How to calculate flashing blank width and metal usage
TunkaForm team · · 5 min read
Short answer
Metal usage for a flashing = blank width × total length of the pieces. Number of pieces = run length ÷ length of one piece (rounded up), and ~4 cm of joint overlap is added to each piece. For example, a 12 m eaves flashing with a 250 mm blank width: 6 × 2.04 m = 12.24 m → 0.25 × 12.24 ≈ 3.06 m² of metal.
What is blank width?
Blank width is the width of the flat metal strip before the flashing is bent. It equals the sum of the widths of all the flashing's segments (flanges, faces, hemmed edges). For example, if an eaves flashing consists of 40 + 120 + 60 + 30 mm segments, its blank width is 250 mm.
Step 1: number of pieces
A flashing is not made as one continuous piece but as pieces of equal length, because the folding machine and transport limit the length.
number of pieces = ⌈ run length ÷ maximum piece length ⌉
A 12 m run with 2 m pieces: 12 ÷ 2 = 6 pieces.
Step 2: joint (lock) overlap
The pieces slot into each other, so an overlap is added to each one - in practice usually 4 cm.
piece length = run length ÷ number of pieces + 0.04 m
12 ÷ 6 + 0.04 = 2.04 m. Forgetting the overlap is the most common reason pieces come out short and have to be cut again.
Step 3: metal area (m²)
metal (m²) = blank width (m) × piece length × number of pieces
0.25 m × 2.04 m × 6 = 3.06 m².
Step 4: fitting the blank width to the sheet width
Several strips are cut from a 1250 mm wide sheet. If the sheet width divides by the blank width with no remainder, waste drops to zero:
| Blank width | Strips from 1250 mm | Remainder | Waste |
|---|---|---|---|
| 200 mm | 6 | 50 mm | 4% |
| 250 mm | 5 | 0 mm | 0% |
| 300 mm | 4 | 50 mm | 4% |
| 312 mm | 4 | 2 mm | 0.2% |
| 400 mm | 3 | 50 mm | 4% |
That is why experienced workers design the blank width, wherever possible, so that 1250 mm divides by it with no remainder.
Common mistakes
- Not accounting for the joint overlap - pieces come out short.
- Calculating the blank width without the hemmed edges (only from the visible segments).
- Using an old dollar rate for the metal price - the order closes at a loss.
The TunkaForm flashing calculator performs these 4 steps in 15 seconds and includes 22+ ready-made flashing templates.
Questions
- How do I know how much metal a flashing needs?
- Multiply the blank width (in metres) by the total length of the pieces. For a 250 mm blank width and 6 × 2.04 m pieces: 0.25 × 12.24 ≈ 3.06 m².
- Why is a joint overlap needed?
- The pieces slot into each other; without an overlap a gap is left between them. Usually 4 cm is added to each piece.