Flat Roof Replacement Cost

The short answer. A new flat roof in 2026 costs roughly £40 to £95 per square metre in felt, £80 to £120 in EPDM rubber and £95 to £150 in GRP fibreglass, supplied and fitted. That puts a single garage roof between about £600 and £2,250 depending on the covering, and a rear extension roof between £1,800 and £6,500. Felt is cheapest and shortest-lived; EPDM is the usual choice for a roof nobody walks on; GRP is the one for terraces and balconies.

Flat Roof Cost by Material

Covering	2026 range per m²	Quoted lifespan	Best for
Torch-on felt (three-layer)	£40 – £95	15 – 20 years	Sheds, garages on a budget
EPDM rubber (single sheet)	£80 – £120	40 – 50 years	Garages, extensions, dormers with no foot traffic
GRP fibreglass	£95 – £150	25 – 40 years	Balconies, terraces, roofs with many details

Sources vary at the edges. Some 2026 guides put felt as low as £40 per square metre and GRP as high as £150; others narrow both. The order never changes: felt, then EPDM, then GRP. Lifespans are the figures manufacturers and roofers quote, and they assume a sound deck and proper drainage underneath.

Flat Roof Cost by Roof

Roof	Typical size	Felt	EPDM	GRP
Porch or bay window	3 – 8 m²	£300 – £600	£500 – £900	£600 – £1,200
Single garage	15 – 18 m²	£600 – £1,700	£1,400 – £2,200	£1,700 – £2,700
Double garage	30 – 36 m²	£1,200 – £3,000	£2,400 – £4,300	£2,900 – £5,400
Rear extension	20 – 30 m²	£1,000 – £2,800	£1,800 – £3,600	£2,000 – £4,500
Large extension or full flat roof	40 – 60 m²	£2,000 – £5,000	£3,500 – £7,000	£4,000 – £9,000

Small roofs cost more per square metre than large ones, because every roofer has a minimum charge, typically £300 to £500, and a porch does not use a day. The single garage row is the most-quoted flat roof job in the country, and the 2026 guides broadly agree on it: about £990 to £1,710 in felt and £1,440 to £2,160 in EPDM for an 18 square metre roof.

Felt, EPDM or GRP

Felt is the traditional covering, now laid as three torch-on layers with a mineral cap sheet. It is the cheapest and the quickest, and on a shed or a garage that will be replaced anyway in twenty years it is a reasonable choice. Its weaknesses are the joints and the upstands, where it cracks with age and movement, and it does not like standing water.

EPDM is a single sheet of synthetic rubber glued to the deck, with no joints on most domestic roofs. It handles movement and temperature well, which suits timber-decked garage roofs, and it is the usual recommendation where nobody will walk on the roof. The lifespans quoted for it are the longest of the three.

GRP is laid wet, resin and glass mat over a new deck, then cured into a single rigid skin with a coloured topcoat. It is walkable, it seals awkward details well, and it looks the neatest. It needs a dry, reasonably warm day to lay, and it can crack if the deck moves, which is why the deck matters more for GRP than for anything else.

Whichever covering you choose, the deck underneath decides whether it reaches its quoted life. New OSB or plywood over sound joists, laid to a proper fall so water reaches the outlet, is what the guarantee actually rests on.

What Changes the Price

Area. A porch, a single garage and a full-width extension are three sizes of job, and the per-metre rate falls as the area grows.

The covering. As above. The gap between felt and GRP on a large roof can be several thousand pounds.

The deck. If the old deck is rotten it comes out and new boarding goes in, at roughly £30 to £60 per square metre on top of the covering. Ask whether the quote assumes the deck is sound and what happens if it is not.

Insulation. When a flat roof is replaced, building regulations in England generally expect it to be insulated to current standards if it was not already. A warm-deck build-up adds material but also cures the condensation that killed the old roof. Budget an extra £30 to £60 per square metre.

Details. Every upstand against a wall, every outlet, rooflight, lantern and pipe is a place to seal. A plain garage roof is quick; an extension with a lantern, two outlets and a parapet is not.

Access. A garage roof is reached from a ladder. A dormer on a three-storey terrace needs a scaffold, and that can double the cost of a small job.

Region. Day rates in 2026 run roughly £150 to £270 outside London, with the South West in the middle of the band.

Warm Deck or Cold Deck

The two ways of insulating a flat roof affect the price and, more importantly, whether the new roof lasts.

A cold deck roof has the insulation between the joists, below the deck, with a ventilated gap above it. It is the traditional build-up and the cheaper one, but the ventilation is often inadequate on an old garage or extension, and moist air from the house condenses on the underside of the cold deck and rots it from inside. That is how a lot of the felt roofs being replaced now actually failed.

A warm deck roof puts rigid insulation boards on top of the deck, with a vapour control layer beneath them and the covering on top. The deck stays at room temperature, there is nothing to ventilate, and condensation stops being a problem. It costs more in materials and it raises the roof by the thickness of the boards, which can matter at a door threshold or a parapet, but it is what a roofer should be recommending on any roof over a heated room.

On an unheated detached garage, a cold deck with proper ventilation is still reasonable. On an extension, a dormer or anything over living space, ask for warm deck and expect the price to reflect it.

Repair or Replace

A flat roof can usually be patched once for £150 to £800 depending on the covering and the size of the failure. It can rarely be patched three times. The honest tests are age and pattern. Felt over fifteen years old, blisters across the surface, a deck that feels soft underfoot, water standing after rain, or a second patch on the same roof: any of those says replace. A new covering costs less than another year of chasing leaks, and it comes with a guarantee the patches never had. Our roof repair costs guide covers the patch end of the range.

Worked Examples

A felted single garage roof in Beacon Heath, blistered and cracked at the upstand. Eighteen square metres, deck sound, ladder access. New EPDM at £1,400 to £2,200. Felt would be £1,000 to £1,700 but is the covering that just failed.

A rear kitchen extension on a Victorian terrace in Heavitree. Twenty-five square metres, one outlet, a long upstand against the old back wall where the leak was, deck partly rotten at that wall. EPDM with a new deck and warm-deck insulation: £3,000 to £4,500. Access through the house adds if a tower is needed.

A dormer on a loft conversion in St Leonard's. Six square metres of GRP with three upstands and a rooflight, three storeys up. The material is a few hundred pounds; the scaffold is most of the £1,500 to £2,500 quote.

A balcony over a garage in Exmouth. Walked on daily and facing the sea. GRP with a non-slip topcoat and stainless trims, £2,000 to £3,500 for twenty square metres, and worth every extra pound over felt.

How to Compare Flat Roof Quotes

The covering, named, with the manufacturer and the guarantee that comes with it.
Whether the deck is being replaced, and at what rate if it is found rotten.
Whether insulation is included and to what thickness.
The fall: how the roofer will make sure water reaches the outlet.
Every upstand, outlet and rooflight listed.
Access and waste included.
VAT position, and whether the guarantee is insurance-backed.

Getting a Quote

This is one of three pages on the site that carries prices, because a flat roof is quoted on its size, its deck and its details. When you are ready, the flat roofing page takes your enquiry and we pass it to a roofing firm covering your part of Exeter or Devon. If the flat roof is part of a whole-house job, what a new roof costs covers the rest. And read how to choose a roofer before you accept anything.

Figures researched from published UK sources in September 2026 and shown as typical ranges, not quotes. Prices vary by roof, access and location.

Flat Roof Replacement Cost in the UK: 2026 Guide