Roof Flashing Calculator
Calculate roof flashing quantities — apron, step, valley, and chimney flashing — from roof dimensions and junction details. Free, no signup.
Angle Finder
Calculate roof angles for flashing, bends, and junctions
Use when two roof planes meet around an internal or external corner (usually a 90° building corner).
Usually 90° for square building corners
Related Free tools
Roof flashing — did you know?
Types of flashing and where they go
Apron flashing: where a roof meets a vertical wall above (e.g. lean-to against house). Step flashing: where a roof slope meets a side wall. Valley flashing: in valleys. Chimney flashing: base + step + apron + back gutter. Damp-proof courses (DPC) under all.
Flashing material types
Lead: traditional, long-lasting (60+ years), expensive. Colour-coated aluminium: cheaper, 30 years. Zinc: 50 years, self-healing. GRP/pre-formed: modern, quick to install. Flashing band/tape: temporary only.
Step flashing calculation
Each step flashing piece covers ~100mm of horizontal wall length. Pieces = wall_length ÷ 0.1. At 2m chimney wall: 20 pieces each side. Each piece: 150mm × 200mm minimum, lapped 75mm.
Valley flashing width
Valley flashing width depends on pitch and water volume. Low pitch (under 20°): 450mm minimum. Normal pitch (20-40°): 300mm. Steep (40°+): 225mm. Always allow 75mm upstand each side. Center the valley trough.
Formulas used
Apron flashing length
apron = wall_length + 150mm each end
Step flashing pieces
pieces = ceil(wall_length ÷ 0.1) × 2 sides
Valley flashing
valley = valley_length + 150mm × 2 ends
Chimney flashing
chimney = base + step + apron + back gutter
Frequently asked questions
How do I calculate step flashing?
Pieces = wall_length ÷ 100mm × 2 sides. 2m chimney: 20 pieces per side × 2 = 40 pieces. Each piece: 150 × 200mm minimum.
What width valley flashing do I need?
Low pitch (<20°): 450mm. Normal (20-40°): 300mm. Steep (40°+): 225mm. Add 75mm upstand each side.
Lead vs aluminium flashing?
Lead: 60+ year life, expensive, heavy. Aluminium: 30 years, cheaper, lighter. Zinc: 50 years, mid-price. Choose by budget and building style.
Best free roof flashing calculator?
One that handles apron, step, valley, and chimney flashing quantities. This tool does all, free.