The short answer
A typical small flat roof on a garage or single-storey extension costs £1,500–£4,000 in 2026, depending on size and material. By the square metre, EPDM rubber runs £80–£120, GRP fibreglass £90–£130, and built-up felt is cheaper up front but shorter-lived. The price covers stripping the old roof, new deck if needed, insulation where required, the waterproof layer and trims. For how the materials compare on lifespan and suitability, see our flat roof materials guide.
Flat roofs cover garages, extensions, dormers and porches across much of the UK housing stock, and the modern systems are a world away from the short-lived felt of decades past. This guide sets out typical 2026 costs by material and size, explains what sits inside a flat-roof quote, and flags what pushes the figure up. All figures are typical illustrations, not quotes.
Flat roof costs at a glance
- EPDM rubber £80–£120 / m²
- GRP fibreglass £90–£130 / m²
- Built-up felt cheaper / m², shorter life
- Small flat roof (garage / extension) £1,500–£4,000
- New timber deck (if needed) added cost
- Insulation upgrade (warm roof) added cost
Cost by material
The three common flat-roof systems sit at different price points. Built-up felt (layered bitumen) is the cheapest to install but has the shortest life. EPDM rubber is a single-ply membrane, often laid in one piece on smaller roofs, at £80–£120 per m². GRP fibreglass is a seamless resin-and-matting system at £90–£130 per m², giving a hard, durable finish. On a small roof the difference between systems may be only a few hundred pounds, so lifespan and suitability often matter more than the upfront gap — our flat roof lifespan guide covers that.
| System | Typical rate | Indicative lifespan |
|---|---|---|
| Built-up felt | cheaper than below | 10–20 years |
| EPDM rubber | £80–£120 / m² | 20–30 years |
| GRP fibreglass | £90–£130 / m² | 20–30 years |
What a flat-roof quote includes
A complete flat-roof price covers stripping the old covering, checking and replacing the timber deck if it is soft or rotten, fitting insulation where the design or regulations require it, laying the waterproof membrane, and finishing with edge trims, drips and outlets. On a re-cover where the deck is sound, the deck and insulation lines may be small or absent; where the deck has failed from years of leaks, replacing it is a meaningful extra. A good quote itemises deck, insulation and membrane separately so you can see what you are paying for.
Warm roof versus cold roof
Modern flat roofs are increasingly built as “warm roofs”, with insulation above the deck, which avoids the condensation problems of older “cold roof” designs. A warm-roof build-up costs more because of the insulation and the slightly raised detailing at the edges, but it performs better and is often what Building Regulations expect on a renewal. Our insulation and ventilation guide explains the difference, and our Building Regulations guide covers when Part L applies.
What pushes the cost up
Beyond size and material, the figure rises with: a failed deck needing full replacement; an insulation upgrade to meet Part L on a warm-roof build-up; access difficulties on upper-floor or dormer roofs needing scaffolding; parapet, lantern or rooflight detailing; and new outlets or guttering. For whether your existing flat roof needs full renewal or just a repair, see our repair-or-replace guide.
Compare flat roof quotes
Flat-roof prices vary with material, deck condition and access. Compare itemised quotes from vetted roofing contractors who have inspected the roof.
Frequently asked questions
How much does a new garage flat roof cost?
A typical small flat roof on a garage or single-storey extension costs £1,500–£4,000 in 2026, depending on size and material. EPDM rubber runs £80–£120 per m² and GRP fibreglass £90–£130 per m². Replacing a failed timber deck adds to the figure. These are typical illustrations, not quotes.
Is EPDM or fibreglass cheaper for a flat roof?
EPDM rubber is usually a little cheaper per square metre at £80–£120, against £90–£130 for GRP fibreglass. On a small roof the difference may be only a few hundred pounds, so suitability and finish often matter more than the upfront gap.
Does a flat roof need insulation?
Modern flat roofs are increasingly built as warm roofs with insulation above the deck, and Building Regulations Part L can require an insulation upgrade on a renewal. A warm-roof build-up costs more but avoids the condensation problems of older cold-roof designs.
Why is replacing the deck an extra cost?
If years of leaks have left the timber deck soft or rotten, it must be replaced before the new membrane goes on — a membrane laid over a failed deck will not last. A good quote lists deck replacement separately so you can see whether it applies to your roof.
Sources & further reading
- NFRC (National Federation of Roofing Contractors) — flat roofing systems and standards
- CompetentRoofer — flat-roof self-certification under Building Regulations
- GOV.UK / Building Regulations Approved Document L — insulation standards for flat roof renewal
This is general information, not advice for your specific property or roof. Costs vary with your home, roof size, material and chosen contractor. Flat roof work should be carried out by a vetted roofing contractor.