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Replace a flat roof.
EPDM, GRP, felt or single-ply membrane flat roof — old roof stripped, fresh decking, new waterproof layer.
✓ Updated May 2026✓ Free quotes✓ Vetted pros
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02 · Read the brief
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Costs, timelines, pitfalls and the right questions to ask before any roofer starts on flat Roof Replacement.
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Hiring a roofer, without the regret.
Four moves that separate a smooth job from a nightmare.
Felt blistering
Hot torch-on felt blisters in summer heat. Worth specifying EPDM or GRP for north-facing roofs (more shade) or paying for premium SBS-modified felt.
EPDM corner failure
Cheap EPDM uses tape at corners — fails after 5–10 years. Welded corners are better.
GRP cracking
GRP can crack from movement. Use flexible primer base.
Insulation skipped
Re-roofing without upgrading insulation misses Part L requirement.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and a typical week-by-week schedule.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
3-layer torch-on felt per m²Traditional UK flat roof
£50–£110/m²
EPDM rubber per m²Single piece, no seams
£70–£140/m²
GRP fibreglass per m²Seamless laminate
£85–£170/m²
Single-ply membrane per m²Sarnafil, Sika
£80–£160/m²
Standard garage roof (20 m²)EPDM or felt
£1.8k–£3.5k
Dormer flat roof (15 m²)Smaller area
£1.4k–£2.8k
!
Quote spread is typically ± 18% — always get 3 quotes.
Timeline
Semi-detached full re-roof · week by week
Typical
4 phases · 3 wk
4 phases · 3 wk
W1
W2
W3
Scaffold erection
Strip + felt + battens
Tile + ridge + verge
Flashings + snags
!
Schedule slips on dependencies — pad each phase by 10–20% for real-world delays.
At a glance
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Roofer explained
Flat roofs on extensions, garages and dormers need replacing every 15–25 years depending on material. Expect to pay £1,800–£8,500 for a UK flat roof replacement, depending on size and material.
What Does the Work Involve?
Old roof covering stripped — felt, asphalt, or old EPDM. Decking inspected; rotten OSB or chipboard replaced. Insulation upgraded if Building Regs notifiable (250mm minimum to meet Part L). New covering laid: felt (3-layer torch-on), EPDM (single-piece rubber), GRP (fibreglass), or single-ply membrane. Edge trims, drips and outlets formed. Tested with hose.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| 3-layer torch-on felt per m² | 50 | 110 | Traditional UK flat roof |
| EPDM rubber per m² | 70 | 140 | Single piece, no seams |
| GRP fibreglass per m² | 85 | 170 | Seamless laminate |
| Single-ply membrane per m² | 80 | 160 | Sarnafil, Sika |
| Standard garage roof (20 m²) | 1,800 | 3,500 | EPDM or felt |
| Dormer flat roof (15 m²) | 1,400 | 2,800 | Smaller area |
| Large extension flat roof (40 m²) | 3,500 | 8,500 | EPDM or GRP |
| Insulation upgrade per m² | 30 | 80 | Building Regs Part L |
How Long Does It Take?
Garage flat roof: 2–3 days. Standard extension: 3–5 days. Larger extension: 5–8 days. Plus insulation upgrade time if needed.
DIY or Professional?
Professional. Hot torch-on felt is fire-risk work. EPDM and GRP are slightly more accessible but still need professional installation for warranty.
Choosing the Right Tradesperson
NFRC member with flat-roof experience. EPDM specialists (Firestone-trained, Permaroof-accredited). GRP installers (Cure-It, Crystic-trained). 15–25 year warranty in writing.
UK Regulations
Building Regs Part L thermal upgrade (250mm insulation equivalent) when replacing flat roof. Part C moisture barrier. Part F ventilation continuity. Listed buildings may have material restrictions.
Common Problems
- Felt blistering: Hot torch-on felt blisters in summer heat. Worth specifying EPDM or GRP for north-facing roofs (more shade) or paying for premium SBS-modified felt.
- EPDM corner failure: Cheap EPDM uses tape at corners — fails after 5–10 years. Welded corners are better.
- GRP cracking: GRP can crack from movement. Use flexible primer base.
- Insulation skipped: Re-roofing without upgrading insulation misses Part L requirement.
Frequently Asked Questions
Best flat roof material 2026?
EPDM rubber — single-piece, no joints, 25–30 year life, easy to install. The modern UK standard.
How long does flat roofing last?
Felt: 15–25 years. EPDM: 25–30 years. GRP: 25–30 years. Cheap torch-on felt: 10–15 years.
EPDM or GRP?
EPDM: single piece, no joints, more forgiving install, slightly cheaper. GRP: seamless, can be coloured, slightly more expensive, less forgiving of substrate flex.
Why do flat roofs fail?
UV degradation, thermal cycling, standing water, poor installation. Most flat roofs that fail early were either poorly installed or not detailed properly at edges.
Can I just patch the felt?
Small patches work for 3–5 years. Multiple patches or felt past 15 years old: full replacement is cheaper than continued patching.
Do flat roofs need ventilation?
Cold-deck (insulation between joists): yes — vents to prevent condensation. Warm-deck (insulation above): no vent needed.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
NEWAI Trade Assistant
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