GRP (fibreglass) and EPDM (rubber) are both excellent flat roofing systems lasting 25 to 30 years in Southampton. GRP gives a hard, seamless finish ideal for foot traffic and complex shapes; EPDM is a flexible single-membrane rubber that handles movement well and installs faster. Both far outlast old felt systems. Expect £80 to £120 per m².
If you are replacing a flat roof on a Southampton property — whether it's the extension on a Bitterne semi, the garage on a Hedge End detached, or a rear kitchen extension in Chandler's Ford — the two main options are GRP fibreglass and EPDM rubber membrane. Both are a significant upgrade over the felt they replace. This guide explains the practical differences so you can make the right choice for your property.
What is GRP Flat Roofing?
GRP stands for glass reinforced plastic — the same composite material used in boat hulls and car body panels. A GRP flat roof is installed by laying woven glass matting over the deck and saturating it with liquid resin, which cures to form a seamless, rigid, fully waterproof shell. Because there are no joints or seams in a GRP installation, there are no weak points for water to penetrate.
BS 6229 (flat roofs with continuously supported flexible waterproof coverings) is the code of practice governing flat roof design and installation in the UK.
Source: British Standard BS 6229
What is EPDM Flat Roofing?
EPDM is a synthetic rubber membrane — the same material used in car door seals and plumbing gaskets. It comes in large sheets that are bonded to the deck with adhesive and heat-welded at the joints. The key difference from GRP is that EPDM is flexible rather than rigid, which gives it different performance characteristics on larger roof areas.
Cost Comparison
GRP vs EPDM — Typical Installed Costs (Southampton 2025)
GRP is typically 10 to 15 percent more expensive than EPDM on equivalent area. The premium reflects higher material cost and a slightly more labour-intensive installation process.
Performance in Southampton's Climate
Southampton's proximity to the Solent means above-average rainfall, persistent humidity and occasional salt-laden air for properties close to the waterfront. Both GRP and EPDM handle this environment well — far better than felt, which is why we recommend replacing any felt flat roof in Southampton rather than repatching it.
The relevant difference is in temperature response. GRP is rigid and handles thermal movement across its expansion joints. EPDM is inherently flexible and absorbs thermal movement across the whole membrane — which is why it performs slightly better on very large roof areas where differential movement is greater.
GRP or EPDM — Which Should I Choose?
| Factor | GRP | EPDM |
|---|---|---|
| Roof area under 25m² | ✓ Recommended | Good option |
| Roof area over 30m² | Good option | ✓ Recommended |
| Seamless finish (no joints) | ✓ Yes | Joints present |
| Foot traffic tolerance | ✓ Higher | Moderate |
| Lifespan | 25+ years | 25+ years |
| Maintenance | None | None |
| Appearance | Hard finish | Soft, rubber look |
| Cost | 10–15% higher | Lower |
What About the Deck Underneath?
Whatever system you choose, the deck boards underneath must be sound before installation. On flat-roofed extensions in Southampton's older housing stock — particularly the 1950s and 1960s properties in Bitterne, Woolston and Thornhill — we find rotten deck boards in a significant proportion of jobs. We inspect the deck during survey and include any necessary board replacement in the quote before work begins, not as an extra on the invoice.
Need Advice on Flat Roofing?
We will survey your flat roof, assess the deck condition and recommend the right system for your property and budget. Free inspection, no obligation.
Call 0238 184 9628Last reviewed: 20 July 2026 by Southampton Roofing Experts