Flat roof options and costs: felt, EPDM and GRP fibreglass compared

Flat roofs, common on extensions, garages and older properties, are covered in a narrower range of materials than pitched roofs, but the choice between them still has a big effect on both the upfront bill and how often the roof needs replacing. The three systems homeowners are most likely to be quoted for are felt, EPDM rubber, and GRP fibreglass, and each sits at a different point on the cost-versus-lifespan scale.

Felt roofing is the cheapest option to install, typically costing around £40-£70 per square metre, and it remains a common choice on smaller structures such as garden sheds and garages where budget matters more than longevity. Its main drawback is a shorter working life: felt roofs are commonly expected to last around 15-20 years before needing replacement, and multiple layers can be prone to cracking, blistering or seams failing over time, particularly under strong sun or where standing water collects. Because of this shorter lifespan, felt can end up being the most expensive option over several decades once repeated replacement is factored in, even though it's the cheapest to install any single time.

EPDM, a synthetic rubber membrane usually installed as a single, seamless sheet, typically costs a bit more upfront, around £50-£90 per square metre. Its main selling point is durability: EPDM has very few joints to fail, flexes well with temperature changes rather than cracking, and installers and manufacturers commonly quote a wide lifespan range of roughly 25 to 50 years depending on the product grade and how well it's installed. That wide range partly reflects genuine differences between EPDM products and partly reflects how much installation quality affects real-world performance, so it's worth asking an installer which specific product they're quoting and what lifespan they'd realistically expect for it.

GRP, or glass-reinforced plastic fibreglass, sits at the premium end of the three, typically costing £80-£130 per square metre. Like EPDM, it's applied as a seamless, fully bonded surface rather than overlapping sheets, which removes one of the main failure points of felt roofing. GRP is commonly quoted with a lifespan of around 30-40 years, and it tends to be a popular choice for roofs that will see foot traffic, such as roof terraces or balconies, or for larger or more complex flat roof shapes where a fully bonded, jointless finish is particularly valuable.

Two other systems come up less often but are worth knowing about. Modified bitumen, a heavier-duty evolution of traditional felt, is commonly quoted with a lifespan of roughly 20-25 years, sitting a little above standard felt. PVC single-ply membranes, similar in concept to EPDM but made from a different material, are commonly quoted at around 25-35 years. Both sit between felt and the higher-end EPDM or GRP systems in terms of durability, and may come up as an option depending on which contractors quote your job.

Looking at total project cost rather than just the rate per square metre helps put these figures in perspective. For a typical 25-30m² garage or extension roof, felt commonly totals around £1,200-£3,500 installed, EPDM roughly £1,800-£3,200, and GRP about £2,400-£4,200. The gap between felt and the other two options narrows considerably at this project size, which is part of why many installers now steer homeowners toward EPDM or GRP even on smaller jobs — the extra upfront cost is often modest next to the difference in expected lifespan.

When comparing quotes for a flat roof, it's worth asking exactly which product is being specified, not just which broad category (felt, EPDM or GRP) it falls into, since quality and warranty terms vary meaningfully within each category. It's also worth asking about the warranty offered, since manufacturer and installer warranties on flat roofing can range from ten years up to twenty or more, and a strong warranty is often a useful signal of how confident the installer is in both the product and their own workmanship.

Frequently asked questions