Lead Flashing Calculator — Rolls, Weight & Code

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.