entire roof surface is a single seamless membrane with no joints, seams or edges for water to find.
Lifespan: 40–50 years in Worthing's coastal environment. GRP is completely impervious to salt air, UV degradation and the humidity cycles that shorten felt's lifespan. There is no ongoing maintenance requirement — no re-coating, no re-felt, no resealing of joints.
Guarantee: 25-year manufacturer's guarantee on the GRP material itself, plus our 10-year workmanship guarantee on the installation.
Cost: Higher upfront than felt. A standard garage flat roof in GRP costs £800–£1,600 in Worthing. An extension of 20–30m² costs £2,000–£3,800.
Installation: Completed in a single day for most domestic applications. The resin is mixed and applied on-site; it cures in hours and is weatherproof within 24 hours.
Coastal performance: Excellent. Salt air has no effect on cured GRP. UV resistance is built into the gel coat. The absence of joints means there is no sealant to degrade and no seam edge for wind-driven rain to attack. This is why GRP is our standard specification for all flat roofs in Worthing's coastal zone.
The Case for Felt
Traditional mineral felt and torch-on felt modified bitumen systems have been the standard flat roofing material for 60+ years. They remain widely used, partly because they're familiar to most roofers and cheaper to install than GRP.
Lifespan: 10–20 years inland. In coastal Worthing — where UV is higher, salt air attacks the bitumen surface, and temperature cycling is more pronounced — expect the lower end of that range. We see felt flat roofs on Worthing properties that are 8–12 years old regularly requiring replacement. Seafront and near-seafront properties are at the extreme end.
Guarantee: Typically 10–15 years on torch-on systems. Older mineral felt carries no meaningful guarantee.
Cost: Lower upfront. A garage felt flat roof costs £500–£900 in Worthing. An extension of 20–30m² costs £1,200–£2,500.
Coastal performance: Poor relative to GRP. Salt air attacks the bitumen surface, causing surface crazing and the progressive loss of aggregate from the surface mineral layer. This exposes the bitumen to UV, which in turn causes embrittlement, surface cracking and eventual loss of waterproofing. Joints and edge details are sealed with mastic, which degrades faster in UV and salt air.
| Factor | GRP Fibreglass | Torch-On Felt |
|---|---|---|
| Upfront cost (garage) | £800–£1,600 | £500–£900 |
| Lifespan (Worthing coastal) | 40–50 years | 8–15 years |
| Maintenance required | None | Inspect / reseal joints 5-yearly |
| Salt air resistance | Excellent | Poor |
| UV resistance | Excellent | Moderate (degrades) |
| Joint / seam vulnerability | None (seamless) | Yes — primary failure point |
| 30-year cost (garage) | £800–£1,600 | £1,500–£2,700 (2–3 replacements) |
EPDM Rubber — The Middle Option
EPDM (Ethylene Propylene Diene Monomer) is a synthetic rubber membrane that performs well in coastal environments and sits between felt and GRP in cost. Lifespan is 20–30 years, with a 20-year material guarantee available. It's flexible, relatively easy to repair if punctured, and performs well on larger flat areas where the cost of GRP becomes significant.
We use EPDM on larger flat roofs (typically over 50m²) where cost is a significant factor, and GRP on smaller domestic applications where the seamless, joint-free finish is worth the premium. EPDM has seamed edges and a bonded perimeter that requires more careful installation than GRP — if the perimeter bond fails, water gets under the membrane. Quality of installation is critical.
The 30-Year Cost Comparison
The upfront cost comparison favours felt. The 30-year cost comparison favours GRP significantly, particularly in coastal Worthing where felt replacement cycles are shorter.
A felt garage flat roof replaced in Worthing every 10–12 years costs approximately £1,500–£2,700 over 30 years (three replacement cycles). A GRP roof installed once costs £800–£1,600 over the same period — and still has 10–20 years of useful life remaining at year 30. The maths are unambiguous.
For any flat roof on a Worthing property — garage, extension, bungalow rear addition — we specify GRP as standard. The coastal environment shortens felt lifespan to a point where the cost advantage is eliminated within 10–12 years. GRP installed correctly requires no maintenance, carries a 25-year guarantee, and will almost certainly outlast the property ownership period of most homeowners who install it. Call 01903 905 128 for a free survey and quote.
Can an Existing Felt Roof Be Overlaid With GRP?
Sometimes, yes. If the existing deck (the plywood or OSB substrate beneath the felt) is structurally sound — no rot, no delamination, no soft spots — it's possible to overlay with GRP rather than stripping back to the rafters. We assess deck condition as the first step of any flat roof survey. Where overlay is viable, it reduces the installation cost and the disruption. Where the deck is compromised, stripping back is the only correct approach.
Call 01903 905 128 for a free flat roof survey across Worthing and the surrounding BN postcode area.
Frequently Asked Questions
Yes, consistently. GRP costs 30–60% more upfront than torch-on felt but lasts 40–50 years in coastal Worthing vs 8–12 years for felt. Over 30 years, a Worthing homeowner will typically replace felt two to three times at equivalent or greater total cost to a single GRP installation.
With correct installation and enhanced marine specification, 40+ years. GRP is completely impervious to salt air and UV degradation — the two primary failure mechanisms for flat roofing on the West Sussex coast.
Yes — if the structural deck (plywood or OSB substrate) is sound. We assess deck condition before quoting. If the deck has rotted from prolonged moisture ingress, we replace it before applying GRP. Call 01903 905 128 for a free survey.
EPDM is a synthetic rubber membrane with 20–30 year lifespan in coastal conditions. It performs significantly better than felt and is viable for larger flat roofs over 50m² where GRP cost becomes significant. For standard domestic flat roofs in Worthing, GRP's seamless joint-free finish is the better specification.
About the author
Worthing Roofing Experts — local roofing specialists serving Worthing and the West Sussex coast since 2003. NFRC registered, fully insured, 10-year workmanship guarantee on all work. Call 01903 905 128 for a free, no-obligation quote.
Last reviewed: 20 July 2026 by Worthing Roofing Experts.