The Short Answer
Re-covering a single garage (about 18m², 6m by 3m) in EPDM rubber costs £1,600 to £2,800 on a sound deck. Replacing a 20m² flat roof over a kitchen extension, with a new deck and insulation, is £2,850 to £4,900. Patch repairs are £120 to £170 per m².
If there's a heated room underneath, replacing the covering usually means insulating it to today's standard under the building regulations, so check every quote includes it. Our prices add a fixed £450 to £900 for trims, outlets and the skip, which is why small roofs cost more per m².
Flat roof cost calculator
A flat roof is priced per square metre plus fixed costs that don't shrink with the roof, which is why small roofs cost more per m².
Estimated total
£1,730 to £3,000
| EPDM rubber, 15m² | £980 to £1,580 |
| Edge trims, outlets, skip and access | £450 to £900 |
| New roof deck (OSB or ply) | £300 to £530 |
Flat roof prices by covering
| Covering | Per m², laid | Garage, 18m², sound deck | Extension, 20m², new deck, insulated |
|---|---|---|---|
| Torch-on felt | £60 to £95 | £1,550 to £2,600 | £2,750 to £4,700 |
| EPDM rubber | £65 to £110 | £1,600 to £2,800 | £2,850 to £4,900 |
| GRP fibreglass | £75 to £120 | £1,800 to £2,950 | £3,050 to £5,100 |
| Extra | Typical cost |
|---|---|
| New roof deck (OSB or ply), per m² | £20 to £35 |
| Insulation, warm deck, per m² | £35 to £60 |
| Rotten joists, allowance | £250 to £1,200 |
| Patch repair, per m² | £120 to £170 |
Felt, EPDM or GRP, and how long they last
| Torch-on felt | EPDM rubber | GRP fibreglass | |
|---|---|---|---|
| What it is | Bitumen membrane, flame-bonded to the deck | Synthetic rubber, usually one sheet, glued cold | Resin and glass fibre, laid wet, then a topcoat |
| What the maker says about life or guarantee | At least 35 years (IKO), and it can be overlaid with a new layer later | Over 50 years (Holcim Elevate, the Firestone range) | 10 to 15-year warranty; 25 years on its premium system, approved installers only (Scott Bader) |
| Per m², laid (our estimate) | £60 to £95 | £65 to £110 | £75 to £120 |
The figures above are best cases from the people who make the materials. My view: choose the installer before the material, ask which system they're fitting by name, and get the maker's guarantee in writing. For a GRP guarantee, check they're one of the maker's approved installers.
When a new flat roof must be insulated
Approved Document L treats replacing the waterproof membrane on a flat roof as renovating a thermal element. If you renovate more than 50% of a roof over a heated room, the whole roof should be brought up to a U-value of 0.16 W/m²K, as far as that's technically possible and pays back within 15 years.
The guidance gives the typical case: existing insulation under 100mm, in poor condition or disturbed by the work means insulating between and over the joists. Insulation on top of the deck raises the roof, so upstands, door thresholds and the fascia have to work with it; the guidance allows a lesser standard where the structure or upstand heights make the full target impractical.
Your roofer can self-certify the work if they're registered for roof coverings with the National Federation of Roofing Contractors (NFRC) or NAPIT; otherwise a building regulations application is needed first.
Repair or replace?
Warning signs on a flat roof, from Which?'s guidance: cracks or splits round the edges, uneven lumps, sagging (water may have built up), and damp patches on the ceiling below, which can mean a tear in the covering.
A repair is the cheaper option while the problem is local and the roof is otherwise sound. If it's been patched repeatedly, or you want to insulate the room below anyway, I'd get a replacement quote alongside. Pitched roof leaks are on roof repair costs; garage and shed re-felting on roof re-felting costs.
A correction
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Frequently asked questions
How much does a new flat roof cost?
About £1,600 to £2,800 to re-cover an 18m² garage in EPDM, or £2,850 to £4,900 for a 20m² insulated extension roof with a new deck. Felt is a little cheaper, GRP a little dearer.
Which is best: felt, EPDM or GRP?
The makers claim long lives for all three: at least 35 years for IKO's bitumen membranes, over 50 years for Elevate's EPDM, and up to a 25-year guarantee on Scott Bader's premium GRP system. I'd choose the installer first and get the maker's guarantee in writing.
How often do flat roofs need replacing?
It depends on the material and the fitting. Makers' own claims run from a 10 to 15-year warranty on a standard GRP system to at least 35 years for modern bitumen membranes and over 50 years for EPDM.
Does house insurance cover flat roof leaks?
Sudden storm damage can be covered; gradual wear usually isn't. If a claim is refused, the Financial Ombudsman Service asks whether there were storm conditions, whether the damage looks like storm damage, and whether the storm was the main cause.
Do I need building regulations for a new flat roof?
If it's over a heated room, yes: replacing the membrane on more than 50% of it means insulating it to current standards where practical. A roofer registered with the National Federation of Roofing Contractors (NFRC) or NAPIT can self-certify.
How much does a flat roof repair cost?
£120 to £170 per m² for a patch repair, on our estimates.
How we priced thisView sources (11, 4 official)
Rules come from the legislation and official guidance, read at source. Prices are our estimates: we check them against trade price data, retailer shelf prices and specialists' price lists, then build them up from their parts so the totals add up. Below are the sources behind the page; the trade price guides we only compare against aren’t listed. Where sources disagreed, we've said so. More on our method.
- OfficialApproved Document L: Conservation of fuel and power, Volume 1 Dwellings (2021 edition incorporating 2023 amendments), GOV.UK. Checked 16 Oct 2026.
- OfficialThe Building Regulations 2010, Schedule 3: self-certification schemes (para 16, roof coverings), legislation.gov.uk. Checked 15 Oct 2026.
- OfficialNFRC Competent Person Scheme: property owners' FAQ, NFRC CPS. Checked 15 Oct 2026.
- IndustryWhat is the lifespan of an IKO bituminous roofing membrane?, IKO. Checked 15 Oct 2026.
- IndustryRubberCover EPDM membrane (residential roofing), Holcim Elevate (Firestone). Checked 15 Oct 2026.
- IndustryCrysticROOF GRP roofing brochure (UK), Scott Bader. Checked 15 Oct 2026.
- IndustryHow much do roof repairs cost? (Which? Trusted Traders survey, November 2025), Which?. Checked 15 Oct 2026.
- OfficialStorm damage (home buildings insurance complaints), Financial Ombudsman Service. Checked 15 Oct 2026.
- Trade guideFlat roof replacement costs, Checkatrade. Checked 15 Oct 2026.
- Trade guideRoof repair cost, Checkatrade. Checked 15 Oct 2026.
- IndustryMasonry paint, PVCu guttering, fascia boards, facing bricks and roof insulation (shelf prices), Wickes. Checked 15 Oct 2026.
