Construction Details in a Tech Pack: Stitch Callouts 101

Construction details in a tech pack are the instructions that tell a factory how to sew your garment: which seam types join each panel, which stitch types to use, the stitch density, and where topstitching goes. Your bill of materials says what the garment is made of and your points of measure say how big it is — construction details in a tech pack are the third pillar, defining how it’s actually put together. Get them wrong (or leave them out) and two factories will sew the same design two different ways.

What counts as a construction detail?

A construction callout is a short, specific instruction tied to one location on the garment. A typical one reads like: “Shoulder seam: 4-thread overlock, 10 SPI, with clear elastic stabilizer tape.” Each callout usually specifies:

• Seam type — how the fabric pieces are joined. A plain seam is the basic two-pieces-sewn-together join; a flat-felled seam (think jeans outseams) wraps the raw edges inside for durability; a French seam encloses the edge for delicate fabrics.
• Stitch type — the thread formation. A lockstitch is the standard straight stitch on wovens; an overlock (or serge) wraps the edge and stretches, which is why it dominates knitwear; a coverstitch is the twin-needle finish you see on t-shirt hems.
• SPI (stitches per inch) — stitch density. More stitches per inch means a stronger, finer seam but slower sewing and slightly higher cost.
• Seam allowance — the distance from the raw fabric edge to the stitch line, typically given in millimeters or fractions of an inch.
• Thread and finish notes — matching thread color (tie this back to your BOM), topstitching placement, and anything decorative.

Demystified once, none of this is intimidating — it’s a vocabulary of maybe a dozen terms that covers the vast majority of garments.

Why can’t the factory just figure it out?

Factories can and will make construction decisions for you — that’s the problem. A sample room defaults to whatever is fastest on their machines, not what your design intends. The classic surprises: a hem topstitched in one row when you imagined two, a neckline finished with a band instead of binding, seams that pucker because the SPI was too low for the fabric.

Every one of those becomes a sampling round: a new sample, more weeks, more cost. Clear construction callouts are the cheapest insurance you can buy in the sketch-to-production process, because they cost you an hour of specification instead of a month of revision.

How do I write stitch callouts a factory understands?

You don’t need an engineering degree or the ISO stitch catalog memorized. Here’s the practical workflow:

1. Work from a flat sketch. Number each seam and detail on your technical drawing — shoulder, armhole, side seam, hems, neckline, pockets.
2. Write one callout per number. Location, seam type, stitch type, SPI, seam allowance. Plain language is fine: “twin-needle coverstitch hem, 25 mm” communicates perfectly.
3. Reference a garment they know. “Flat-felled like a jeans outseam” or “binding like a standard tee neckline” anchors your intent in something universal.
4. Call out the invisible parts. Stabilizer tape at shoulders, interfacing in collars and plackets, bartacks at stress points like pocket corners and belt loops. These are the details factories most often skip when unspecified.
5. Flag anything non-standard. If a detail is decorative or unusual — contrast topstitching, raw edges on purpose, exposed seams — say so explicitly, or the factory will “fix” it.

A worked example: basic hoodie

For something like a hoodie tech pack, a solid construction section covers roughly: shoulder seams (overlock with stabilizer tape), armhole and side seams (4-thread overlock), hood seams and facing, kangaroo pocket attachment with bartacks at the openings, drawcord eyelets and channel stitching, cuff and waistband attachment (overlock plus optional coverstitch), and the bottom hem. That’s eight to ten callouts — an hour of work that pre-answers every question the sample room would otherwise guess at.

Simpler garments need fewer. A t-shirt tech pack usually gets by with six or so: shoulders, neckline, sleeve attachment, sleeve hems, side seams, bottom hem.

What are the most common construction mistakes beginners make?

The pattern we see with new independent designers is consistent: they specify the visible and forget the structural. The checklist worth running before you send anything to a factory:

• No SPI anywhere — the factory picks, and density affects both look and durability.
• Knit garments without stretch seams — a lockstitch side seam on a stretchy fabric pops the first time someone pulls the garment on.
• No stabilization callouts — shoulders and necklines on knits grow out of shape without tape.
• Missing bartacks — pocket corners and stress points fail in wear-testing.
• Decorative details with no note — anything intentional but unusual gets “corrected” by a well-meaning sample room.

If your construction notes survive this checklist, you’re ahead of most first tech packs a factory receives.

Where do construction details live in the tech pack?

Construction callouts sit alongside your flats as a numbered annotation page, plus a notes table — one row per callout, keyed to the numbers on the sketch. A structured tool keeps the callouts, sketch numbers, and BOM thread colors consistent with each other automatically; that’s exactly the kind of cross-referencing techpacks.app handles for you, and you can preview your first pack to see how a finished construction page reads before committing to anything.

FAQ

What does SPI mean in a tech pack?

SPI stands for stitches per inch — how dense the stitching is. Higher SPI generally means a stronger, cleaner seam but slower sewing. Most woven garments sit around 10–12 SPI; calling it out removes guesswork.

Do I need to know ISO stitch and seam numbers?

No. Plain language plus a clear diagram works for most factories. ISO codes (like stitch type 401 or 504) are a precise shorthand worth learning over time, but a labeled sketch communicates just as well.

What’s the difference between a seam and a stitch?

A stitch is the thread formation itself (lockstitch, chainstitch, overlock). A seam is how two pieces of fabric are joined using stitches (plain seam, flat-felled seam, French seam). A construction callout usually specifies both.

How detailed should construction notes be for a simple garment?

Even a basic t-shirt needs callouts for shoulder seams, neckline binding, sleeve and bottom hems, and any topstitching. That’s typically 6–10 callouts. If a stranger couldn’t sew it from your notes alone, add detail.