How Much Does Flat Roof Replacement Cost?
Flat roof replacement typically costs £1,500 to £7,500 in the UK, depending on size and material. Per square metre, expect £50 to £60 for EPDM rubber, £70 to £100 for GRP fibreglass, and £40 to £70 for felt. A single garage (around 20m²) in EPDM often lands between £1,500 and £2,500 fitted.
| Project | Typical size | EPDM fitted | GRP fitted |
|---|---|---|---|
| Single garage | ~20m² | £1,500 to £2,500 | £2,000 to £3,500 |
| Double garage / large extension | ~40m² | £2,500 to £4,500 | £3,500 to £6,000 |
| Dormer roof | ~15m² | £1,200 to £2,200 | £1,800 to £3,000 |
| Porch / bay window | ~5m² | £600 to £1,200 | £800 to £1,500 |
EPDM vs GRP vs Felt: Which Material?
| EPDM rubber | GRP fibreglass | Felt (torch-on) | |
|---|---|---|---|
| Cost per m² fitted | £50 to £60 | £70 to £100 | £40 to £70 |
| Lifespan | 25 to 30 years | 25 to 30 years | 10 to 15 years |
| Best for | Best value; DIY kits exist | Seamless finish; complex shapes | Budget jobs; short-term fixes |
| Watch out | Needs careful detailing at edges | Weather-sensitive installation | Shortest life; seams can fail |
Why Do Quotes Vary So Much? The Quote Decoder
The number one frustration in forums: one roofer quotes £800, another £4,500 for the “same” job. The variance is almost always scope, not dishonesty. Check whether each quote includes:
- Strip or overlay? Stripping back to decking costs more but reveals rot; overlaying is cheaper but only valid on sound decks (and not for every material).
- Decking replacement: new OSB3 boards add £20 to £40/m²; quotes assuming sound decking look cheaper until rot is found.
- Insulation: warm-roof upgrades (required by building regs on 50%+ renewals) add cost but cut heat loss. See our flat roof insulation guide for the numbers.
- Edge trims and details: cheap quotes often skimp here; failures start at edges.
- Scaffolding and waste removal: included or “extra on the day”?
- Guarantee: manufacturer-backed vs a handshake promise.
Is my quote fair? Quick checklist
- Get 3 quotes and compare scope line by line, not just totals
- EPDM at £50 to £60/m² fitted is the market anchor; far below needs scrutiny
- Ask what happens if the decking is rotten (per-m² extra agreed upfront)
- Confirm insulation and building regs compliance in writing
- Check the guarantee: who backs it if the firm folds?
Repair or Replace? The Decision Framework
- Repair when: localised damage (one blister, one failed seam), roof under 10 years old, decking sound. Typical repair: £200 to £600.
- Overlay when: membrane tired but decking sound, budget tight. Adds 10 to 15 years at 60 to 70 percent of full replacement.
- Full replacement when: widespread failure, ponding that will not drain, spongy decking, or the roof is past its material lifespan. Compare with new roof costs for pitched roofs.
Guarantees: Read the Small Print
Three different guarantees exist and they are not the same: manufacturer (covers the membrane, often 20 to 25 years, conditional on certified installation), workmanship (the roofer’s own promise, only as good as the firm), and insurance-backed (pays out even if the firm folds; always ask for this on jobs over £2,000). A “25-year guarantee” from a one-van outfit without insurance backing is worth little.
Building Regulations
Renewing more than 50 percent of a roof triggers Approved Document L: the roof must meet U-value 0.18 W/m²K, which usually means adding insulation (a warm roof build-up). Your roofer should handle building control notification; confirm it is included, not assumed.
Can You DIY a Flat Roof?
EPDM kits (£300 to £600 for a garage size) are genuinely DIY-able for competent homeowners: one-piece membranes, water-based adhesive, no naked flames. GRP is not a DIY material (resin chemistry, weather windows). Felt needs torch-on skills and fire awareness. DIY saves labour but forfeits manufacturer guarantees that require certified installers.
Flat Roof Cost FAQs
How much does a flat roof replacement cost in the UK?
Typically £1,500 to £7,500 depending on size and material: EPDM £50 to £60/m², GRP £70 to £100/m², felt £40 to £70/m² fitted.
How much does a flat garage roof cost to replace?
Around £1,500 to £2,500 in EPDM for a single garage (~20m²), or £2,000 to £3,500 in GRP.
Is EPDM or GRP better for flat roofs?
EPDM wins on value and simplicity; GRP wins on seamless finish for complex shapes. Both last 25 to 30 years when installed well.
Can you overlay a new flat roof on the old one?
Sometimes, if the decking is sound and the material allows it. It adds 10 to 15 years at lower cost, but hides deck condition: get the deck checked first.
How long does a flat roof last?
EPDM and GRP: 25 to 30 years. Torch-on felt: 10 to 15 years. Lifespan depends heavily on installation quality and detailing.
Do I need building regulations for a flat roof replacement?
Renewing over half the roof triggers Part L insulation requirements (U-value 0.18). Your roofer should notify building control.
Why is one flat roof quote so much cheaper than another?
Scope differences: strip vs overlay, decking replacement, insulation, trims, scaffolding and guarantees. Compare line by line, not totals.
What guarantee should I get on a flat roof?
Ask for insurance-backed guarantee on jobs over £2,000, so you are covered even if the firm ceases trading. Manufacturer warranties need certified installation.
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