28 Mar 2023 • • by Heather Lou

How to Sew Flat Felled Seams Two Ways! {Video}


Learning to create durable, strong seams is a great skill for garments that you want to stand the test of time. Flat fell seams are one of the sturdiest seam finishes you can sew, and add strength and longevity to clothing. Typically found in jeans and workwear, they can be used on any garment where you want a strong, enclosed seam with an attractive topstitched finish. Their unique construction results in a professional and neat look inside and out! In this blog post, we show two different ways to create flat felled seams - this might just become your new favourite way to finish your next pair of jeans or jean jacket!

What is a Flat Fell Seam

A flat fell seam is a very strong and durable seam finish where the seam allowance is completely enclosed with no visible raw edges and then sealed with a line or two of topstitching. Flat felled seams are traditionally used for hardwearing garments that require a durable, long-lasting finish such as jeans, jean jackets, overalls, and other workwear. While French seams are also a clean, enclosed seam, flat fell seams are anchored to one side of the garment with 1-2 lines of stitching, making them incredibly stable and strong. In this post, we are talking specifically about sewing flat fell seams with jeans when you need two lines of topstitching, but if you prefer a simpler finish, check out this slightly simplified post for sewing flat fell seams on the Kelly Anorak.

Where to Use Flat Fell Seams

In general, it's easiest to flat fell straight seams. When making jeans, you can flat fell the following areas:

  • Attaching yoke to back leg, seam pressed down.
  • Attaching back legs together along seat curve.
  • Inseam or side seams, but not both. Once you sew the legs into a tube, it's basically impossible to topstitch both in place, so pick one or the other for your flat fell. In ready-to-wear, it is generally the inseam that is flat-felled, and it is always pressed forward towards the front leg.
  • The front curve is almost always finished with a "faux flat fell" (more on that below!) rather than a traditional one, probably to reduce bulk and make the zipper fly installation easier.

It is possible to flat fell curved seams, but they are a bit trickier. You may need to notch the trimmed/shorter seam to get it to lay flat. Either way, use lots of pins to secure before topstitching!

Flat Fell Seams vs. Faux Flat Fell Seams

In a traditional flat fell seam, one side of the seam allowance is trimmed down so the other seam can be wrapped around it and stitched in place. Before the advent of sergers, this was the strongest way to finish a seam. Nowadays, you can get the look of a flat fell seam without all the labour simply by trimming your seam allowance, serging or zigzagging the raw edges, and then topstitching in place from the right side. We call this a "faux flat fell seam" since it looks like a traditional one from the right side of the garment.

Before we go into the details of the construction for flat-felled seams, here are a few tips for topstitching.

Topstitching Tips for Flat Fell Seams

Beautiful topstitching is crucial for durable flat-felled seams - here are some tips to get super professional-looking and long-lasting results.

  • Use heavy and durable thread for your heavy-duty fabrics, just as you would use thinner thread for lighter fabrics. If sewing denim or canvas, look for heavy-duty, extra strong, or denim topstitching thread.
  • Remember the fabric, thread, and needle trifecta. Adjust the size of your needle according to the fabric and thread used.
  • If you're using topstitching thread, use regular thread in the bobbin unless you're using an industrial machine. You'll have a much easier time adjusting the tension and getting an even stitch.
  • If your machine doesn't love topstitching thread (or you ran out!) a quick and simple alternative to re-create the look and strength is to use two strands of regular thread when threading the needle.
  • Balanced thread tension is crucial for achieving clean durable stitches. Heavier thread often requires higher needle tension. Practice on a scrap until you get nice, even lines of stitching.
  • Longer stitch lengths make it easier to achieve nice even stitches, but can be trickier around corners. A stitch length of 2.5-3 is a good all-around length.
  • Use an edge-stitching foot if you have one. The guide aligns with the edge of the seam to give you perfectly even stitch lines.
  • Take it slow, especially when sewing through multiple layers, and manually turn the hand wheel for additional control.
  • Use a hump jumper to level the presser foot as you climb or descend a seam to keep the presser foot level.

How to Sew a Traditional Flat Fell Seam

If you prefer to learn by video, check out our quick Youtube video for this technique. Otherwise, read on!

To start, ensure your seam allowance is a minimum 5/8" wide. If your pattern uses 3/8" seams, you'll have to sew a faux flat fell instead.

Sew the seam at 5/8" with right sides together. Trim one side of the seam to 1/4", trimming the side that you intend to press towards, ie. the side that will eventually be topstitched in place. Use duckbill scissors to trim, or a rotary cutter if the seam is straight.

Fold the long seam around the shorter trimmed seam. Press flat. If you have a clapper, use it to help "seal" the seam in place.

Press the seam to one side while maintaining the folded seam allowance.

At this point, it's time to topstitch the seam in place. It's easier to sew from the wrong side since you can make sure you're catching the folded edge of the seam with your stitching, but unless you are extremely precise, it may not look even on the right side. For this reason, we prefer topstitching from the right side. Pin the seam in place from the right side to ensure it's laying flat.

For that classic "jeans look" you'll want to sew two rows of topstitching. Start topstitching 1/8" from the seam line - an edge stitch foot is very helpful here.

Sew another line of topstitching approximately 1/4" away, making sure you are catching the folded seam below. You should be able to feel it with your fingers as you go. If your machine is skipping stitches or getting stuck in all the layers (denim is thick after all!) try a new, sharp needle. You can also try gently hammering the seam from the wrong side to flatten it out, or use regular thread in lieu of topstitching thread.

If you don't want a double line of topstitching, simply skip the first row at 1/8" as described above, and stitch one time 1/4" from the seam line to anchor the flat fell in place.

How to Sew a Faux Flat Fell Seam

This method works for seam allowances that are 3/8"- 5/8" wide.

To start, sew the seam at the seam allowance with right sides together, and use your serger to finish and trim the seam to 3/8".

Press the seam to the side to which you want to topstitch. Pin the seam in place from the right side to ensure it's laying flat.

Start topstitching 1/8" from the seam line - an edge stitch foot is very helpful here. Sew another line of topstitching 1/4" away.

Flat Fell Seams Video Tutorial

If you're a more visual learner, check out this short video to see it in action!

Practice Your Flat Fell Seams with These Patterns!

Practice your new skill set! Jackets, jeans, even button-down shirts can all be finished with flat fell seams. Strong seams will mean these clothes just get better with age... like both fine wine and sewists!

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