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What do flat roofs cost, and what is in the price?
The Federation of Master Builders puts the average cost of a flat roof replacement at £8,000 and publishes installation ranges by roof area [S1]. Your own price depends on the size, the system chosen, the state of the deck and the access, so treat these figures as a guide rather than a quote.
Last reviewed:What does a flat roof cost?
Flat roofs cover garages, porches, dormers, bays and many rear extensions. When one starts to leak, the first question is usually how much a new one costs.
The Federation of Master Builders gives an average cost of £8,000 for a flat roof replacement [S1]. It advises budgeting an extra 10 to 15% as a contingency [S1]. It also publishes installation ranges by roof area, credited to one of its member firms [S1].
| Roof area | Example FMB gives | FMB installation range |
|---|---|---|
| 20 m² | Small porch or garage | £2,800 to £3,600 [S1] |
| 30 m² | Small extension | £4,200 to £5,400 [S1] |
| 50 m² | Medium extension | £7,000 to £9,000 [S1] |
| 70 m² | Larger extension or garage block | £9,800 to £12,600 [S1] |
| Contingency | Any size | An extra 10 to 15% [S1] |
| VAT | Ask on every quote | Registration is required above £90,000 of taxable turnover in the last 12 months [S5] |
These are guide figures, published by FMB, not a quote for your roof [S1]. FMB says costs vary with location, size, access and the chosen system, among other factors [S1].
Our roofers page covers the trade, and the roof repair cost guide explains smaller repairs. QuoteZila starts in Cheshire, with Crewe, Nantwich and Sandbach first, and anyone in the UK can post a flat roof job.
What is inside a flat roof price?
A flat roof is not just the top layer you can see. FMB lists the layers that make up a typical flat roof [S1]:
- Roof deck. The structural boards over the joists. FMB says OSB costs around £25 per board, depending on size and depth [S1].
- Vapour control layer. A sheet on the warm side that slows moist indoor air from reaching the colder layers.
- Insulation boards. Rigid boards above the deck to cut heat loss.
- Cover board. A thin rigid layer over the insulation to give a smooth base.
- Waterproof layer. The surface you see, such as bituminous felt or a rubber membrane.
- Edge trims and gutters. FMB gives a price of £10 to £20 per 2.5 m length of edge trim [S1].
- Flashings. Where the roof meets a wall. FMB says a three metre strip of lead flashing is typically around £25 [S1].
A quote that lists these layers separately is easier to compare than one with a single figure. If a layer is missing, ask why.
How do you know a flat roof needs attention?
FMB's roofing guide lists splits, blisters, lifting edges or persistent standing water on a flat roof among the signs of a problem [S8]. Inside, look for stains on the ceiling under the roof, damp patches near the walls, or a musty smell in the room below.
FMB also says water can travel before it becomes visible indoors, so a stain may be some distance from the defect [S8]. On a flat roof that often means the leak is at an edge, an outlet or a wall flashing rather than in the middle of the roof.
Look from an upstairs window or from the ground, and take photos. Do not walk on the roof to check it. A soft or spongy feel underfoot is a sign the deck may be wet, and a wet deck may not hold your weight.
Where does most of the money go?
FMB says labour can make up around 40% of the total flat roof replacement cost [S1]. It also says a small flat roof typically takes two to three days to replace, while an average-sized roof can take up to a week [S1].
The rest goes on materials, access and waste. Stripping an old flat roof produces bulky waste, and getting a skip in or a van close can be awkward. GOV.UK's waste code says a tradesperson who works on your property is responsible for the waste they produce, and the cost of its disposal should be included in what they charge [S7].
Which flat roof systems are common?
Bituminous felt. Layers of reinforced felt, heat-sealed together, often finished with mineral chips for protection from sunlight. FMB describes it as typically used for the waterproof layer [S1].
EPDM rubber membrane. A synthetic rubber sheet laid in large pieces, with few joins. FMB describes it as durable and long-lasting [S1].
Glass reinforced plastic (GRP). A liquid resin and glass mat laid on site to form a seamless surface. Our fibreglass roof guide covers it in detail.
Liquid coatings. Brushed or rolled onto a sound existing roof as a repair or refurbishment. They are not a fix for a rotten deck.
Lead and other metals. Used on small, visible roofs such as bays and dormers, and on some period homes.
Ask each roofer which system they recommend for your roof, and why. Different roofers may favour different systems, so ask each one to price the same system as well, so you can compare.
Repair, re-cover or replace?
Repair means patching a split, a blister or a lifting edge. It suits a roof that is otherwise sound.
Re-cover means laying a new waterproof layer over the existing roof, or over a thin overlay. It can work if the deck and insulation are dry. It can also hide a wet deck.
Replace means stripping back to the joists, checking them, and building the roof up again layer by layer. It is the dearest option and the one where the deck and insulation are renewed.
Planning Portal says some flat roof repairs do not need building regulations approval, but if a roof with integral insulation is replaced, you may be required to upgrade the insulation [S2]. It also says repairs or re-covering will not normally need a building control application if less than 25 per cent of the building envelope and less than 50 per cent of the roof is affected [S3].
Our flat roof replacement guide goes deeper into the replacement job itself.
Does it matter where the flat roof is?
The same area of flat roof can cost more or less depending on where it sits.
Garages and outbuildings are often easy to reach, with room for a skip or a van nearby. HSE lists asbestos cement products, including roofs, among the places asbestos can be found in buildings built or refurbished before the year 2000 [S9]. Mention the age of the garage in your job, and do not break or cut an old roof sheet yourself.
Rear extensions often join the house wall, with a flashing along the join and sometimes a roof window or lantern to work around.
Dormers and bays are small, but they sit high up or at the front of the house, so access takes more planning and the finish is on show.
Balconies and roof terraces need a surface you can walk on, guarding at the edge and careful drainage, which is a different job from a plain roof.
Tell the roofer which of these you have, and send a photo that shows how they would get to it.
Why is a flat roof still work at height?
A garage roof may look low and easy to reach, but falls from roof edges and through weak decks happen at any height. HSE's guidance for workplaces says to avoid work at height where it is reasonably practicable to do so, and otherwise to prevent falls using the right type of equipment [S6]. HSE: work at height. QuoteZila is not affiliated with this organisation. HSE explains more in its introduction to working at height.
This guide does not give a method for working on a flat roof. Ask each roofer what edge protection and access they will use.
What should a flat roof quote include?
Ask every roofer to price the same job, and to list:
- the roof area and the system, by product name
- whether the old roof is stripped back to the joists, or re-covered
- the deck, vapour control layer, insulation and cover board
- edge trims, gutters, outlets and flashings
- a rate for replacing rotten joists or deck found during the job
- access equipment and edge protection
- waste removal
- the guarantee offered, in writing, and who backs it
- whether VAT is included
Citizens Advice suggests trying to get written quotes from at least 3 different traders before you decide on one, and says some traders charge for quotes, so ask first [S4]. It suggests offering to buy materials yourself if a trader asks for a deposit to pay for them, and always getting a receipt for any deposit [S4].
If the flat roof is part of a new extension rather than a replacement, our flat roof extension cost guide covers that wider job.
Jobs you might post
These are examples, not taken from real job posts.
- "Single garage with an old felt roof, leaking in two places and soft underfoot near the back. Please quote to strip and replace, including the deck if needed."
- "Kitchen extension flat roof, around ten years old, ponding water after rain. Please inspect and quote to repair or replace."
- "Small porch roof, felt lifting at the edge. Please quote for a rubber membrane with new edge trims."
Posting a flat roof job
Add photos of the roof, the edges and any stains inside, and give the rough size if you know it. Your job goes to up to five matched local businesses [QZ2]. QuoteZila is free for customers and local businesses [QZ1].
Honest status: QuoteZila is new. We're starting in Cheshire, beginning with Crewe, Nantwich and Sandbach, and growing area by area. Anyone in the UK can still post a job or register a business. How fast you get quotes depends on how many local businesses have joined near you. Businesses can register and receive jobs straight away. We don't check insurance, qualifications or registrations, so please check these yourself before you hire.
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Questions about flat roof prices
What is the average price of a flat roof?
The Federation of Master Builders gives an average cost of £8,000 for a flat roof replacement. That is an average across sizes and systems. A small garage roof and a large extension roof will sit at very different points.
How much does a small flat roof cost?
FMB lists an installation range of £2,800 to £3,600 for a 20 m² roof, which it describes as a small porch or garage. Your price depends on access, the system used and the state of the deck underneath.
Which flat roof system lasts longest?
It depends on the product and how well it is laid and looked after. FMB says a well maintained flat roof can last over 20 years. Ask each roofer what system they recommend for your roof, how long they expect it to last and what guarantee they give in writing.
Do I need approval to replace a flat roof?
Often. Planning Portal says that in most situations replacing a roof needs Building Regulations approval, and that replacing a flat roof with integral insulation may mean upgrading the insulation. Ask each roofer how they will deal with sign-off.
Why is one flat roof quote much lower than the others?
Check what it covers. A lower price may re-cover over the old roof instead of stripping it, leave out insulation, or skip new edge trims and flashings. Compare each quote layer by layer, not just by the total.
Does QuoteZila check roofers?
No. We don't check insurance, qualifications or registrations, so please check these yourself before you hire. You can report a quote or a business from your job page.
This page is general information, not professional or legal advice. QuoteZila does not check Gas Safe, WaterSafe, NFRC, NICEIC or other registrations against the register on your behalf: look them up yourself.
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Related trades
Cost guides
Related pages
- Flat Roof Replacement: What to Compare. The Federation of Master Builders flat roof guide of 7 November 2025, updated March 2026, puts the average replacement at £8,000. QuoteZila does not set prices.
- Gutter Cleaning: What a Quote Should List. A gutter cleaning quote in Cheshire should name each elevation, the downpipes and the access. A clean and a roof repair are different jobs.
Sources
- QZ1: QuoteZila product fact. QuoteZila home and FAQ copy.
- QZ2: QuoteZila product fact. QuoteZila FAQ How does getting quotes work.
- S1: Flat roof replacement cost guide for UK homeowners, Federation of Master Builders. Accessed 2026-11-02. https://www.fmb.org.uk/resource/flat-roof-replacement-cost-guide-for-uk-homeowners.html
- S2: Building regulations: Re-roofing, Planning Portal. Accessed 2026-11-02. https://www.planningportal.co.uk/permission/common-projects/roof/building-regulations-re-roofing
- S3: Building Regulations: Work to an existing roof, Planning Portal. Accessed 2026-11-02. https://www.planningportal.co.uk/permission/common-projects/roof/building-regulations-work-to-an-existing-roof
- S4: Before you get building work done, Citizens Advice. Accessed 2026-11-02. https://www.citizensadvice.org.uk/consumer/getting-home-improvements-done/before-you-get-building-work-done/
- S5: Register for VAT, GOV.UK. Accessed 2026-11-02. https://www.gov.uk/register-for-vat
- S6: Introduction to working at height safely, Health and Safety Executive. Accessed 2026-11-02. https://www.hse.gov.uk/work-at-height/introduction.htm
- S7: Waste duty of care: code of practice, GOV.UK (Defra). Accessed 2026-11-02. https://www.gov.uk/government/publications/waste-duty-of-care-code-of-practice/waste-duty-of-care-code-of-practice
- S8: The ultimate guide to roofing, Federation of Master Builders. Accessed 2026-11-02. https://www.fmb.org.uk/find-a-builder/ultimate-guides-to-home-renovation/the-ultimate-guide-to-roofing.html
- S9: Asbestos: frequently asked questions, Health and Safety Executive. Accessed 2026-11-02. https://www.hse.gov.uk/asbestos/faq.htm