Felt & Membrane Calculator

Works for roofing felt, breathable membrane, self-adhesive underlays, and flat roof membranes

Membrane / Felt Type

Breathable membranes can be laid directly on rafters; traditional felt requires counter-battens above.

Roll Width (m)

Roll Length (m)

Side Lap (horizontal overlap) (mm) Minimum 150 mm for most membranes

End Lap (roll-to-roll overlap) (mm) Minimum 150 mm - check manufacturer spec

Effective Coverage per Roll

- enter roll dimensions above

Waste Factor (%)

10%

10% standard; 15% for complex roofs with hips, valleys, or many penetrations

Cost per roll

Labour per m²

Delivery (flat fee)

Total Roof Area
-
m²

Rolls to Order

inc. waste

Coverage per Roll

m² effective

Total Coverage

m² (ordered)

Slope	Eaves Length	Rafter Length	Area	Courses	Rolls

Breathable membrane - The modern standard for pitched roofs. Laid directly on rafters; no ventilation gap required. Also called HR (High Resistance) or LR (Low Resistance) membrane.
Traditional roofing felt - Non-breathable (BS 747). Requires a 50 mm ventilation gap above the felt. Still used on some projects and extensions to match existing roofs.
Self-adhesive underlay - Used in high-wind zones or under slate and plain tiles as a second defence. Bonds directly to the deck without nailing.
Flat roof membrane - EPDM, GRP, or torch-on felt for flat or very low-pitch roofs. Wider rolls and larger laps than pitched-roof products.
Vapour control layer (VCL) - Fitted on the warm side of insulation in warm-roof builds to prevent condensation within the structure.

Side lap - Horizontal overlap between runs. Minimum 150 mm for most products; increase to 225 mm in exposed locations.
End lap - Overlap where one roll ends and the next begins. Minimum 150 mm; stagger joints between runs.
Eaves drape - Allow 50-75 mm overhang into the gutter at eaves.
Ridge - Membrane should lap over the ridge by at least 150 mm each side.
Always check the manufacturer's data sheet - some premium membranes specify tighter laps.

Eaves length - Horizontal measurement along the guttering line
Rafter length - Measured along the slope from eaves to ridge (not horizontal). Add 150-200 mm extra for ridge lap and eaves drape.
For flat roofs, add upstands - typically 150 mm up abutment walls.
Order at least 10% extra for waste, cuts, and any damaged rolls.