How much does it cost to replace a flat roof in 2026?

Flat roof replacement cost in the UK comes down to three things: the system you choose, the size of the roof and what is hiding underneath the old covering. In Cambridge, where almost every Victorian terrace seems to have a flat-roofed rear extension or dormer and post-war semis often have a flat-roofed garage, Checkatrade’s 2026 data puts a typical new flat roof at £1,430–£2,988. This guide breaks that down per square metre by system, works through garage, extension and dormer examples, and lists the extras that make the difference between two quotes.

How much does it cost to replace a flat roof in 2026?

Checkatrade’s flat roof replacement guide and its newer EPDM and GRP guides give these supply-and-fit rates:

Flat roof system	Price per m²	Average per m²	Typical lifespan
Torch-on felt (bitumen)	£50–£80	£65	15–20 years
EPDM rubber	£80–£100	£90	30–50 years
GRP fibreglass	£95–£120	£107.50	25–30 years (recoat every 10–15)
Fully insulated GRP warm roof	about £210	–	as GRP
Liquid plastic coating	£20–£28	£24	not published
Mastic asphalt	£50–£70	£60	20–30 years
Green roof	£75–£110	£100	not published

The main flat roof table was last refreshed in December 2024, so it is a little dated; the September 2026 GRP guide’s figure of about £210 per m² for a fully insulated GRP roof is the better guide if your roof needs insulating, which most replacements do.

Checkatrade’s own lifespan figures vary between its guides (one says felt lasts roughly 10–15 years, another 15–20), so treat them as typical ranges, not promises.

What does the per-m² price include?

Usually just the new covering laid on a sound deck. The fully insulated figure is closer to a whole build-up from the deck to the finish. Removal of the old roof, replacement decking, scaffolding, the skip and new trims or gutters are normally priced separately, which is why two quotes for the same roof can look so different.

How much does a new flat roof cost for a garage, extension or dormer?

Real jobs are priced as a whole, not just by area. This table works through typical sizes using Checkatrade’s EPDM rate of £80–£100 per m² for the covering only, alongside a published whole-job benchmark. The sizes are illustrative, not standard.

Roof	Example size	EPDM covering only	Published whole-job benchmark
Single garage	15 m²	£1,200–£1,500	Re-felt a flat garage roof: £1,000–£2,000
Rear kitchen extension	20 m²	£1,600–£2,000	Cambridge new flat roof average: £2,059
Larger extension	32 m²	£2,560–£3,200	UK flat roof of 32 m²: £2,800–£7,500
Dormer roof	10 m²	£800–£1,000	Scaffolding usually extra

The benchmarks come from Checkatrade’s re-felting guide, its Cambridge cost data and its roof replacement guide. MyBuilder’s 2026 guide puts a flat GRP or membrane roof at £3,000–£6,000.

Garage roofs

Garages are the simplest flat roofs: usually ground level, easy access and no internal ceilings to protect. Checkatrade’s re-felting figures assume felt at about £50 per m² plus labour at about £200 a day. On 1960s and 1970s garages around Arbury, King’s Hedges and Chesterton, check the roof sheets before anyone starts: HSE lists asbestos cement roofs among materials that may be found in buildings built or refurbished before 2000, and its advice is not to disturb them.

Rear extensions

Extensions are the most common flat roofs in Cambridge, especially behind the terraces of Romsey, Petersfield and De Freville. Expect extra cost for roof lanterns or rooflights (Checkatrade puts a flat roof dome at £300–£400), new upstands against the house wall and lead or trim details at the abutment.

Dormers

Dormer roofs are small, but often the dearest per square metre. They need scaffolding, the details are tight and every upstand has to tie neatly into the main pitched roof. A dormer on a three-storey townhouse can cost more in access than in materials.

What extras add to the cost of a flat roof replacement?

Checkatrade’s flat roof calculator uses these typical figures for the extras:

• Removal of the existing roof: about £2,000.
• Scaffolding: about £900 a week.
• Skip: about £300.
• Insulation: about £960.
• New gutters: about £775.

The deck underneath

The timber or board deck is the unknown in every flat roof quote. If it is soft, rotten or delaminated, it has to be replaced before the new covering goes on, and that can only be priced properly once the old covering is lifted. A good quote says how replacement decking will be charged if it is needed.

Insulation and Building Regulations

Approved Document L treats “replacing the waterproof membrane on a flat roof” as renovating a thermal element. If more than 50% of the roof is renovated, the whole roof should be upgraded to 0.16 W/m²K where that is technically and economically feasible. Cambridge’s shared building control service, 3C Building Control, says approval is needed for “replacement roofs (even if this only new or replacement ‘felt’)”.

In practice that usually means a warm roof: insulation boards laid over the deck, under the new waterproof layer. It costs more than a straight re-cover, but it keeps the timbers warm and dry and cuts heat loss. Our guide to warm roof vs cold roof explains the difference.

Should you repair or replace a flat roof?

A patch repair is often the sensible first step. Checkatrade’s Cambridge data puts a typical flat roof repair at £584–£1,738, averaging about £1,002. Its UK repair rates per m², for areas up to about 3 m², are:

Repair	Price per m² (UK)	Price per m² (Cambridge)
Bitumen felt	£100–£140	£94–£135
EPDM rubber	£120–£170	£116–£167
GRP fibreglass	£130–£180	£123–£177

Repair it if the leak is local (a split, a failed upstand or a lifted edge) and the deck is sound. Replace it if the covering is blistered or cracked all over, the deck feels spongy underfoot, the roof leaks in new places each winter, or it is felt that is already 15 years old or more.

Some ponding is normal: the Confederation of Roofing Contractors says “all flat roofs will have ponding”. Water that is still standing weeks after rain, though, deserves a closer look.

Which flat roof system gives the best value?

Divide the average price per m² by the typical lifespan and the picture changes. Using Checkatrade’s figures, torch-on felt costs roughly £3.25–£4.33 per m² per year of life, GRP about £3.58–£4.30 (before any recoat), and EPDM about £1.80–£3.00. These are rough sums, not quotes, but they explain why EPDM is so popular for extensions and garages.

How Cambridge weather affects your choice

East Anglia is one of the driest parts of the UK, but it is hard on flat roofs in other ways. The Met Office’s summer 2025 regional figures named East Anglia the warmest and sunniest district in the UK’s warmest summer on record, and Cambridge averages around 40 days of air frost a year. Heat ages bitumen felt, and Checkatrade notes that GRP can be “prone to cracking over time in very cold conditions”, while EPDM’s flexibility copes with expansion and contraction. Each system can work well here when it is detailed properly. Our EPDM vs GRP guide compares them in depth.

We fit all three main systems, so our advice is not tied to one product. Read more about our EPDM rubber roofing, GRP fibreglass roofing and felt roofing, or see our main flat roofing page.

Is it worth converting a flat roof to a pitched roof?

Sometimes, but it is a bigger project than a replacement. Checkatrade puts a flat-to-pitched conversion at £3,000–£4,500, averaging about £3,750, before any structural design or decoration inside. It also changes the planning picture: replacing a flat roof with a pitched one can count as a roof enlargement, and in Cambridge’s conservation areas roof extensions need planning permission. It will need Building Regulations approval too, because new roof timbers are structural work.

For most extensions and garages, a well-detailed flat roof in EPDM or GRP with proper falls is the simpler and cheaper answer. A pitched conversion makes more sense when a flat roof has failed repeatedly, or when you want the look or the loft space and are happy to go through the approvals.

How do you compare flat roof quotes?

Two flat roof quotes can differ by thousands for the same roof, usually because one includes things the other leaves out. Check that each quote states:

1. The system (felt, EPDM, GRP or liquid) and how it is fixed or bonded.
2. Whether the old covering is stripped and taken away, and the skip.
3. The insulation thickness and whether the roof will meet Building Regulations.
4. How replacement decking will be charged if rot is found.
5. Upstands, trims, outlets, gutters and any rooflights or lanterns.
6. Scaffolding or access equipment, and VAT.
7. The guarantee and what it covers.

Get a fixed price for your roof

Per-m² rates are useful for budgeting, but the deck, the details and the access decide the real price. We inspect your flat roof by ladder or drone, check the deck and the ceiling below, and give you a detailed written quote with repair and replacement options and a fixed price. The survey is free with no obligation. Call 01223 902596 (Monday to Friday, 8am to 5pm) or request a free quote.