So what exactly is a flat-fell seam?? Well, if you're wearing jeans right now, just take a peek at your crotch! The inner leg seam on a pair of jeans, and indeed the vast majority of RTW trousers, is a flat-fell seam. Technically speaking, a flat-fell seam is a very strong seam that has two lines of stitching visible from the right side, and no bulky excess seam allowance inside. Sounds complicated, right? Super wrong! Guys, I bring you excellent tidings - the flat-fell seam could not be easier to pull off...
Where-to, Why-to & How-to
This technique is ideally applied to the inseam of a pair of trousers or shorts. The seam itself provides super strength and durability, plus there's no pesky excess seam allowance on the inside to rub and irritate your skin. As an extra bonus, the flat-fell seam will not fray as the excess seam allowance is completely encased, rather like a French seam.
Start by identifying the "right side" and "wrong side" of your fabric. The right side is the side you want to show from the outside.
Pin your fabric, wrong sides together, along the raw edges to be seamed.
Stitch with your usual seam allowance, as dictated by the pattern you're working from. This will probably be 5/8" or 15mm.
We now need to trim the excess seam allowance to about 5-8mm from just one length. Press the seam allowance so that wider length covers the length you just trimmed.
Fold and press the wider length over, so that it starts to encase the narrower length. Pin into place.
To secure it all down and finish your flat-fell seam, topstitch as close to the pressed fold as possible, preferably 1-2mm from the fold.
Give it one last press and you're done! It should look like this from the right side:
And like this from the wrong side: