Flatlocking is a great technique to learn on your serger/overlocker and you don’t need any special accessories to do it. The resulting seam lies flat so it is a great option for clothing for kids who find a lot of their seams itchy. Flatlocking works on both woven and stretch fabrics, but because the raw edge is somewhat exposed inside the stitches, fabrics that fray a lot aren’t ideal. Flatlocking can be done on seams or hems and gives a great ready-to-wear look.

Flatlocking is often seen on activewear and gives a wetsuit its signature smooth seaming. An industrial flatlocking machine actually cuts the fabric as it stitches to perfectly butt the two fabric edges together, but you can get a very close approximation on your home over locker and I’ll show you all the steps.

My fabric and thread choices for this tutorial aren’t the prettiest, but I wanted to make each thread and fabric really obvious for you to most easily see how it all works. Here are the threads I used:
Needle thread: white
Upper Looper thread: orange
Lower Looper thread: black

Flatlocking works by having a very high tension on the lower looper and a very loose tension on the needle. The lower looper pulls the needle thread across the back of the seam to form the stitches or ‘ladders’ across the back. The upper looper thread forms the loops on the top of the seam.

Because the seam is pulled open at the end, the width of the seam adds to the overall width of the finished piece. In most cases, I don’t worry about the small difference, so aim for the needle to stitch along my intended seam line and use the knife to trim off the excess.

For seams, it is up to you whether you want the knife engaged or not. For hems, you want to disengage the knife. This will prevent you accidentally cutting into the fold of the fabric. This will make more sense in the hemming section below.

Ok…now let’s get flatlocking! First off, remove your right needle as you only need one needle for flatlocking. Note: you can also make a narrow flatlock by instead removing the left needle.

It does take a bit of playing to get your tensions correct, but once you know them write them down somewhere so next time you can set your machine for flatlocking in a flash! Your manual may also have suggested settings for flatlocking.

To start with, set your tensions to:
Needle: 0
Upper Looper: normal (mine is set to 3)
Lower Looper: 9

On the left is how the top of my seam looks with these default tension settings. The picture on the right, shows the underside. You can see that the white thread, which is the needle thread is a bit too loose and not very neat.

By increasing the tension on my needle thread to 1, I now have a much neater stitch. This is what you are after! If you find that your seam is bunching up, it is likely that the lower looper tension is too high. Reduce it one number at a time, until your seam no longer bunches up. I find my lower looper tension best at about 5.5 – 6. These are the settings you can see in the picture of my overlocker above.

Now for the exciting bit! Pull the two pieces of fabric apart!

The seam now lies completely flat! The two pieces of fabric lap over one another and the stitches encase the seam. The picture on the left shows the top, and the picture on the right shows how it looks underneath. As you can see the top has ‘loops’ and the bottom has ‘ladders’. You can choose which of these you want to show on the right side of your seam.

For ‘loops up’ as shown on the left, you want to put your fabric pieces wrong sides together (opposite to usual). For ‘ladders up’ as shown on the right, you want to put your fabric pieces right sides together. Notice that for ‘ladders up’ you only really need to change the needle thread to the colour you want, which is handy if you want to use a colour that you don’t have 3 cones for.

Hems with the Flatlock stitch
As with flatlocked seams, you have the option do a ‘loops up’ hem or a ‘ladders up’ hem. I like the look of a ‘loops up’ hem better, but the ‘ladders up’ is easier to stitch.

Loops Up Hem
I personally prefer the look of this as I think it looks more like ready-to-wear than ‘ladders up’ but it can be tricky to catch the raw edge in the hem because you can’t see the raw edge as you stitch.

Press up your hem the amount instructed in your pattern. Then, fold it up again and press. This will result in a double turned hem.

You should have your fabric wrong side up, with the double turn hem to your right. Flatlock along the edge of the hem. Pull fabric apart and press.

Ladders Up Hem
I find this easier to do because you can see the raw edge as you’re stitching so it’s easy to make sure you catch it inside the stitches.

Press up your hem the amount instructed in your pattern. Now flip your fabric over so it is right side up and press the hem up again by the same amount. The raw edge should be lined up with the pressed fold and you will have created a zig zag.

With the right side up, flatlock along the edge of the pressed hem. Pull fabric apart and press. You can see in my picture on the right that I have a tiny fold in the middle of the hem. This is quite common and you may like the look, you may not. To prevent it, you need to move your fabric slightly to the left when stitching. This will allow more of the stitching to hang off the side of the hem. It can also help to make sure the raw edge of the hem slightly hangs over the fold when pressing.

If you have a blind hem foot for your serger, you may find it easier to use it for flatlocking as it will help you line up the edge in the exact spot. I did all my seams and hems with my regular foot on as I don’t have any other feet!

Flatlocking Stretch Fabrics and Knits
You can flatlock knits and stretch fabrics too. When flatlocking seams that will take a lot of stretching, such as cuffs, neck lines and most horizontal seams, you need to give the lower looper thread some give. To do this, make a long chain both at the start and end of your stitching. I make mine about 6 inches long for seams on kidswear, but the longer your seam is, the longer you want your chains. Hold your fabric from behind as it comes out from your serger’s foot. Holding it taut will prevent the seam from bunching up.

Once you have finished your seam, stretch it out as it will be when worn. Any extra thread needed in the seams to accommodate the stretch will be taken from the start and end chains.

Here I have flatlocked a seam on a rash vest, made from nylon lycra. The seam was stretched out after stitching so you can see it looks a little more slack than a seam flatlocked onto woven non-stretch fabric. The picture on the left shows the right side of the seam and the picture on the right shows the underside.
It is very important to make secure knots at the ends of your stitches, especially for seams and hems that will be stretched. If the lower looper thread comes loose, the whole stitch will unravel.

I stitched all seams for the rash guard with ‘loops up’ and the hems ‘ladders up’. The fabric piece you have uppermost when seaming is the fabric that will be seen ‘inside’ the flatlock stitching from the right side. I wanted the green fabric to be in the seam on the raglan sleeves so made sure that was the uppermost fabric when working my seams.

Unpicking the Stitches
Sometimes, you may need to do some ripping, but it’s actually pretty easy to do. Cut the chains off right at the edge of the fabric and gently tug the threads to find the lower looper thread. This will be the tight thread that runs straight along the edge of the seam. Get a good hold of it and pull it out. It should come straight out and then you can just pull off the other two threads.

There’s a lot of information in this tutorial, but I really wanted to get it all down in one place for you. If you have any questions, please leave a comment!