Serger School: Flatlock Savvy – Sew Daily

|Sponsored| Functional, decorative, and comfortable, the flatlock is one of the most versatile stitches on a serger. Eager to give this stitch a try? Learn the best tips and techniques to achieve a perfect flatlock stitch on your serger.

Flatlock Basics

On the upper side, the stitch looks similar to a regular overlock stitch, and on the reverse side, there is a row of horizontal lines, also called ladders. Both sides of this stitch can be used on the outside of your makes.

A serger can sew both a 2- and 3-thread flatlock version. The 2-thread uses only the lower looper, plus needle thread, and is easy to open up flat, making it an excellent choice for decorative purposes. The 3-thread flatlock is more durable but also bulkier and more difficult to get entirely flat, especially on thicker fabrics.


The flatlock can be used to sew together seams, for hemming, and even for purely decorative purposes. This stitch is suitable for activewear, children’s wear, underwear, quilting, home décor, and more.


Note that the suggestions below are only guidelines. Always check your serger manual for recommended settings and be prepared to experiment with different adjustments to get the best result depending on the project.

Differential feed adjustments: Start with N and adjust if needed. On knits, you might need to go higher to prevent the seam from stretching out.

Cutting knife: The knife should be engaged when sewing seams on fabrics that fray or if the edges are uneven. For purely decorative stitching or when sewing knits, the knife can be disengaged.

3-thread Flatlock

  • 1 needle thread + upper and lower looper thread
  • Width:
    • 3–4 mm (narrow flatlock, right needle)
    • 5–7 mm (wide flatlock, left needle)
  • Tension:
    • Needles: 1–2
    • Upper looper: 3–5
    • Lower looper: 7–8
  • Length: 2–3
3-stitch flatlock stitch on a serger

2-thread Flatlock

  • 1 needle thread + lower looper thread
  • Engaged upper looper converter (check the manual)
  • Width:
    • 3–4 mm (narrow flatlock, right needle)
    • 5–7 mm (wide flatlock, left needle)
  • Tension:
    • Needles: 1–2
    • Upper looper: 3–5
    • Lower looper: 7–8
  • Length: 2.5–3.5
2-stitch flatlock stitch on a serger


For a beautiful flatlock stitch with excellent coverage, try a heavier specialty thread in the loopers. Twisted cotton, rayon embroidery thread, and woolly nylon are all great options. You might need to lower the looper tension and increase the stitch length when sewing with thicker threads for the best result. On a 3-thread flatlock, only use specialty thread in the upper looper to make the stitch less bulky.

In the needle, a regular serger thread is generally the best choice.


Always start with a sample using the same fabric and threads to test the settings.

1. Place the fabric pieces together, according to which stitch you want on the outside.

  • Overlock stitch on the outside: Place wrong sides facing up, with the right side facing up.
  • Ladder stitch on the outside: Place right sides together, with the wrong side facing up.

2. Insert the fabric under the presser foot.

3. Align the fabric edges with the knife or move the fabric slightly to the left, away from the knife, to make the seam easier to open flat. There is generally no need to cut away fabric unless it’s prone to fraying or if the edges are unevenly cut (1).

Learn how to make a flatlock stitch on a serger.

4. If sewing on the round: To finish, overlap the beginning with a few stitches. Release the threads and pull a tail that is at least 4″ long. The beginning tail can be trimmed away (2). Secure the seam by hand sewing the thread tail into the seam.

Learn how to make a flatlock stitch on a serger.

5. If sewing a flat seam: Release the threads and pull a tail that is at least 4″ long. Tie a knot on both the beginning tail and end tail. After opening the seams, secure it by hand sewing both thread tails into the stitch on the reverse side (3).

Learn how to make a flatlock stitch on a serger.

6. Open the seam by pulling the fabric pieces apart so that the seam lies flat (4). You might need to pull some areas several times to get the stitch to open properly.

Learn how to make a flatlock stitch on a serger.

Troubleshooting the Flatlock Stitch

Seams that don’t open flat are the number one issue when sewing a flatlock serger seam. Luckily there are several ways to fix this problem.

Move the fabric edges away from the knife. Let the needle thread extend beyond the fabric edge by moving the edges to the left while serging. Make sure at least half the seam is still covering the fabric. This is a very effective method to flatten the seam, but it also makes the flatlock slightly less durable (5).

Troubleshooting the flatlock stitch on a serger.

Adjust the tension. Start by increasing the lower looper tension. If this doesn’t help, also try lowering the needle tension slightly. But check closely so that the tension is still balanced when the seam is opened.

Increase the stitch length. A longer stitch length is quite effective for getting the 3-thread flatlock stitch to lie flatter. It’s also helpful for seams sewn with heavier thread as it removes bulk. However, a longer stitch length can make the seam less durable. There will also be more fabric showing through between the threads.

Use woolly nylon in the lower looper. This flossy stretch thread actually increases the tension slightly, so if the lower looper tension on the serger is already set to max, switching to woolly nylon will usually fix the balance issue. Other heavier threads can have the same effect.

Flatten out curled edges inside the ladders. If sewing close to the knife, the outer fabric edge will often be curled inside the ladder, which can prevent the seam from lying flat. To fix this issue, insert a blunt hand-sewing needle underneath the curls and flatten out the edges (6). But be aware that the curled edges can actually make the flatlock seam more durable and will also prevent the fabric from fraying, so uncurling the edges isn’t always the best choice.

Troubleshooting the flatlock stitch on a serger.

Other Common Problems

Snagged Threads

  • Keep the project away from hooks, zippers, needles, and other sharp objects.
  • Use a mesh washing bag, wash separately or wash by hand to prevent snags.
  • Using a shorter stitch length can also make the seam less prone to snags.

Stitch Unravels

  • Make sure the thread tails are properly secured.
  • Increase the needle tension slightly. There is a fine balance between making the seam lie completely flat while still keeping it secure.

Loose or Unbalanced Stitch

  • Check that the machine is threaded correctly. Since a flatlock often requires different threading methods compared to regular overlock stitches, this is often the cause of stitch issues.
  • Adjust the tension settings. Usually adjusting the needle tension is enough to balance the stitch. If the needle thread is too loose, increase the tension in small increments and vice versa if the thread is too tight.
  • Move the fabric edges closer to the knife. This helps sewing together the edges evenly, which also balances the stitch.

Want to learn more about using a serger? Check out our serger resource page for tips, tricks, and tutorials! You’ll be succesfully serging in no time!

Sponsored by BERNINA of America | BERNINA provided theL890 serger |

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