Flat roof explained: types, lifespan and cost

EPDM rubber, GRP fibreglass or traditional felt (which flat roof material lasts longest and costs least?

What is a flat roof?

A flat roof isn't completely flat (it has a slight pitch (typically 1-3 degrees) for drainage. True flat roofs pool water and fail early. Garages, extensions, commercial buildings and garden sheds commonly use flat roofs because they're simpler to build than pitched roofs.

Three materials dominate the UK market: EPDM rubber (Ethylene Propylene Diene Monomer), GRP fibreglass (Glass Reinforced Polyester), and traditional torch-on felt. Each has different lifespans, costs and installation requirements.

EPDM lifespan

20-30 years

GRP lifespan

25-30 years

Felt lifespan

10-20 years

Typical cost (installed)

£40-100/m² depending on material

Minimum pitch

1:80 (Building Regs)

How long does a flat roof last?

EPDM rubber roofs last 20-30 years according to manufacturer warranties (Firestone, Permaroof). GRP fibreglass lasts 25-30 years per Which? guidance. Traditional three-layer felt lasts 10-20 years, though Checkatrade reports many fail after 15 years.

Lifespan depends on installation quality. EPDM fails early if seams aren't bonded correctly. GRP cracks if the resin isn't mixed to specification. Felt degrades faster if torch-on layers aren't overlapped properly. A well-installed felt roof can outlast a poorly installed EPDM roof.

UV exposure, ponding water and foot traffic all shorten lifespan. A flat roof on a south-facing extension gets more UV than a north-facing garage roof. Standing water (from blocked gullies) degrades all three materials. If you're walking on the roof regularly to access something, expect shorter lifespan.

What is the best material for a flat roof?

For extensions or garages, EPDM or GRP. EPDM is cheaper (£40-70/m² installed) and easier to DIY if you're competent. GRP is more durable (25-30 years) and better for foot traffic, but costs more (£70-100/m²) and needs professional installation (resin mixing is finicky).

For sheds or outbuildings, torch-on felt is sufficient. It costs £10-40/m² installed and lasts 10-20 years. You'll replace it sooner than EPDM, but the upfront cost is lower. If the shed isn't a critical structure, felt makes sense.

EPDM comes in large sheets (up to 15m × 6m), so small roofs can be done in one piece with no seams. GRP is applied as liquid resin + fibreglass matting, so it forms a seamless membrane. Felt comes in rolls and is torch-welded in overlapping layers, creating seam points that can fail.

How much does a flat roof cost?

Felt: £10-40/m² installed. EPDM: £40-70/m² installed. GRP: £70-100/m² installed. These are 2026 Checkatrade averages including labour.

A typical 20m² extension roof costs:

• Felt: £200-800
• EPDM: £800-1,400
• GRP: £1,400-2,000

Costs vary by access difficulty. A ground-level garage roof costs less than a first-floor extension roof (scaffolding adds £300-600). Awkward shapes (dormers, valleys) add 15-30% because they need more cuts and seams.

Can I install a flat roof myself?

EPDM: yes, if you're competent with tools. It's the most DIY-friendly material. You cut the membrane to size, bond it to the deck, and seal the edges. Mistakes (unbonded seams, wrinkles) can cause leaks, but the process isn't complex. Expect 2-3 days for a small roof.

GRP: not recommended unless you've done it before. Resin mixing ratios are critical. Too much catalyst and it cures too fast (you can't spread it). Too little and it stays tacky. Temperature affects cure time. Professional installers get it right because they've done it hundreds of times.

Felt: possible but requires a blowtorch. You overlap layers and heat-weld them together. Fire risk if you're inexperienced. Also harder on your knees than EPDM (you're kneeling on the roof for hours). Most people hire a roofer for felt.

What can go wrong with flat roofs?

Ponding water is the most common issue. If the roof doesn't have enough pitch (minimum 1:80 per Building Regs), water pools. Standing water degrades EPDM, GRP and felt. It also grows algae and looks bad. Fix: increase pitch or add drainage gullies.

Blistering happens when moisture gets trapped under the membrane. More common with felt than EPDM/GRP. Blisters weaken the roof and eventually puncture. Fix: cut out the blister, dry the deck, patch with new membrane.

Seam failure happens with EPDM if seams aren't bonded correctly, or with felt if layers aren't overlapped enough. Water gets in through the seam. Fix: re-bond the seam or replace the section.

UV degradation affects all three materials, but felt worst. Felt goes brittle after 15-20 years of sun exposure. EPDM and GRP are more UV-stable. Fix: replace when the membrane cracks.

Do flat roofs need planning permission?

Usually no, if it's on an extension that's already within permitted development. But if the extension itself needs planning permission (e.g., it's too large, or you're in a conservation area), the roof is part of that application.

Building Regulations always apply. Your flat roof must meet Approved Document C (drainage, insulation, ventilation). Most local authorities want to see a minimum pitch of 1:80. Check with Building Control before starting.

Last reviewed: 2026-05-16