A flat felled seam has no visible seam allowance inside or out. Instead, the seam allowance is enclosed within the seam. Flat felled seams are very strong and durable, which is why they're often used on sturdy fabric, such as denim. Commercial sewing machines typically have a system that does this seam in one step. But it can be accomplished at home with a straight-stitch sewing machine, scissors, and an iron. Strong thread is also an important component, as is pressing to get the seam to lie flat. A flat felled seam is an intermediate-level skill, but it won't take much time once you understand the steps.
If you've never sewn a flat felled seam, grab some scraps of fabric and practice before attempting it on your intended item. Practicing will allow you to understand each step and gain the confidence to make a professional-looking flat felled seam.
Equipment / Tools
- Sewing machine
- Iron and ironing board
- Strong thread
Sew the Seam on the Right Side of the Fabric
To begin sewing a flat felled seam, place the wrong sides of the fabric together. Then, sew the seam on the right side of the fabric. This is unlike a standard seam, which is sewn on the wrong side of the fabric. Every step for a flat felled seam will be seen from the right side.
Thoroughly press the seam the same way it was sewn to set the stitching on the fabric.
Trim the Back Seam Allowance
Find the front and back of the item. Trim the back seam allowance in half, cutting neatly and as straight as possible. Sharp scissors are a must, so you have a smooth edge and aren't fighting fraying threads as you create the seam.
Press the entire seam allowance to the back side. Press it open and then to the back if necessary to get a smoothly ironed seam that faces the back of the item.
Enclose the Seam Allowance
Press the larger seam allowance in half, turning under the raw edge so it just about meets the originally sewn seam. This will cause the wider seam allowance to enclose the trimmed seam allowance.
Next, press the turned-under seam allowance flat. Use a press cloth if necessary to protect your fabric. But make sure you thoroughly iron the seam allowance, so the fold is well pressed before you sew it in place.
Sew Down the Enclosed Seam Allowance
When you sew the final step, you will want the stitching to be parallel to the original seam line and not to go off the folded edge. This final stitching will enclose the seam allowances, so fraying is not possible and the finished seam is strong.