Flashing Calculator
Add each flashing location - step, apron, valley, abutment, back gutter, ridge. Mix materials across sections.
10% for straightforward work; 15-20% for complex chimneys with many cuts
Flashing Sections
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Results
Total Flashing Area
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m² (inc. waste)
Total Linear Metres
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lm (inc. waste)
Section Breakdown
| Section | Type | Material | Length | Width/Girth | Area | Lead Weight |
|---|
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Frequently Asked Questions
How much flashing do I need for a chimney?
A chimney needs flashing on all four sides: front apron, two stepped side flashings, and a back gutter. For a 600 mm wide chimney, the front apron is ~600 mm wide with a 150 mm upstand and 150 mm cover over the tiles. Stepped flashings run the full chimney height, each step sized to the brick course height. Allow 15% for waste and fixing.
What lead code should I use for roof flashing?
Code 3 (1.32 mm): soakers. Code 4 (1.80 mm, 20.41 kg/m²): standard for stepped flashings, aprons and cover flashings. Code 5 (2.24 mm): valley and parapet gutters. Code 6+: large valley gutters and flat roof coverings. The Lead Sheet Association (LSA) publishes detailed code selection guidance.
What is the difference between a flashing and a soaker?
A soaker is a small lead piece fitted under each individual tile at an abutment, folded to go under the tile and up the wall - hidden from view. A step flashing (cover flashing) is a continuous lead strip dressed over the soakers and into the wall mortar joints in steps - visible from outside. Together they form a two-part weatherproof junction.
What Building Regulations apply to roof flashings?
Flashings must comply with Part C (Moisture Resistance) - all junctions between roof, walls, chimneys and upstands must be weatherproofed to prevent moisture ingress. Lead flashings must follow the Lead Sheet Association Manual. For extensions requiring approval, flashing details are inspected by the building inspector at frame and roof stages.
What are the alternatives to lead flashing?
Aluminium: lighter, cheaper, less durable, harder to dress. Copper: very durable, used on heritage and high-spec projects. Zinc: long lifespan, popular on contemporary roofs. Self-adhesive bitumen tape (Flashband): quick but shorter lifespan, not suitable for all applications. Lead remains the preferred choice for most UK residential work due to its 50-200 year lifespan and malleability.
What waste allowance should I use for roof flashing?
Allow 10-15% waste for cutting, dress allowances and laps at joints. Lead is sold in rolls (typically 3 m or 6 m) in standard widths. Important: always add the upstand depth + cover depth to the visible face width when calculating total lead width - a 100 mm visible flashing needs approximately 250-300 mm total width including upstand and fixing allowance.