How Much Flat Roof Felt / Membrane Do I Need?
Quick Answer
Roof area + 150mm overlap per joint + 75mm edge upstand
Based on: flat roof membrane or torch-on felt. A 4m × 3m flat roof needs ~14–15m² of material (including overlaps).
How We Calculated This
Flat roof covering needs to account for overlaps, upstands, and drip edges:
- Roof area: 4m × 3m = 12m²
- Side overlaps: 150mm at each joint (typically 1 joint on a 4m-wide roof)
- End overlaps: 150mm where rolls meet end-to-end
- Edge upstands: 75–150mm turned up at abutments
- Drip edge: 50–75mm overhang at the gutter
- Total including waste: ~14–15m²
Flat Roof Membrane Types
- Torch-on felt (modified bitumen): Most common for domestic flat roofs. 2 or 3 layer system. ~£8–15/m².
- EPDM rubber (e.g., Firestone): Single-ply, glued down. 20–30 year lifespan. ~£10–18/m².
- GRP fibreglass: Seamless, very durable. 25–30 year lifespan. ~£15–25/m².
- PVC single-ply (e.g., Sarnafil): Heat-welded, commercial grade. ~£15–20/m².
- Liquid applied: Painted on, ideal for complex shapes. ~£12–20/m².
Build-Up for a 3-Layer Felt System
- Vapour control layer: 1 layer across the entire deck
- Insulation: Rigid PIR or EPS boards (to meet Part L U-value)
- Underlay felt: 1 layer, nailed or bonded
- Top sheet: Torch-on mineral felt, lapped 150mm
Tips
Flat roofs must have a minimum fall of 1:40 (preferably 1:60 as built, allowing for deflection). Always use a proper detail at abutments — turn the membrane up the wall at least 150mm above the finished roof level. Check Building Regulations for insulation requirements (Part L). For roofs accessible for maintenance only, EPDM or torch-on felt is usually the most cost-effective option.