Last reviewed: 19 September 2026

Flat roof cost in the UK

Flat roofs on garages, dormers and extensions are usually priced per square metre, then adjusted for edges, outlets, the condition of the deck and whether insulation has to be brought up to current standards.

Covering cost, not construction cost

The figures on this page are for replacing a flat roof covering on an existing garage, dormer or extension. They are not the cost of building a new extension, adding a storey, or constructing a new structure from the walls up. Those are different projects with different budgets.

Cost per m² by system

Worked size examples

A single garage of about 20 m² in EPDM often plans around £1,500–£2,800 before a rotten deck or full scaffold. A 35 m² rear extension is more commonly £2,500–£5,500 depending on system and insulation. Small bays and dormers can look expensive per m² because edge work dominates.

The three common systems

Felt

Built-up or torch-on bitumen is still widely used, especially on simpler, lower-budget roofs. Lifespan is shorter than EPDM or GRP when specified cheaply. Hot-works rules may apply to torch-on systems.

EPDM rubber

A flexible single-ply sheet, often chosen for garages and extensions. Performance depends on seams, corners, outlets and upstands — not just the membrane brand.

GRP / fibreglass

A resin laminate with trims, often chosen where a harder finish or more complex shape is needed. It has to be laid in suitable conditions. Walk-on terraces are a different specification from a simple garage.

What else changes the price

• Roof area and how interrupted it is by lanterns, pipes or walls.
• Stripping the existing covering and whether the deck is sound.
• Firring pieces or new falls if water is ponding.
• Insulation to meet Building Regulations on a replacement.
• Edge trims, drip details and guttering while access is up.
• Scaffolding or tower access, especially above a first floor.

If the rest of the house is pitched, read new roof costs as well. Localised leaks may still be a repair. The calculator can combine area, system and access into a planning range.