Lead Flashing Calculator: Rolls, Pieces and Weight to Order
By the Professional Roofers team
Updated 2026 · Independent cost guide
Lead flashing is sold by the roll, but it has to go on in short lapped pieces: no piece longer than 1.5 metres, or 1 metre where it is bonded to felt or a single-ply membrane, because lead moves with every swing in temperature. Enter the length you need to cover and this works out how many pieces that means, how much lead the laps add, how many rolls to buy and what it all weighs.
Work out the lead flashing to order
How the calculation works
Lead expands and contracts by about 0.03mm per metre for every degree of temperature change. Fixed in one long length, a flashing tears itself loose or splits at the fixings, which is why the Lead Sheet Training Academy limits flashings to 1.5 metre pieces, and to 1 metre where the lead is bonded along one edge. The calculator splits your run into the fewest pieces that stay within that limit, then adds one lap for every joint between pieces.
Example: a 4.2m wall abutment in 300mm Code 4. Two 1.5m pieces cannot reach, so it takes 3 pieces. The two joints add 0.2m of lead at 100mm each, giving 4.4m fitted, or three pieces of about 1.47m. With a 10% cutting allowance that is 4.84m to order, which is one 6m roll weighing about 36.7kg. The calculator always rounds up to whole rolls and never lets a single piece span two rolls.
Weight matters more than people expect. A 6m roll of 300mm Code 4 is about 36.7kg, awkward to carry up a ladder and part of why leadwork is priced as it is. For what each code is used for, see lead code numbers explained; for fitting and repair costs, see our lead flashing and leadwork guide, and if you are weighing up the theft risk, lead flashing alternatives.
Choosing the width
The strip has to turn up into the mortar joint (usually 25mm into the chase, with the upstand above the tiles) and then lap down over the roof covering. How far it must cover the tiles depends on the tile profile, the pitch and how exposed the roof is, and low-pitch or exposed roofs need more cover. Measure an existing flashing before you order, or ask your roofer which width the detail needs.
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