Felt & Membrane Calculator

Q: How many rolls of roofing felt or breathable membrane do I need?

Divide the roof area by the net coverage per roll. Net coverage = (roll width - horizontal overlap) x roll length. For a standard breathable membrane roll of 1.5 m x 50 m with a 150 mm overlap, net coverage = (1.5 - 0.15) x 50 = 67.5 m2. Divide your total roof area by 67.5 and round up to get the number of rolls. Add 10-15% for waste at gable ends, eaves laps, and complex details.

Q: What is the difference between roofing felt and breathable membrane?

Traditional roofing felt (Type 1F bitumen felt) is not vapour-permeable and requires a ventilation gap between the felt and the insulation to prevent condensation. Breathable membrane (vapour-permeable underlay or VPU) allows water vapour to pass through from inside the roof space to outside, reducing the need for ventilation. Under BS 5534 and current Building Regulations guidance, breathable membrane is now preferred for pitched roofs as it allows warm roof and close-coupled rafter insulation designs without a ventilation gap.

Q: What overlap should I use for roofing felt and breathable membrane?

For breathable membrane on pitched roofs, a minimum 150 mm horizontal (side) lap and 150 mm end lap is standard. In exposed locations (high wind zones), increase side laps to 200 mm. Traditional bitumen felt (Type 1F) requires minimum 75 mm side laps. At the eaves, the underlay should overhang into the gutter by at least 50 mm. Always check the manufacturer's specific lap requirements as they can vary between products.

Q: Do I need roofing felt or membrane under roof tiles?

Yes - UK Building Regulations Approved Document C and BS 5534 require an underlay (felt or breathable membrane) under all pitched roof tile and slate coverings. The underlay acts as a secondary defence against wind-driven rain and condensation. It is fixed horizontally across the rafters from eaves to ridge, with each course lapping over the one below. Without an underlay, rain can be driven under the tiles by wind and reach the roof structure.

Q: What roofing felt or membrane should I use for a flat roof?

Flat roofs require a dedicated waterproofing system rather than pitched roof underlay. Common flat roof systems include: torch-on bitumen felt (mineral-surfaced cap sheet over base layer), EPDM rubber membrane, GRP (fibreglass), and hot-melt systems. Under UK Building Regulations and BS 6229 for flat roofs, the waterproofing must achieve a minimum 1:80 fall across the roof surface to prevent ponding. Our felt and membrane calculator covers pitched roof underlay - for flat roof coverings, use the flat roof joist calculator.

Q: What waste allowance should I use for roofing felt or membrane?

Allow 10% waste for a straightforward gable roof with few penetrations. Use 15% for roofs with hips, valleys, dormers, chimneys and skylights where more cutting and lapping is required. Membrane rolls can be tricky to handle in wind - some waste is inevitable. Always buy slightly more than calculated, as it is much easier to return an unopened roll than to make a second purchase for a small shortfall.

How much roofing felt or breathable membrane do I need?

Divide the roof area by the net coverage per roll (roll width minus the horizontal overlap) to get the number of rolls per slope. For a standard breathable membrane roll 1.5 m wide with a 150 mm overlap, the net coverage per run is 1.35 m. A 50 m roll therefore covers 1.35 x 50 = 67.5 m² net. Add 10-15% for waste, end laps and complex roofs. Under UK Building Regulations Approved Document C and BS 5534, pitched roofs must use a vapour-permeable underlay (breathable membrane) or traditional Type 1F felt with adequate ventilation to prevent condensation in the roof space. Our calculator covers both standard felt and breathable membrane for pitched and flat roofs, with overlap and waste built in.

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

Roof Slopes

Waste Factor (%)

10%

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

Job Costing (optional)

Cost per roll

Labour cost

Rate x total area

Delivery (flat fee)

UK Market Prices Fills in the material cost fields with current UK market prices. You can review and edit each price before applying.

Results

Total Roof Area
-
m²

Rolls to Order

inc. waste

Coverage per Roll

m² effective

Total Coverage

m² (ordered)

Slope Breakdown

Slope	Eaves Length	Rafter Length	Area	Courses	Rolls

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Frequently Asked Questions

How many rolls of roofing felt or breathable membrane do I need?

Net coverage = (roll width - overlap) x roll length. For a 1.5 m x 50 m roll with 150 mm overlap: (1.5 - 0.15) x 50 = 67.5 m² per roll. Divide your roof area by this figure and round up. Add 10-15% for waste at gables, eaves laps and complex details.

What is the difference between roofing felt and breathable membrane?

Traditional felt (Type 1F) is not vapour-permeable - requires a ventilation gap between felt and insulation. Breathable membrane (VPU) allows water vapour to pass through, reducing ventilation requirements. Under BS 5534 and current Building Regulations guidance, breathable membrane is now preferred as it allows close-coupled rafter insulation without a ventilation gap.

What overlap should I use for roofing felt and breathable membrane?

Breathable membrane: minimum 150 mm horizontal lap and 150 mm end lap. In exposed/high-wind zones: increase to 200 mm. Traditional bitumen felt: minimum 75 mm side laps. At eaves: overhangs into gutter by at least 50 mm. Always check the manufacturer's specific lap requirements.

Do I need roofing felt or membrane under roof tiles?

Yes - Approved Document C and BS 5534 require an underlay under all pitched roof tile and slate coverings. The underlay is a secondary defence against wind-driven rain and condensation, fixed horizontally from eaves to ridge with each course lapping over the one below. Without underlay, rain driven by wind can reach the roof structure.

What roofing felt or membrane should I use for a flat roof?

Flat roofs need a dedicated waterproofing system, not pitched roof underlay. Common systems include: torch-on bitumen felt, EPDM rubber membrane, GRP (fibreglass), and hot-melt systems. Under BS 6229, the surface must achieve a minimum 1:80 fall to prevent ponding. Use our flat roof joist calculator for flat roof projects.

What waste allowance should I use for roofing felt or membrane?

Straightforward gable roof: 10% waste. Roofs with hips, valleys, dormers, chimneys or skylights: 15%. Membrane rolls are difficult to handle in wind and some waste is inevitable. Always buy slightly more than calculated - returning an unopened roll is much easier than making a second trip for a small shortfall.

UK Building Regulations BS 5534 / BS 747 / BS 8747

Breathable membrane Traditional felt Self-adhesive underlay Flat roof membrane Vapour control layer

Min side lap150 mm

Min end lap150 mm

Ventilation gapNot required

StandardBS 5534

Breathable (HR/LR) membranes comply with BS 5534 and can be laid directly on rafters without a ventilation gap. Minimum 150 mm side and end laps; increase to 225 mm in severe exposure zones.

Membrane Types

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.

Lap & Overlap Guide

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.

Measuring Tips

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.

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Felt & Membrane Calculator

How much roofing felt or breathable membrane do I need?