A sheet metal shop drawing is the communication bridge between the contractor who specifies trim and the fabricator who makes it. If you can't read one confidently, you risk approving a profile that looks right on paper but installs wrong on the job site. This guide walks through every element of a typical sheet metal shop drawing so you can review and approve them without second-guessing.
What a Shop Drawing Shows
Unlike an architectural drawing that shows how trim fits into a building assembly, a sheet metal shop drawing shows the cross-section profile of a single piece of trim — as if you sliced through it with a saw and looked at the end. It answers these questions:
- How long is each leg (flat section) of the profile?
- At what angle does each bend occur?
- What is the total developed width (flat material consumed)?
- What material and gauge will be used?
- How will exposed edges be finished (hemmed, returned, raw)?
Most shop drawings also include a title block with the job name, material spec, revision number, and the fabricator's contact information.
The Cross-Section Profile: Reading Leg Dimensions
The cross-section is drawn to scale (or with explicit dimensions) and shows each flat leg of the profile connected by bend lines. Dimensions are typically called out in inches and fractions — for example, 4½", 2¾", ¾". Each dimension label tells you the length of that flat segment before the next bend.
Common leg names you'll see:
- Top flat / Top leg — horizontal surface on top of coping or cap trim
- Face / Outside face — vertical or angled exposed surface
- Return / Inside face — leg that goes back under or inside the assembly
- Hem — a short folded-back leg (typically ½"–¾") used to stiffen and finish an exposed edge
- Cleat slot — a small inward fold used to receive a standing-seam cleat
Key point: When you see a dimension on a shop drawing, it refers to the outside measurement of the leg unless explicitly noted otherwise. For tight profiles, this matters — inside dimensions are slightly shorter than outside dimensions due to material thickness and bend radius.
Understanding Bend Directions
Bends are typically shown as angled lines in the cross-section, with the bend angle noted. The two terms to understand:
- Bend up / Bend down — from the perspective of the flat sheet lying on a brake. A "bend up" means the leg rotates upward from the flat reference plane.
- Hem (or return hem) — a 180° fold back onto the previous leg. Used on exposed edges for safety and stiffness.
Most architectural trim uses 90° bends, but sloped applications (like rake trim or sidewall flashing) may use 95°, 100°, or custom angles to match a specific roof pitch. These will always be called out explicitly on the drawing — don't assume 90° if the angle isn't labeled.
Developed Width: The Most Important Number
The developed width is the total flat sheet width consumed to make one linear foot of the profile. It's calculated by summing all the leg lengths (excluding hems, which fold back onto themselves). This number drives the material cost per linear foot more than any other single measurement.
| Profile Type | Typical Developed Width | Notes |
|---|---|---|
| Simple drip edge (2" face, 3" top) | 5–6" | Plus hem on face = ~5¾" |
| Standard coping (4" face, 8" top, 4" face) | 16–18" | Returns and hems add width |
| Complex architectural coping | 20–28" | Multiple returns, standing seam slots |
| Z-bar siding trim | 4–6" | Depends on reveal depth |
| Eave trim (L-trim) | 6–10" | Face + soffit leg + hem |
When comparing quotes, verify that the developed width in each quote matches the drawing. A shop that prices a "6" coping" on face dimension alone is quoting something very different from one quoting the full 18" developed width.
Material and Gauge Notes
The title block or general notes section of a shop drawing will specify:
- Material type — galvanized steel, Galvalume, aluminum (.032 or .040), Kynar-painted steel
- Gauge or thickness — 24 ga, 26 ga, .032", .040"
- Coating / finish — mill finish, painted, Kynar 500, PVDF
- Color — if painted, the color code (e.g., "Musket Brown" in the specified paint system)
Verify these match what your spec or purchase order calls for before approving the drawing. Gauge and material changes can substantially affect both cost and installed performance.
Revision Clouds and Delta Marks
On revised shop drawings, changes are highlighted with a revision cloud — a bubble-shaped outline drawn around the changed area — and a delta symbol (△) with a revision number. Always check the revision history before approving: if you're looking at Rev 3, make sure you know what changed in Rev 1 and Rev 2.
A common mistake is approving a revision without reading the revision note, then discovering the fabricator changed a leg dimension to make the profile easier to brake-form. That change may affect fit on the job.
How Trimgy Generates Shop-Ready Profiles
Trimgy's drawing grid works at the same level of precision as a shop drawing — 1/8" increments — so the profile you draw is the profile the fabricator sees. There's no translation layer where dimensions get rounded or interpreted. When you click "Order," the profile geometry goes directly to fabrication without a shop drawing approval cycle.
For contractors who want to shortcut the drawing-to-approval cycle, drawing directly in Trimgy eliminates the back-and-forth entirely.
Try drawing a profile in Trimgy →
Frequently Asked Questions
What does a sheet metal shop drawing show?
A sheet metal shop drawing shows the cross-section profile of a single trim piece — as if you cut through it and looked at the end. It shows the length of each flat leg, the angle of each bend, the total developed width (flat material consumed), the material type and gauge, and how exposed edges are finished. It answers how the piece is made, not how it fits into the building.
What is developed width on a sheet metal drawing?
Developed width is the total flat sheet width consumed to produce one linear foot of the trim profile. It is calculated by adding up all the leg lengths in the cross-section (excluding hems that fold back on themselves). Developed width is the primary driver of material cost per linear foot — a profile with a 16" developed width costs roughly twice as much in material as one with an 8" developed width, all else equal.
Are dimensions on shop drawings inside or outside measurements?
Dimensions on sheet metal shop drawings typically refer to outside (overall) measurements of each leg unless explicitly noted otherwise. For most architectural trim, the difference between inside and outside dimensions is small (a few hundredths of an inch due to material thickness and bend radius). However, for tight-fitting profiles where pieces must nest together, this distinction matters — confirm with the fabricator if your application requires inside dimensions.
What does a revision cloud mean on a shop drawing?
A revision cloud is a bubble-shaped outline drawn around any area of a shop drawing that changed from the previous revision. It is accompanied by a delta symbol (△) and a revision number. Always read the revision note and review all previous revisions before approving — a common error is approving a revision without checking what changed, then finding the fabricator altered a leg dimension to ease fabrication in a way that affects how the trim fits on site.
Do I need to approve a shop drawing before my trim is fabricated?
On traditional custom orders, yes — the shop submits a drawing for your approval before cutting material, which adds 1–2 days to the front end of every order. Some online ordering platforms (including Trimgy) eliminate this step by having you draw the profile directly on a precision grid, so the geometry you specify goes straight to fabrication without a separate approval cycle.