Flat roof looking tired, or worse, leaking?
Replacing it can stop damp, improve insulation, and set you up for low-maintenance living for years. Costs vary with location, size, access, and the chosen system, among other factors, but an investment now is an investment in your family’s future comfort
How much does a flat roof replacement cost?
A quote from a trusted builder is the best way to know exactly how much it’ll cost to replace or upgrade your flat roof. Prices depend on several factors, including the location and size of your property, the quality of finish, and the materials you choose. However, the table below provides a general guideline.
| Flat roof area | Installation range* |
|---|---|
| 20 m² (small porch/garage) | £2,800 – £3,600 |
| 30 m² (small extension) | £4,200 – £5,400 |
| 50 m² (medium extension) | £7,000 – £9,000 |
| 70 m² (larger extension/garage block) | £9,800 – £12,600 |
* Installation costs available on the website of FMB member company Aspect.
For more information on installing a new roof, read our guide on new roof costs.
How to choose a good roofer
When looking for a roofer, make sure they can provide you with a written, itemised quote. Also ask for proof of insurance and always ensure you have a signed contract in place before work starts.
Beware of cowboy roofers who promise rock-bottom prices and push for cash. Red flags include the absence of a company address, reluctance to specify products or warranties, and pressure to start 'today'.
At the FMB, all of our members are vetted and their work has been inspected before they can join the FMB - so you can be sure of a job well done. Find trusted roofers in your area by entering your postcode below.
What makes up the flat roof replacement cost?
There are several elements to the construction of a flat roof:
- Roof deck
- Vapour control layer (VCL)
- Insulation boards
- Cover board
- Waterproof layer
- Edge trims and gutters
- Flashings
Material costs
Roof deck
The roof deck is the structural sheet fixed over joists or trusses that everything else sits on, providing the primary support. Common deck materials include Oriented Strand Board 3 (OSB 3), a load-bearing, moisture-resistant engineered wood sheet, or structural plywood. Some older roofs use timber boards, and concrete slabs are occasionally used. OSB 3 costs around £25 per board, depending on size and depth.
VCL
The VCL sheet sits on the warm side of your roof or ceiling build-up. It slows moist indoor air from reaching the colder layers, helping prevent hidden condensation, which could damage your insulation or timbers. Depending on the thickness of the VCL sheet, expect to pay £27 for a 25 metre roll.
Insulation board
Insulation boards are rigid boards that sit above the deck to cut heat loss. Polyisocyanurate (PIR) foam is favoured over other materials due to its high thermal performance and lightweight nature. Typical prices are from £25 per m2.
Cover board
Cover or overlay boards are a thin, rigid layer laid on top of the insulation to give a smooth, rigid surface for the waterproof coating. It evens out joints in the insulation and improves durability. OSB 3 sheets are typically used as cover boards, costing around £25 per board.
Waterproof layer
The final layer is the outer, waterproof layer, typically made of bituminous felt. Bituminous felt is a flexible roofing sheet made from a reinforcement material soaked in bitumen and laid in layers that are heat-sealed with a torch, forming a durable, waterproof flat-roof surface. It’s then covered with slate chips to provide UV protection. Often called a mineral capsheet, it costs around £45 per roll.
Ethylene Propylene Diene Monomer (EPDM) is a synthetic rubber sheet used as the waterproof layer on a flat roof, as an alternative to bituminous felt. Durable and long-lasting, it's supplied in large rolls, typically costing around £15 per m2.
Edge trims and gutters
Edge trims finish the roof parameter, secure the membrane, and guide the rainwater into the gutters. Expect to pay between £10 and £20 per 2.5m length.
Gutters catch rainwater and carry it to downpipes, protecting walls and foundations from runoff, and cost around £5 for a two metre length.
Flashings
Lead flashing creates a watertight seal at junctions, such as where a flat roof meets a wall, or along seams. A typical price for a three metre strip is around £25.
For a more detailed look at roofing materials, read our Ultimate Guide to Roofing.
Additional flat roof costs
In addition to the materials, several other factors affect your budget, with labour making up the bulk of the flat roof cost.
Labour
A small flat roof typically takes two to three days to replace, whereas an average-sized roof can take up to a week. According to FMB member company Aspect, expect to pay around £85 per hour for labour. Although this figure can vary with geographical location, quality workmanship designed to last is well worth the short-term cost, as it saves money in the long term.
Scaffolding and access
Scaffolding provides a temporary, rigid platform, giving builders safe access to your flat roof. It can be sheeted to keep rain and debris off the work area, particularly useful with roofing work, and ensures the job meets safety regulations.
Expect to pay around £400 per day, depending on the amount of scaffolding required and its placement. If it has to sit on the public pavement, your builder may need permission from your local council in the form of a licence, which will add to the cost.
Waste removal and disposal
Stripping an old flat roof generates a lot of bulky waste. Your builder will be able to estimate the skip size required, but as a rough guide, a mid-size six-yard skip typically starts from £200.
Our members are rigorously monitored, which includes a thorough vetting, reassuring you that you have an experienced, professional team working on your flat roof or any other home improvement projects.
Having your flat roof replaced or upgraded boosts your family’s day-to-day comfort, delivering weatherproofed protection and a cosy home for the future.
For further information about roofs, how to maintain them and how to recognise when it's time to replace them, read our ultimate guide to roofing.