Salamandra
Log in Bag 0

Process ·

Flatlock or Overlock Seams on a Heavy Hoodie?

Flatlock or overlock on a heavy hoodie? What ISO 4915 and ASTM D6193 say about stitch types, how to spot each seam, and why flat matters under a coat.

Turn a hoodie inside out and look where the sleeve meets the body. You'll find one of two things. Either a raised ridge of looped thread wrapped around the cut edge, or a flat band of thread that lies level with the cloth on both sides. The first is an overlock seam. The second is usually called a flatlock. They're built differently and they behave differently under a jacket, and that's worth knowing before you buy a heavy hoodie for winter.

Stitches have numbers, and the numbers are standardised

Factory sewing runs on a numbering system, not loose words. ISO 4915:1991, Textiles, Stitch types, Classification and terminology classifies, designates, describes and illustrates the stitch types used in hand and machine sewn seams. ISO lists it as last reviewed and confirmed in 2021, so the 1991 edition is still current. The American equivalent is ASTM D6193, Standard Practice for Stitches and Seams, which identifies the category, formation and general purpose of stitch and seam types, with drawings of each.

The first digit of a stitch number gives the class. These are the ones that matter on a hoodie:

  • 301, lockstitch. The ordinary home sewing machine stitch. Thread manufacturer Coats describes it as the most common stitch type, and notes that it looks the same on top as underneath.
  • 401, chainstitch. Coats says it gives higher productivity and greater seam elongation than a lockstitch.
  • 504 and 514, overedge. This is the overlock. Coats describes 504 as three threads, one needle and two loopers, with excellent extension, and says it does not unravel easily. 514 is the four thread version, with two needle lines.
  • 516, safety stitch. A 401 chainstitch and a 504 overedge sewn together in one pass. Five threads.
  • 602, 605 and 607, coverstitch. The 600 class are the coverstitches. Coats notes that 605 has high seam elasticity and uses more thread than average, and that the older 606 flatlock has largely been replaced by 607, which it calls a flatseam.

So "flatlock" in shop talk usually means a 607 seam, even though the name first belonged to a different stitch number. When a brand says flatlock, the word is being used loosely.

What an overlock seam is doing

An overlock machine trims the edges of two layers, sews them together and wraps the raw edge in thread, all in one movement. The two layers end up face to face, so the joined edge sits as a small fin on the inside, bound in looper thread. From the outside you see one neat line where the pieces meet.

It's fast, it stretches with knit cloth, and it's the default on most sweatshirts at most price points. Nothing wrong with that. The catch on heavy cloth is bulk. That fin is two thicknesses of fabric plus thread. On dense fleece it can become a ridge you feel through a base layer, especially across the shoulder where a backpack strap or a coat seam presses on it.

What a flatlock seam is doing

A 607 seam needs a dedicated machine. A 2023 paper by Ineta Nemeša in Tekstilna industrija sets out how these machines work. The 607 stitch is formed with four needle threads, a top cover thread and a bottom looper thread, so it covers both faces of the cloth and the edges with a lot of thread. The paper names three seams these machines make:

  • Lap seams, where one edge overlaps the other, used for light to medium thickness cloth.
  • Butt seams, where the two cut edges meet edge to edge, described as suitable for joining heavy materials and for joins reinforced with tape.
  • Cover seams, used for decoration.

The paper also notes that flatlock machines trimming heavy cloth such as fleece use a single edge trimmer rather than one that cuts both edges. If you're weighing up the cloth itself, our note on loopback terry and brushed fleece covers the difference.

The practical result of a butt seam is that there's no fin. The two edges sit side by side and the thread bridges them, so the seam ends up close to the thickness of the cloth itself.

How to tell which one you've got

  1. Turn the hoodie inside out. Look at the side seam and the armhole.
  2. Feel for a ridge. An overlock leaves a raised edge of looped thread you can pinch between two fingers. A flatlock lies flat.
  3. Count the lines from the outside. An overlock usually shows one line of stitching, two at most. A 607 shows four parallel needle lines with a zigzag of cover thread across them.
  4. Check both faces. A flatlock looks busy on both sides. An overlock looks clean outside and busy inside.
  5. Look at the hem and cuffs separately. Plenty of hoodies use a coverstitch on the hem and an overlock everywhere else. One flat seam doesn't make a flatlock garment. Cuffs take their own kind of wear, which we cover in fixing stretched cuffs.

Which one is stronger

This is where common advice runs ahead of the evidence. You'll read that flatlock is "stronger" or "more durable", full stop. ASTM D6193 doesn't put it that way. Its significance section lists five characteristics of a properly made seam: strength, elasticity, durability, security and appearance. It then lists what drives seam strength: the fabric, the seam configuration, the stitch type, the stitch density, the thread tension and the thread strength. Stitch type is one variable out of six. A tight, dense overlock in good thread on stable fleece can outlast a loose flatlock in cheap thread, and the other way round.

Testing doesn't settle it cleanly for knits either. The seam strength method people quote most, ASTM D1683, Failure in Sewn Seams of Woven Apparel Fabrics, is written for woven cloth. Hoodie fleece is knitted. The method built for stretchy textiles, ASTM D3787, Bursting Strength of Textiles, Constant-Rate-of-Traverse Ball Burst Test, covers textiles with a high degree of ultimate elongation and explicitly mentions knitted goods, but it's a bursting test, not a ranking of seam types. We couldn't find a published standard that sets a pass mark for a hoodie seam, or a peer reviewed study comparing 504 and 607 seams on heavy cotton fleece.

What's fair to say is narrower. A flatlock has no fin, so there's less bulk to rub. It covers both faces of the join in thread. And a butt seam on heavy cloth is a recognised technique, not a gimmick. Those are construction facts, not performance guarantees.

Why it matters more in October than in June

In summer a hoodie is often the top layer. In a Canadian autumn it moves inside, under a shell, then a parka, with a backpack strap on top. Every layer presses the seams against you. A fin at the shoulder you never noticed in September starts to register by the third week under a jacket.

What to ask before you buy

Product pages rarely give the full picture, so it helps to know which questions the standards point to. If a brand claims flatlock, these are the things worth checking or asking:

  • Which seams are flat? The shoulder, armhole and side seams take the pressure under a coat. A flat hem alone doesn't count for much.
  • Butt seam or lap seam? Both can be made on a flatlock machine. The Nemeša paper describes the butt seam as the one suited to heavy material.
  • What thread, and how dense? ASTM D6193 names thread strength, thread tension and stitch density as drivers of seam strength. A brand that knows its seams should be able to say something about them.
  • Does "flatlock" mean 607? The word is used loosely. Four needle lines with a cover thread across them is the thing to look for.

Looking after either seam

Seams fail at the thread first, and heat and abrasion are what wear thread. A few habits help with both types:

  • Wash inside out so the seam threads aren't scrubbing against zips and buttons on other clothes.
  • Don't wring a heavy hoodie. Wet fleece is heavy, and twisting it loads every seam at once.
  • Dry it flat or on low heat.
  • Deal with a pulled or broken stitch early, before it catches on something.

Our full washing notes are on the care page.

The short version

Overlock is a looped edge that leaves a fin inside. Flatlock, usually stitch 607, is a flat band that leaves none. Both stretch. Neither is proven stronger in every case, and ASTM D6193 lists six things that drive seam strength, not one. On a heavy hoodie that will spend the winter under a coat, a flat seam earns its place through comfort and lower bulk. That's why we cut cloth to be worn hard and keep the seams flat.

Sources

PreviouslyHow to Wash a Hoodie Without Shrinking It NextHow to Store Heavy Sweats Over the Summer
All pieces