Now I wouldn’t exactly call this lazy or a guide, but I figured I would make a quick tutorial on how to do a proper flat felled seam since they’re incredibly useful if you work with a lot of bottom-weight materials and/or make a lot of pants. So let’s get to it!
A flat felled seam, or “jean seam”, is a self-enclosed seam used primarily in the construction of menswear and denim jeans—as you might have already guessed. With a flat felled seam, the unfinished edge is covered by overlapping the fabric within the seam allowance and top stitching the folded edge down. Because there’s two rows of stitching, the resulting seam is incredibly durable and resistant to splitting under stress. It also reduces excess bulk in the seam when working with heavy fabrics, such as denim or wool, and allows you to not need a lining for garments such as pants and skirts.
Before I go into how to do a proper flat felled seam, let’s talk about its sibling, the mock felled seam.
Mock Felled Seams
Mock felled seams look functionally the same as a flat felled seam on the outside of a garment, but differ in that the raw edge isn’t enclosed within the seam allowance. They are done by sewing the seam as you normally would, right sides together, then pressing both sides of the seam allowance to one side and top stitching it down about 1/4″ from the seam line. Most fast fashion jeans are constructed this way, and it’s a preferred method with cosplayers due to its efficiency and speed.
The downside to this method is the exposed unfinished edge on the interior that may unravel and look unsightly, which means that you may need a lining if you have no way to finish it, and the fact that heavier fabrics will result in a bulkier seam that could be more difficult to maneuver through a machine. Having a serger or pinking shears can solve the first, but not so much the second.
Flat Felled Seams
Okay, on to the real deal. The example used in the photos below is my cowl for my Elma cosplay from Miss Kobayashi’s Dragon Maid.
First, you want to sew your fabric wrong sides together. If you’re familiar with French seams then it’s the same dealio. Make sure you press your seam! I always press it closed first to ensure the stitching is flat, then I press it open. I had a bunch of extra seam allowance since I made a giant tube that was pinned where it needed to be sewn, I trimmed the whole thing down to 1/2″ before pressing.
Decide on which side you want your overlap to be. In my example it didn’t matter since the cowl can go on either way, so this is something you’d pay more attention to when doing something like pants so that your stitching matches up properly.
Trim one side to about a 3/16″ seam allowance. Because I had two layers of pretty heavy fabric, I graded the seam allowance on the other side to make it less bulky.
Fold the other edge down towards the seam and press . Then, fold that folded edge over the trimmed edge. The final width should be about 3/8″ from the stitching to the edge of the fold.
From there it’s a matter of top stitching along the folded edge and you’re done!
If anyone has any questions on the process or if something isn’t clear, feel free to send me an @ on Twitter or leave a comment below!