Calculate how many rolls of roofing felt you need with lap allowance
Roofing felt -- also known as underlay or sarking felt -- is a critical layer in any pitched roof construction. Installed over the rafters and beneath the tile battens, it provides secondary weather protection against wind-driven rain and snow that penetrates between the tiles. Without adequate felt, moisture can reach the roof timbers, insulation, and ceiling below, leading to rot, damp patches, and costly repairs. In the UK, roofing felt is sold in standard rolls measuring 1 metre wide by 15 metres long (15 square metres gross per roll). However, the actual coverage per roll is less than 15 m2 because adjacent strips must overlap to create a watertight seal. The standard lap overlap is 150mm, though this should be increased for low-pitch roofs or exposed locations. Getting the quantity right matters -- running short mid-way through a roofing job means leaving timbers exposed while you source additional rolls, and having too many leftover rolls is wasted money. Modern breathable roofing membranes have largely replaced traditional bitumen-based felt on new builds, as they allow moisture vapour to escape from the roof space while still preventing water ingress. Both types are sold in the same roll sizes and this calculator works for either product. Building Regulations and BS 5534 require an underlay beneath all pitched roof coverings in England and Wales.
To calculate the number of roofing felt rolls you need: 1. Enter the roof length in metres. This is the distance along the ridge of the roof. If your roof wraps around corners or has multiple sections, measure each section separately and add the totals together. 2. Enter the roof width (slope length) in metres. This is the distance from the eaves to the ridge measured along the slope, not the horizontal span. Felt is laid horizontally starting from the eaves and working upwards, so the slope length determines how many rows of felt are needed. 3. Enter the lap overlap in millimetres. The default of 150mm is standard for most UK installations. Increase to 200-225mm for roof pitches below 15 degrees or in exposed locations with high wind-driven rain. The minimum overlap under BS 5534 is 75mm at rafter supports and 150mm between supports. 4. Adjust the waste allowance. The default 5% covers minor wastage from cutting at verges, hips, and valleys. Increase to 10% for complex roof shapes with dormers, skylights, or many hip and valley junctions. 5. Review your results. The calculator shows the number of rolls needed, the total felt area including waste, and the approximate number of fixing nails required. Order rolls to the next whole number -- you cannot buy partial rolls.
The roofing felt calculation accounts for the reduced effective width caused by the lap overlap: Effective width per strip = (1000mm - lap overlap) / 1000. With a 150mm overlap: (1000 - 150) / 1000 = 0.85 metres effective width. Effective coverage per roll = effective width x roll length = 0.85 x 15 = 12.75 square metres. Total area needed = roof length x roof width x (1 + waste percentage / 100). For a 10m x 5m roof with 5% waste: 10 x 5 x 1.05 = 52.5 m2. Rolls needed = ceiling(total area needed / effective coverage per roll). 52.5 / 12.75 = 4.12, rounded up to 5 rolls. Fixing nails are estimated at approximately 50 per roll. Felt is secured to the rafters using galvanised clout nails (typically 20mm x 3mm) at roughly 150mm centres along each rafter where the felt crosses it. The exact number depends on rafter spacing, but 50 per roll is a practical ordering guide. For breathable membranes, the manufacturer may specify different lap requirements. Always check the specific product data sheet. Some membranes have printed overlap guides on the material itself to make installation easier.
When choosing roofing felt or membrane, consider the ventilation requirements of your roof. Traditional non-breathable felt requires a 50mm air gap between the felt and any insulation, plus ventilation at eaves and ridge to prevent condensation. Breathable membranes allow the air gap to be omitted, enabling thicker insulation between the rafters. This distinction is particularly important for loft conversions where maximising insulation thickness is desirable. All roofing underlays used in England and Wales must comply with BS 5250 (management of condensation in buildings) and be installed in accordance with BS 5534. In Scotland, the requirements are similar but reference the Scottish Building Standards. Always use a membrane or felt that carries BBA (British Board of Agrement) certification for your specific application.