Flatlock vs overlock seams in activewear: where each fits

What is the difference between flatlock and overlock?

An overlock seam joins two plies face to face and wraps their edges in looper threads, leaving a seam allowance that stands up or folds to one side. A flatlock seam butts or slightly overlaps the edges and covers them top and bottom, so the join lies almost flat on both faces.

In ISO 4915, overlocks belong to class 500, overedge stitches, and flatlock to class 600, cover stitches. ASTM D6193 uses the same stitch numbers and classifies the seams that carry them: an overlocked side seam is a superimposed seam, a flatlocked one a flat seam. ISO 4916 notes that a seam designation only becomes a meaningful specification once the stitch type is added, so write both. Doing so also stops an overlock opened out flat, which some workrooms call flatlock, being substituted for a 607 flatseam.

Where does flatlock earn its extra cost?

Where a seam is pressed into skin, rubbed repeatedly or stretched hard. Coats describes the 600 family as giving flat, comfortable seams with high seam elongation, which matters most where a garment is tight and the wearer moves for a long session. It earns less on garments that hang away from the body, or on brushed fleece, where a seam ridge is rarely felt through the fit or the soft interior. Typical zones where it pays:

• Side and underarm seams of close-fitting tops and base layers.
• Inside leg, crotch and gusset seams of tights and leggings.
• Shoulder and back seams under backpack or hydration-pack straps.
• Panel seams on compression garments, where a ridge would add a line of pressure.
• Visible style lines, where a contrast flatseam is part of the design.

When is overlock plus coverstitch the better choice?

On loose tees, polos, training shorts, hoodies and any seam that is not pressed into skin, where a clean outside and efficient sewing matter more than a perfectly flat inside. Coats notes that the three-thread 504 overlock has excellent extension, does not unravel easily and runs at high speed; the four-thread 514 adds a second needle for a more secure seam. A coverstitch then hems the edges. Where the ridge matters, the overlocked allowance can be topstitched flat with a coverstitch, but that two-pass seam uses about as much thread as a flatseam, so its case rests on appearance and machine availability rather than cost.

On seams that are only overlocked, thread is the visible saving. Coats’ consumption ratios, at 7 stitches per centimetre, are about 14 metres of thread per metre of seam for a 504 overlock and 18 for a 406 coverstitch, and Coats puts the 607 flatseam at about 32. Flatseamers are also specialist machines, so ask how many the factory runs and whether flatlock operations will set the pace of your order. SenLin supplies a costed bill of materials with each quotation; check that the thread your seam list calls for is the thread being priced.

Across a typical range, the starting points look like this; your fabric, fit and budget decide the final list.

How should seams be written into the tech pack?

Give each sewing operation its own line, so that factories quoting the same garment are pricing the same construction. For each one, state:

• Location: side seam, underarm, inside leg, shoulder, hem or neck binding.
• Stitch type number and seam type, for example a 607 flatseam on the side seams.
• Thread fibre, construction and size for needle and looper positions.
• Stitch density per centimetre, with a tolerance.
• Seam allowance or trimmed width, and the needle gauge for cover stitches.
• Reinforcement, such as bartacks at pocket openings and where seams cross at the crotch.
• Thread colour, matched or contrast, and which face is the right side.
• Confirmation of the whole list on the approved sample before bulk, with photographs of the inside.

What should you check on the sample?

Turn it inside out, wear it, stretch it and wash it before approving. A seam that looks neat on a hanger can rub, crack or wave once a body is inside it. Check these points:

• Comfort: run a hand along every skin-contact seam, then wear-test a full session of the intended activity.
• Edges: flatlock edges should be fully covered, with no raw edge or fibre whiskers escaping.
• Stretch: pull each seam to its working extension and look for cracked threads or skipped stitches.
• Flatness: coverstitched hems should not tunnel into a ridge, and overlocked seams should not wave.
• After washing: recheck puckering, fraying and thread ends after the agreed care cycle.
• Strength: numeric strength and extension requirements need their own test plan, covered in the activewear seam strength guide.