Pick the method
Parallel element lines (straight length, pipe, miter): parallel line. Right cone or frustum: radial line. Anything else: triangulation.
Then the same tail for every piece: add seam allowance outside the pattern, choose the joint by metal, check a non-90 or radius piece with its maker, and inspect.
A flow chart. 1 asks: are all element lines (bends) parallel, with the same cross-section along the length? Yes leads to parallel line. No leads to 2: do all element lines meet at one point on centre? Yes leads to radial line, no to triangulation. All three join 3 add seam allowance outside the pattern, 4 choose the joint by metal, 5 check non-90 or radius pieces with the maker, 6 check the arc or true lengths, 7 form and inspect.
- 1
- Element lines (bends) all parallel, same cross-section along the length?
- 2
- Do all element lines meet at one radius point on centre (a right cone or frustum)?
- 3
- Add seam allowance outside the stretch-out, never in the divisions
- 4
- Choose the joint by metal and thickness from the governing specification
- 5
- Check a non-90 or radius piece with the maker: Petersen refers non-90 miters to a separate print approval; Drexel says to consult it for radius work
- 6
- Check the pattern: arc equals circumference, or lines equal the TLD
- 7
- Form, join and inspect
Sources
- Pattern Development: Sheet Metal Level 1 - Radial Line, Triangulation and Parallel Line parts (2021, CC BY 4.0) - BCcampus (Brian Coey)
- PAC-Continuous Cleat Coping: Standard Accessories (DWG 41011-38588 Rev A, 06/06/22) - Petersen Aluminum Corporation (PAC-CLAD)
- Manufactured Fascia, Drip Edge and Gravel Stops, Section 07 71 19 guide specification (September 2019) - Drexel Metals, Inc.
- UFGS 07 60 00 Flashing and Sheet Metal (August 2023, Change 1 05/25) - USACE / NAVFAC / AFCEC (Unified Facilities Guide Specifications, via WBDG)