What lead code should I use for chimney flashing?

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.

How many soakers do I need?

You need one soaker per tile course on each side of the chimney. The number equals the chimney depth divided by the tile gauge, multiplied by 2 (for both sides). Soakers sit behind each tile and under the step flashing.

What size should step flashings be?

Step flashings should overlap each other by at least 65mm and be turned into the mortar joint by 25mm. Each step piece is typically 180mm wide and one tile gauge plus overlap in length.

Do I need a back gutter?

Yes. The back gutter sits behind the chimney (on the upslope side) and diverts water around the chimney stack. It should be at least 150mm wide on the flat and 150mm up the chimney face.

How much does chimney lead flashing weigh?

Code 4 lead weighs 20.41 kg/m² and Code 5 weighs 25.40 kg/m². A typical chimney flashing set requires 2–4 m² of lead, weighing 40–100kg depending on chimney size and lead code.

TradeCalculator

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.

Chimney Width

Width of chimney stack (across the slope)

Chimney Depth

Depth of chimney stack (up the slope)

Lead Code

Code 4 is standard for most chimneys

Tile Gauge

Batten spacing / tile gauge

Price Per kg Lead (optional)

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 soakers.

The formula

Step flashings per side = Chimney depth (mm) ÷ Tile gauge (mm)

Soakers = Step flashings × 2 sides

Total lead (kg) = Total area (m\u00b2) × Lead weight (kg/m\u00b2)

Lead code weights

Code 3: 14.97 kg/m\u00b2
Code 4: 20.41 kg/m\u00b2
Code 5: 25.40 kg/m\u00b2

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.