Flatlock stitch: why you should know this sewing technique

  • Serger Pepper - One Thimble 7 -Flatlocking made easy-

This post about the flatlock stitch was originally written by me for One Thimble issue 7

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Let’s learn how to sew a flatlock stitch!As featured in One Thimble

As you might know, I have a thing for sergers: I need to know everything about them, and I LOVE to share what I learn, because I can’t understand overlockers fear, and I’m here to defeat it!
Overlockers comes with several different stitches: some are perfect for hemming lightweight fabrics, like rolled hem, others can be used to stitch together and finish two (or more) layers of fabric in one time, like the 4-thread overlock, others are great for finishing seam allowances, like the 2 (or 3)-thread overlock.

Today, you’re going to discover all the secrets of the flatlock stitch, made using a regular overlocker (vs. an industrial flat-lock machine).

Flatlock seams’ most noticeable quality is that they have almost no bulk. You can see them in stretch sportswear, where seam allowances in the inside of the garment may hurt athlete’s skin.

Serger Pepper Flatlock Stitch on RTW detail

With this kind of join, there are no seam allowances hanging around, they’re all wrapped inside the seam, with raw edges side by side.

Which side is your favorite?

Flatlock has two nice-looking sides: one with the upper looper’s thread zig-zagging, the opposite with a nice ladder stitch (displaying the needle thread).

Serger Pepper - One Thimble 7 - Flatlock right-wrong-ladder -

If you want the loops showing on the right side of the fabric, sew fabric with wrong sides together; to show the ladder stitch, put it right sides together.

Decorative only?

Flatlock can also be used as a purely decorative stitch, folding the fabric and sewing along that fold. In this case, I would suggest you choose a 2-thread flatlock to reduce the bulk.

Cool Guy Tip: Choose to showcase a nice thread from your stash or, maybe, slip a narrow ribbon under the ladder stitches for a fancy touch!

Straighten the edges first!

Flatlock stitch is sewn without using the blade: the fabric won’t even go next to it so you may technically leave it engaged but I’d suggest you put it out of the way if you’re flatlocking on the fold… better safe than sorry!
If you’re using the flatlock stitch to attach together two pieces of fabric (instead of using it above a fold for ornamental purposes only), you’d better start straightening up the raw edges, making them even, before you start sewing.
Serger Pepper - One Thimble 7 - Flatlock 1 -

Pro Tip: Use a rotary cutter and mat*.

Seam Allowances

You may need to trim away seam allowances, depending on how wide are the ones already included in your pattern pieces: you’re going to stitch at above 1/8-3/16″ from the raw edge.

Choose the right foot

While you can use a standard serger foot (on the right in the picture below) to sew a flatlock seam, it will be way easier if you choose an adjustable blind hem foot* (pictured on the left). Make sure you choose a replacement foot that is compatible with your serger brand and model!

Serger Pepper - One Thimble 7 - Flatlock serger feet -

  1. If using a standard foot, align the fabric raw (and trimmed) edge between the needles (test your serger to identify some reference points, like the little hole you see in mine).
  2. If using an adjustable blind hem foot, try on scraps to decide how much you need to move the adjustable slider to the right, then close the tiny screw on it: no more guessing!

Serger Pepper - One Thimble 7 - Flatlock blind hem foot guide -

Generally speaking

A flatlock is sewn leaving loose the needle’s thread tension (around zero) because you need to pull the fabric to open the seam after you’ve sewn it. This way it will lay flat and the needle’s thread won’t pull the fabric.

Tip: this is a stitch that needs a little fiddling to be perfect: you’d better tune it on scraps before you actually sew on our finished garment. If you spend that little time practicing you’ll be amazed from results, substantially improving your sewing level.

Flatlock seams are rad and I want to add them everywhere!|tweet this|

When sewing a flatlock, opposite to a regular overlock, you need to serge leaving the stitches hanging off of the fabric; this will leave enough room to the fabric to lay flat, once you’ve gently pulled it on the sides.

Serger Pepper - One Thimble 7 - Flatlock on the edge -

Pro Tip: Add strength (but also a little bulk) to a flatlock by sewing a regular seam first, then fold it along the seam, enclosing seam allowances. Flatlock on top of it. Press flat, then trim seam allowances close to the flatlock stitching.

2 vs. 3

Let’s compare a 3-thread flatlock to a 2-thread flatlock, and have a glimpse at how to create them.
Since each overlocker is different and changing fabric/thread combo will make settings change (sometimes significantly), consider the following settings as starting points.

Note: Next to each item there’s a setting range, in brackets the settings I used for flatlock stitch you see.

3-thread flatlock

  • It’s the stronger between the two;
  • it uses more thread.

Serger Pepper - One Thimble 7 - Flatlock right-wrong-ladder -

left needle 0-3 (0.5) pink
upper looper 2-5 (4.5) violet
lower looper 6-9 (7) purple

differential feed 1-1.5 (1)
stitch length = your choice
left or right needle (usually the left one, but you can experiment)

2-thread flatlock

  • It’s the weaker between the two, not supposed to be used for piecing but for decoration, sewn on the fold;
  • less thread consuming.

Serger Pepper - One Thimble 7 - Flatlock 2-thread -

To convert your overlocker to a 2-thread stitch, you need to snap-on a converter to your upper looper, to by-pass it.

Each brand/model has a different one but they all serve the same purpose – check your manual if you’re in doubt.

Serger Pepper - One Thimble 7 - Flatlock 2 thread converter -

left needle 0-3 (0) pink
lower looper 6-9 (6) purple

differential feed 1-1.5
stitch length = your choice
left or right needle (usually the left one, but you can experiment)

Keep in mind that:

  1. the loop side will be the looper’s one
  2. the ladder is made with the needle thread.

Choose threads wisely, depending on which side you’re going to show to the world!

Cheat flatlock stitch, anyone?

Flatlock seams are cool but they need some testing time to become perfect. What if you want to achieve a similar look, spending a fraction of the time?

  • Start with a regular 3-thread overlock (with balanced tension settings) sewn putting wrong sides together, then press the seam to one side.

Serger Pepper - One Thimble 7 - Faux- Flatlock -

  • Edgestitch it to the fabric below it using a thread color matching with the fabric.
  • Going straight will be easier using an adjustable edgestitch foot on your sewing machine, but it can be done with a standard foot and a good eyeballing.

Serger Pepper - One Thimble 7 - Faux- Flatlock edgestitching -

  • Let’s compare it to a flatlock seam (on the right):

Serger Pepper - One Thimble 7 -Flatlock vs. Faux- Flatlock -

Hemming with flatlock

An interesting use of the flatlock is for hemming. It works like a charm for an almost invisible hem, showing ladder stitch on the right side of the garment (either in a matching color to almost disappear or in a contrasting color with decorative purpose).
The best part is that you’re going to hem and finish in a professional looking way the inside too, all at once (and who doesn’t like time-saving tricks?).

Here too, using the adjustable blind hem foot will be a huge life-saver, but you can absolutely do without it, using a standard serger foot.

  1. Fold the fabric to the wrong side for the hem width, and press the fold.
  2. Fold back the fabric, aligning the new fold right above the raw edge.
  3. Serge on this edge, using the flatlock stitch (either 2 or 3 threads, I used the 3). Try to catch with the left needle as little fabric as you can, for even more invisible results.

Serger Pepper - One Thimble 7 - hems -

  1. Unfold the fabric, gently pulling it to set stitches, press and admire your lovely hem inside (on top, in the picture) and outside (on the bottom, in the picture).

Using a matching thread the result will look more precise, blending on your fabric. Here, I have used contrasting thread to help you see it and I would have pulled just a tiny bit more, to better set the needle’s thread:

Serger Pepper - One Thimble 7 - flatlock hem inside and outside -

Now you know (almost) everything about the flatlock stitch. Please ask questions, if you have any: I’ll do my best to help you!

Where would you use the flatlock? Do you have a project in mind to test it? I have “just a few” 😉


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About the Author:

Hi there! I love sewing, creating my own patterns and refashioning pre-loved clothes... If you love the same things, why not be friends? See you on Pinterest http://Pinterest.com/MammaNene


  1. Amy Mayen March 2, 2016 at 20:07 - Reply

    Great info, Irene!!

  2. Ramona Walter March 8, 2016 at 16:37 - Reply

    Thank you for the info. Was never really interested in flat locking before but I’m going to give it a try.

  3. Louise March 8, 2016 at 17:01 - Reply

    I have a babylock eclipse, which does not have flat lock feature. I am new at using my serger, after reading your post, are you saying I can do the flat lock stitch you demonstrated without flat lock feature on my serger? I really have learned a lot about serving from your blog! tHank you!

  4. Katie March 8, 2016 at 17:55 - Reply

    I love the blind hem foot for your serger! What brand is it? I wonder if it will work on other brands? That guide looks very helpful!

  5. rlisa March 8, 2016 at 18:44 - Reply

    I don ‘t like so much this Kind of seam, cause i love really coverlock, But your is Great post And you have intrigued To try ????

  6. kerrie March 8, 2016 at 19:35 - Reply

    now i can refer to this when I’m ready to tackle it!
    now getting the threads worked out is the toughest part…thanks to you!!

  7. Wow, Irene, what an informative post! I didn’t know how to do any of this, so I will definitely come back to this when I’m ready to give it a go. Thanks so much! 🙂 Lisa
    (And how funny are you?! You put in all the time for this long post, and then forget to publish it? Haha!)

  8. […] Flatlocking Made Easy! by Serger Pepper […]

  9. ZuzannahLB March 13, 2016 at 05:07 - Reply

    Thank-you. I was staring a my serger a week ago, determined to try flatlock… only to chicken out. Your clear instructions have given me the courage to try tomorrow!

  10. Deirdre M April 3, 2016 at 07:24 - Reply

    I think I’m finally going to try this! I just wish it was possible to do it in a way where the blade could trim off the seam allowance so I don’t have to trim it first. Thanks for this post!

  11. […] for your already expensive machine. This one does NOT! Four Signs you should Invest in a Serger Flat locking One of my very favorite posts! MUST Print I promise, you will be so thankful you printed this. […]

  12. Shannon June 24, 2016 at 16:10 - Reply

    Great post! This is so much more clearly explained than any other place I’ve ever seen this technique described. I unfortunately had to figure out what the heck my manual was trying to explain by trial and error….I so wish this post was available a few years ago when I was teaching myself, it would have been so helpful!! But in the end I suppose all that trial and error was good for me, I did eventually figure it out and for certain garments I absolutely love this stitch and would use nothing else.

  13. Bre September 2, 2016 at 14:38 - Reply

    What’s the best way to tie or knot off a flat lock so it doesn’t come un done when you surge over the tail

    • Mamma Nene September 3, 2016 at 13:00 - Reply

      Hi Bre!
      What if you sew perpendicularly the seam, with a short-length triple straight stitch (or triple zig-zag if you need it to stretch) before you serge over the tail?
      This works for me…
      Happy flatlocking,
      Irene // Serger Pepper Designs

  14. Courtney October 28, 2016 at 13:36 - Reply

    Hi! I am trying to repair my daughters swim shirt, the hem on the bottom has unraveled partway. It looks like the stitch that was used on it has three rows of stitching anchoring down what looks like the loops from a cover stitch underside on both top and bottom…..it doesn’t look like it was stitched more than once so I can’t tell if it’s a fancy flatlock or a special coverstitch. I have a singer pro five which does flatlock but only the basic kind (2 or 3 thread). Any idea what it might be and how to duplicate it?

  15. Nena December 7, 2016 at 18:58 - Reply

    Question for clarification please! — You say Pro Tip: Add strength (but also a little bulk) to a flatlock by sewing a regular seam first, then fold it along the seam, enclosing seam allowances. Flatlock on top of it. Press flat, then trim seam allowances close to the flatlock stitching. Are you putting fabric wrong sides together to seam? And are you taking a quarter or eighth of an inch seam (or trimming the seam allowance to that) . I need to make leggings for an active 13 year old granddaughter who is more tomboy than girly but is willing to wear leggings as long as her tops are not frilly. The knit she chose has just barely enough to get the leggings out of it so I don’t have much to experiment with. And, confession time, knits are just not my thing.

    Thank you for sharing your knowledge. Not much on flatlocking out there. Most serger manuals skim over the stitch as well as the dealers when they teach you how to use the serger.

    • Mamma Nene December 11, 2016 at 12:49 - Reply

      hi Nena and thanks for stopping by!
      First of all, sorry for the late reply (these holidays are killing me!)
      Then, to answer your flatlock question (hoping I am in time): I would not choose this kind of seam to sew for the first time for a pair of leggings, especially if you’re short in fabric.It’s a kind of stitch you indeed have to practice, before it comes out perfect, in the fabric you have chosen. Since knits aren’t your thing, I’d suggest you sew them using a triple straight stitch on your sewing machine, using a stretch needle, then finish seam allowances with a 3-thread overlock on your serger. This way you’ll ensure plenty of strength, durability, and stretch.

      You can start practicing with the flatlock stitch on your serger, using a woven fabric first, just to see how it goes, and try both the wrong sides together and right sides together ways (to see if you like better the ladder look on the inside or the outside). Then you can move on to knit/jersey fabric: use old T-shirts to practice: if you’re like me, you’ll need plenty of fabric until you get the exact look you like (and this will also include playing with 1/4″ or 1/8″: depending on the fabric you’re using, you’ll choose the one that looks better!). This way (using old clothes instead of our precious fabrics) you’ll master this skill in no time, for the next time you’ll need it. Don’t forget to keep the final piece of fabric+seam and to note down into it the settings you have finally chosen.
      I hope this helps. Please ask again, if you need any clarification, I’ll do my best to answer you faster than this time <3
      Happy sewing,
      Irene // Serger Pepper Designs

  16. Jaime Lakatos September 12, 2017 at 02:10 - Reply

    What is the seam used for sportswear? It looks like a flat lock loop side on both sides of the fabric??

    • Irene November 2, 2017 at 18:00 - Reply

      It’s a coverstitch seam, which you get by using a different machine (the coverstitch).

  17. Rebecca Grace July 28, 2018 at 18:22 - Reply

    Irene, I’m thinking of trying a serger flatlock for joining two pieces of fluffy wool batting together to make a piece that is large enough for my oversized King quilt. The goal is to “melt” those two pieces of batting together where the edges abutt, but to do so without creating any discernible ditch or ridge along that batting seam that might be noticeable in my finished quilt. The traditional way that quilters join batting scraps is with a whipstitch done by hand, but that would take a REALLY LONG TIME for a quilt this big. Have you ever used the flatlock stitch for this purpose, or do you know of any other quilters using their serger successfully for this purpose? Obviously I’d go with the 2-thread version, with longest stitch length option and layer the batting in my quilt with the ladder stitches on top and the “threadier” side of the batting seam facing the backing, and use the lightest weight serger thread in my stash. What do you think?

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