Tuesday, January 27, 2015

How to Sew a Flatlock Stitch with Your Serger

After last week's post about my yoga pants, I had a lot of questions about how I did the flatlock stitching that you saw. (See that post here.)  Today I am going to share with you how I did it.If you have a serger, you can do it. You don't need any other special equipment.

Flatlocking was one of the things that was on my list of things I have been wanting to learn, but I was a little intimidated by it. And when I turned to the internet for tips, as I often do, I didn't find much. So I am excited to share with you my technique. I would love to hear how this works for you and see what you make with it.
You can see the flatlock stitching here on these yoga pants.

What is Flatlock Stitching?
Okay, so first, what is flatlock stitching, or flatlocking? A flatlock stitch is so named because it makes a stitch that is flat. The fabrics at the seam will either be butted up against each other or will overlap each other inside the seam so that there is no seam allowance on the inside. This is a nice feature when you have garments that are tight-fitting and/or when they will be seeing a lot of movement close to the body. This way, there is no seam on the inside to cause chafing or discomfort. So, this stitch is most often seen on activewear and swimsuits.

3-Thread Flatlock Stitch with your Serger
For this tutorial, I am going to talk specifically about using your serger to do a 3-thread flatlock stitch at a seam. Some machines are capable of doing a 2-thread flatlock stitch. There are also ways to use the flatlock stitch on a fabric fold for a decorative effect outside of a seam. I'm not going to get into those stitches today. Also, know that many clothing manufacturers use a specialty flatlocking machine to get those perfect stitches you see on store-bought garments. So, our stitch will be a good imitation of that, but it's not exactly the same.

The stitch we are going to make with this tutorial will look like loops on one side and ladders, or parallel lines, on the other side. You can use either one you like for the right side. I like the sporty look of the loops on the outside. Inside this stitch, the seam allowances will overlap and lay flat on top of each other.

*Edit - Note that this tutorial shows a lapped flatlock seam as opposed to a butted seam. This means that the seam allowances overlap each other instead of butting against each other. Thus, the stitch is not entirely flat, but it is much stronger than a butted flatlock stitch and shouldn't allow skin or underlayers to show through the seam when stretched. 

Sewing the Flatlock Stitch

1) Set up your machine: You will want your machine set up so that you are using only one needle, usually in the far left position. Now, you will want to adjust your tensions so that the needle tension is very low, the upper looper tension is normal, and the lower looper tension is high. On my Brother 1034D, the tensions look like this (4 is standard):
Left Needle: 1
Right Needle: -
Upper Looper: 4
Lower Looper: 7

Of course, you will want to practice this on scraps until the tension seems right to you. It will be a little different for each machine and for each fabric.

2) Sew your seam together.  Use your regular presser foot and keep your knife engaged. You will want to make sure you are cutting off some of the fabric at the edge. Of course, at this point, your tensions will not look normal and you may have loops hanging off the edge of your fabric. That's okay.

3) After you pull your fabric out from under your machine, you can grab each side and gently pull your fabric apart until the seam is flat. 

Important Tips to Remember

  • Before you sew, you will need to decide whether you prefer to have the loops or ladders on the outside of your garment. If you sew the garment as usual, with right sides together, you will get the ladders on the right side and your needle thread will show. If you sew with wrong sides together, you will see the decorative loops on the right side of the fabric and the upper looper thread will be visible.
  • Seam Allowance - You may need to adjust the seam allowance from your pattern. Since we are going to pull that seam allowance apart and overlap it, you will gain back some room in your seam. I could try to go into the math here, but just trust me when I say you will want to add about 1/8" to your seam allowance.
  • Leave long thread tails - Make sure to leave long thread tails at the beginning and end of your seam. The tension used for this stitch will cause the stitches to want to bunch up a bit and they will stretch out later. Make sure you stretch them out gently before you go over them with another stitch. And make sure you leave enough thread tails so that your stitch won't come undone at the ends when you do this. The longer your seam, the longer your thread tails will need to be. 
  • Reinforce - It is a good idea to reinforce any high stress areas like the crotch and armpit area with another stitch, like a zigzag stitch for extra strength. Also, check any areas where your flatlock stitch crosses another seam. These areas can be tricky, so reinforce these places if necessary.
  • Get the fit right - It is really hard to adjust your seam after you have sewn it with a flatlock stitch. You don't want to have to unpick it if you can help it! So, it may be a good idea to sew up a practice garment first, so that you won't have to adjust the fit later.
  • Fabric Choice - This stitch may be hard to do with very lightweight fabrics. You may want to choose a heavier weight fabric. Fabrics that do not have a lot of curl will also be easier to handle.
  • Practice! It's always a good idea to practice on lots of scraps first. This is a good way to get your technique down, and also to make sure your tensions are right. 

I would love to hear what you think. If you try this out, leave me a comment and let me know what you've made. Links to things you have made with this technique would be great, too. Thanks!


  1. Thank you!!!! I have always wanted to know how to do this

  2. very cool!! I will have to try this!

  3. Thank you for this fine tutorial. I'll definitely try it very soon.
    Kind greetings from the EU (A)

  4. Do you have suggestions besides reinforcing to make it so seam joints don't unravel? No matter what I do my seam joints (ex armpit) unravel even if I am sure to catch all the threads. Thanks!

  5. Thanks for your insight for your fantastic posting. I’m glad I have taken the time to see this.Yoga Apparel and Accessories