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Lead Flashing Calculator — Rolls, Weight & Code
Calculate lead flashing quantities for stepped, apron, valley and chimney flashings. Uses UK lead codes and standard roll sizes.
Total length of flashing needed
Adds 10% extra length for dressing and trimming
10% is standard for lead work
Enter your supplier price per roll
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How We Calculate This
Our lead flashing calculator determines the amount of lead you need based on the flashing type, total length and lead code. It calculates total area, weight and number of standard rolls required.
The formula
Total area = Length × Width
Weight = Area × Weight per m² (from lead code)
Length with waste = Length × (1 + wastage%)
Rolls = roundup(Length with waste ÷ Roll length)
The wastage allowance is added to the linear metres before rounding up to whole rolls, so the dressing and trimming margin is applied once to the metres you actually cut — not multiplied on top of an already rounded-up roll count.
UK lead codes (BS EN 12588)
- Code 3: 1.32mm thick, 14.97 kg/m² — light flashings, soakers
- Code 4: 1.80mm thick, 20.41 kg/m² — standard flashings (most common)
- Code 5: 2.24mm thick, 25.40 kg/m² — valleys, exposed locations
Standard widths
- Stepped flashing: 150mm wide (Code 4)
- Apron flashing: 300mm wide (Code 4)
- Valley flashing: 450mm wide (Code 5)
- Chimney set: front apron 300mm Code 4, two side flashings 150mm Code 4, back gutter 450mm Code 5
Chimney sets are ordered as three rolls
A chimney set is made from three different roll widths and two different lead codes, so a 450mm Code 5 back gutter cannot be cut from a 300mm Code 4 roll. This calculator therefore sizes each component separately and lists the rolls needed for each width: front apron (300mm Code 4 × chimney width), two side flashings (150mm Code 4 × your entered side length), and back gutter (450mm Code 5 × chimney width). For one-off jobs a pre-formed chimney flashing kit for your chimney size is often the simplest option.
Maximum piece length & laps
Individual lead pieces should be no longer than 1.5m to avoid thermal-fatigue cracking (Lead Sheet Association / NHBC 7.2.20). Where a run exceeds 1.5m it must be split into pieces with a 100mm lap (150mm where exposed) at each join. The calculator adds this lap allowance automatically for runs over 1.5m, and wedges/clips should be fixed at no more than 450mm centres.
Frequently Asked Questions
Code 4 (1.80mm thick, 20.41 kg/m²) is the standard for most flashings including stepped flashings, apron flashings and chimney work. Code 3 is lighter and used for small, simple flashings. Code 5 is heavier and recommended for valley gutters and exposed locations where extra durability is needed. Always comply with the Lead Sheet Association (LSA) guidelines.
Measure the raking length along the abutment where the roof slope meets the wall. Each stepped piece typically covers one course of brickwork (about 75mm rise). The flashing strips are usually 150mm wide (tucked into the bed joint and lapping at least 150mm over the tiles). Lead pieces should be no longer than 1.5m to avoid thermal-fatigue cracking (Lead Sheet Association / NHBC), so a long raking run is made up of several pieces with a 100mm lap at each join. Order enough length to cover the full raking measurement plus the laps and about 10% wastage.
A standard chimney flashing set includes: front apron (Code 4, ~300mm wide × chimney width), stepped side flashings (Code 4, ~150mm wide × chimney depth on each side), and a back gutter (Code 5, ~450mm wide × chimney width). Pre-made chimney flashing kits are available for standard chimney sizes. You will also need to order ridge tiles and main roof tiles — see our Ridge Tile Calculator and Roof Tile Calculator.
Lead flashing is sold by the roll in standard widths and lengths. Common roll lengths are 3m and 6m. The width varies depending on the application — 150mm for stepped flashings, 300mm for apron flashings, 450mm+ for valleys. Lead is also sold by weight (per kg). Prices vary significantly with the lead market price.
Yes — alternatives include EasyFlash (a lead-free, self-adhesive flashing), Ubiflex (non-lead flexible flashing) and zinc or copper flashings. These are lighter, easier to work with and avoid health concerns associated with lead. However, lead remains the traditional standard and is preferred for conservation and heritage work. Check local Building Control requirements.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.