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Chimney Flashing Calculator — Lead, Soakers & Step Flashings
Calculate lead flashing for chimneys: apron, step flashings, back gutter and soaker count by lead code and tile gauge.
Width of chimney stack (across the slope)
Depth of chimney stack (up the slope)
Batten gauge: plain tile ~100mm, slate ~320mm, interlocking ~340mm
Upstand up the chimney face (default 150mm; cover over tiles fixed at 150mm)
Head lap between step flashings (min 65mm)
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How We Calculate This
This calculator works out the lead quantities for a full chimney flashing set: front apron, step flashings on both sides, back gutter and (for double-lap coverings) soakers. The apron, steps and back gutter are ordered in your chosen lead code; soakers are added separately as Code 3 lead.
The formula
Step flashings per side = Chimney depth (mm) ÷ Tile gauge (mm)
Soakers = Step flashings × 2 sides (plain tile / slate only)
Total lead (kg) = Total area (m²) × Lead weight (kg/m²)
The front apron is sized with the chosen upstand up the chimney face plus a 150mm cover over the tiles, per the Lead Sheet Association front-apron detail. Soakers are only used with double-lap plain tiles or slates — single-lap interlocking tiles are weathered with a stepped cover flashing alone.
Lead code weights (BS EN 12588)
- Code 3: 14.97 kg/m² (1.32mm) — soakers, sheltered work
- Code 4: 20.41 kg/m² (1.80mm) — standard flashings
- Code 5: 25.40 kg/m² (2.24mm) — back gutters, exposed
Frequently Asked Questions
Code 4 (1.80mm) is standard for most UK chimney flashings. Use Code 5 (2.24mm) in exposed locations or for back gutters. Code 3 (1.32mm) is only suitable for sheltered positions with short runs.
Soakers are only used with double-lap coverings — plain tiles (~100mm gauge) and slates. Single-lap interlocking tiles use a stepped cover flashing alone with no soakers. Where soakers apply you need one per tile course on each side, so the count equals chimney depth divided by tile gauge, multiplied by 2 (for both sides). Each soaker is roughly 175mm wide (about 75mm up the wall and 100mm over the tile) and a length of one tile gauge plus the head lap plus a short turn, and is usually formed in Code 3 lead.
Step (cover) flashings should overlap each other by at least 65mm (the head lap) and be turned into the mortar joint by about 25mm with a minimum 75mm upstand. Each step piece is typically 180mm wide and one tile gauge plus the head lap in length, usually formed in Code 4 or Code 5 lead.
Yes. The back gutter sits behind the chimney (on the upslope side) and diverts water around the chimney stack. It needs an upstand of about 100mm to the back of the chimney, a 150mm clear gutter sole, and around 225mm carried up the roof slope, over the tilt fillet and ending in a welt — roughly a 0.475m developed girth. It should also extend about 225mm past each side of the chimney to shape the corners and lap over the tiles. Code 5 lead is recommended for back gutters.
Code 3 lead weighs 14.97 kg/m², Code 4 weighs 20.41 kg/m² and Code 5 weighs 25.40 kg/m² (BS EN 12588). A full domestic chimney flashing set — apron, step flashings, back gutter and (for plain tiles or slate) soakers — typically works out to roughly 1.5–3 m² of lead, around 25–65kg, depending on chimney size, tile gauge, covering and lead code. A small 0.6m × 0.45m plain-tile stack is near the bottom of that range (about 1.4 m² / 27kg in Code 4) and a large 1.5m × 1.0m stack near the top (about 2.7 m² / 62kg in Code 5).
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.