A correctly installed GRP flat roof carries a typical service life of 25–30 years, against 15–20 years for traditional felt. — GRP manufacturer guidance
The Two Modern Flat Roof Systems
If your Horsham rear extension or garage flat roof needs replacing, you'll be choosing between two modern systems: GRP fibreglass and EPDM rubber. Both are a significant upgrade over the traditional felt systems they typically replace. Both carry 25-year manufacturer guarantees. But they have different characteristics, different strengths and different cost profiles.
| Factor | GRP Fibreglass | EPDM Rubber |
|---|---|---|
| Manufacturer guarantee | 25 years | 25 years |
| Installation method | Wet laminate — bonded in situ | Adhesive or mechanical — sheet bonded |
| Seams | Seamless — fully bonded laminate | Seams at joins — potential failure point |
| Cost (typical extension) | £900–£2,500 | £700–£2,000 |
| Large area cost | Higher — materials cost more per m² | More economical at scale |
| Foot traffic resistance | Excellent — rigid surface | Good, but surface can abrade |
| West Sussex climate performance | Excellent — UV stable, handles freeze-thaw | Good — some UV degradation at seams over time |
| Repairability | Simple — patch laminate applied | Good — patches bond well to EPDM |
Why We Recommend GRP for Most Horsham Properties
GRP fibreglass is our primary recommendation for flat roofs on Horsham domestic properties — rear extensions, garages and smaller outbuildings. The seamless finish is the key advantage: GRP is applied as a liquid resin and fibreglass laminate that cures to a fully bonded, jointless surface. There are no seams. On EPDM installations, the seams between sheets are the most common failure point over the life of the roof — they require periodic inspection and occasional resealing.
In Horsham's damp climate, where roofs see consistent moisture through autumn and winter, a seamless system provides meaningful additional security. We see GRP systems installed 20–25 years ago still performing correctly on Horsham properties when properly installed with correct drainage.
When EPDM Is the Right Choice
EPDM becomes the better recommendation when the flat roof area is large — above approximately 40–50m². At this scale, GRP laminating costs become significant and EPDM offers comparable performance at meaningfully lower cost. EPDM is also more flexible than GRP on roofs with complex shapes or penetrations (pipes, vents, drains) where the sheet material can be formed around obstacles more easily than a wet laminate.
For commercial-scale flat roofs and large rear additions on Horsham's bigger properties, EPDM is frequently the right specification.
Both Replace Like-For-Like on Failed Felt
The majority of flat roof replacements we carry out in Horsham are replacing failed traditional felt systems on 1970s–90s rear extensions and garages. Both GRP and EPDM are appropriate replacements — the choice comes down to area size, budget and the specific details of the installation. We survey the existing deck condition, advise on the appropriate system and provide written quotes for both options where relevant.
We strip the existing felt system, inspect the deck and replace any soft or damaged boarding before installing the new system. This is included in our quoted price — we don't discover additional costs after work starts. The condition of the deck determines the final price, which is why we survey in person before quoting.
Choosing Between EPDM and GRP for Your Horsham Flat Roof
For most Horsham residential flat roofs — extensions, garage roofs, bay roofs — GRP fibreglass is our standard recommendation. The seamless cured system has no joints to fail, handles West Sussex's autumn and winter rainfall without edge-lift risk, and the 25-year guarantee covers the full installation. GRP is also compatible with standard timber and plywood deck construction, which is what the majority of Horsham residential flat roofs use.
EPDM has specific advantages for larger roofs (over 80m²) and for roofs with complex penetrations — multiple pipes, skylights or rooflights — where the flexible sheet can be cut and bonded around obstacles more easily than GRP lamination. EPDM is also the correct specification for roofs where future dismantling is planned (temporary structures, green roofs). For a standard Horsham rear extension or garage, GRP is the correct choice.
We specify both systems and advise based on the specific roof, not on which generates the higher margin. If EPDM is appropriate for your application, we will say so.
Getting the Work Done in Horsham
All surveys in Horsham are free with no call-out fee. We attend within 3–5 working days for planned surveys and aim to reach emergency call-outs within 2–4 hours. Every job receives a written, itemised, fixed-price quote before any work starts — no verbal estimates and no bills that exceed the accepted written quote.
We are NFRC registered and carry £5 million public liability insurance. All work carries a written 10-year workmanship guarantee backed by a named, verifiable local contractor. Guarantee and insurance documentation is provided on request before any survey visit.
We cover Horsham and the surrounding West Sussex area including Roffey, Broadbridge Heath, Southwater, Billingshurst, Pulborough, Storrington, Henfield, Crawley, Burgess Hill, Haywards Heath, East Grinstead and Worthing. Call 0140 358 1272 to arrange a free survey.
Flat Roof Quote — Horsham
Free survey — we assess your deck, recommend the right system and give you a written price for GRP or EPDM.
📞 Call 0140 358 1272Last reviewed: July 2026 by Horsham Roofing Experts.