Flat Roof Cost Calculator
Enter your flat roof size, material type, job scope, and UK region — get an instant, detailed cost estimate for replacement or new installation including stripping, labour, drainage, and optional extras.
Calculate your flat roof cost
Fill in your roof details below for an instant cost estimate — including material supply, labour, old roof stripping, insulation, drainage, and optional extras like roof lights or green roof build-up.
Your flat roof details
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Your Cost Estimate
Full breakdown including all works & extras
Fill in your flat roof details and click Calculate to get your personalised cost estimate — including material supply, labour, strip-out, insulation, drainage, and all selected extras.
Choosing the right flat roof material
The material you choose affects upfront cost, lifespan, maintenance needs, and suitability for your roof’s shape and pitch. Here’s what you need to know about each option.
Mineral felt
The traditional budget option. Three-layer mineral felt is the most affordable flat roofing system but has the shortest lifespan — 10–15 years with good installation. Prone to cracking and ponding issues. Best for small, low-budget jobs like garden buildings.
£30–£45/m² · 10–15 yr lifespanEPDM rubber
Ethylene Propylene Diene Monomer — a single-ply rubber membrane that’s become the dominant modern flat roofing system. Seamless, highly flexible, and extremely durable. Excellent around penetrations and in cold climates. The best all-round value option.
£40–£65/m² · 25–50 yr lifespanGRP fibreglass
Glass Reinforced Polyester forms a rigid, seamless monolithic surface. Extremely durable, hard-wearing, and can be walked on. Ideal for balconies, roof terraces, and complex shapes. Requires more specialist installation than EPDM — cracking can occur if poorly installed on large spans.
£50–£80/m² · 30–40 yr lifespanTorch-on felt
High-performance modified bitumen felt applied with a gas torch. Two or three layers create a robust waterproofing system. Better than traditional cold-applied felt. Requires gas-safe equipment — not suitable for some timber substrates without a vapour barrier. Good mid-range option.
£40–£60/m² · 15–25 yr lifespanLiquid membrane
Cold-applied liquid waterproofing systems (e.g. Sikalastic, Kemperol) are brushed or rolled onto the decking, forming a seamless reinforced membrane. Excellent for complex shapes, penetrations, and repairs. Can be applied over existing surfaces without stripping. Growing in popularity.
£55–£90/m² · 25–30 yr lifespanZinc / lead sheet
The premium, architectural option. Standing seam zinc or traditional lead is extremely long-lasting and aesthetically striking. Often used on heritage buildings, contemporary architecture, and high-specification extensions. Requires highly skilled specialist roofers — labour costs are high.
£90–£140/m² · 60–100 yr lifespanFlat roof costs by size & material
Typical 2024 UK costs for a complete flat roof replacement including strip-off, new OSB decking, insulation, material, and labour — Midlands/North England pricing. Add 20–35% for London and South East.
| Roof size | Mineral felt | EPDM rubber | GRP fibreglass | Torch-on felt | Liquid membrane |
|---|---|---|---|---|---|
| 10 m² (small) | £700–£1,100 | £900–£1,400 | £1,100–£1,800 | £800–£1,300 | £1,200–£1,900 |
| 20 m² (garage) | £1,200–£1,900 | £1,600–£2,500 | £2,000–£3,200 | £1,400–£2,200 | £2,100–£3,400 |
| 30 m² (extension) | £1,700–£2,700 | £2,300–£3,600 | £3,000–£4,800 | £2,000–£3,200 | £3,100–£5,000 |
| 50 m² (large) | £2,600–£4,100 | £3,500–£5,500 | £4,700–£7,500 | £3,100–£4,900 | £4,800–£7,800 |
| 100 m² (commercial) | £4,500–£7,200 | £6,200–£9,800 | £8,500–£13,500 | £5,600–£8,900 | £8,800–£14,000 |
How to get the best flat roof for your money
Follow these practical tips from roofing professionals to make a smart decision and avoid the most common — and costly — flat roof mistakes.
Always check the decking first
Before any new membrane goes on, the timber decking underneath must be sound. Rot or movement in the decking will cause any new membrane to fail prematurely — within a year in some cases. A reputable roofer will lift a section of the existing covering and inspect before quoting. Never accept a quote that doesn’t mention decking inspection.
Choose EPDM or GRP over felt
Mineral felt may be cheaper upfront, but replacing it every 10–15 years adds up. A well-installed EPDM or GRP roof costs 30–50% more initially but lasts 2–3 times longer. Over 30 years, the modern alternatives are almost always cheaper on a whole-life cost basis — and far less disruptive to replace.
Drainage is critical
A flat roof must have adequate falls (minimum 1:80 gradient) to drain properly. Ponding water accelerates deterioration of any membrane. Ensure your roofer checks falls, clears or upgrades outlets, and uses tapered insulation if needed to create positive drainage. Ask specifically about outlet sizes — undersized outlets are a very common fault.
Insist on a written guarantee
A reputable roofer will provide a written workmanship guarantee of at least 10 years, backed by the material manufacturer’s warranty. EPDM and GRP systems from leading manufacturers come with 20–25 year product warranties — but only if installed by accredited contractors. Check accreditation before signing any contract.
Cost estimates you can trust
Our flat roof cost calculator uses 2024 UK trade pricing data from specialist flat roofing contractors, NFRC rate surveys, and material supplier pricing — adjusted for your material choice, job type, access difficulty, insulation specification, and UK region.
We break costs down into material supply, labour, strip-out, insulation, and extras so you can compare quotes line by line — and spot anything suspicious before work begins.
- ✓2024 UK labour and material rates built in
- ✓6 flat roof materials with individual cost models
- ✓Regional price variation across all UK nations
- ✓Job type (replace / repair / new) factored separately
- ✓Insulation, drainage, decking & extras all included
- ✓No ads, no sign-up, no data stored — runs in your browser
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