A flat-felled seam is an easy technique to master and very useful. It gives a very strong seam finish to a garment. It is often used on trousers and denim jeans as well as shirts for extra durability. It can also be used as a decorative seam finish as it is formed on the right side of the fabric.
I’ve used two different cotton fabrics and a contrasting thread so hopefully its easy for you to see what’s going on and how I’m making the seam
With the wrong sides of the fabric together, stitch along the seam line. Take the full width of the seam allowance. On most commercial patterns this will be 15mm or 5/8 inch, but please check first as they can vary. Press the seam open.
Trim the inner seam allowance to about 3 mm or 1/8 inch. Take care not to cut into the sewing line. You should be able to judge this by eye, there is no need to measure for exact accuracy.
Press the outer seam allowance under by 6mm or ¼ inch; making sure you keep it uniform all the way along all of your seams. Be careful to press like seams in the same direction as one another (e.g. both shoulder seams should be pressed towards the front).
Stitch this folded edge to the garment using a top stitch. Top stitching usually uses a longer stitch length than a seam line stitch. Take a note of the stitch you are going to use, either straight or decorative and ensure you keep using the same one through out the whole garment. This can be colour matched to your fabric or you can use a decorative thread or stitch for added effect.
I like to use different types of seams to achieve different looks on the projects that I make. The flat-felled seam is a useful technique to master for use on shirts, especially men’s formal or dress shirts. It gives a clean, defined look with its rows of straight topstitching. It is also a good choice for children’s wear as the extra line of stitching add a lot of strength to the seams. Sports wear often uses visible, decorative seams too for both durability and decoration. If you are making sports clothing in a stretch fabric though, be sure to use the correct stretch stitch on both the seam line and the top stitching. A zigzag or flat lock stitch can look good here.