The flat felled seam is a flat, comfortable seam suitable for medium-weight fabrics and garments requiring maximum durability. A flat felled seam makes sportswear and casual wear look very professional and if you have an active little toddler running around, these seams are hardwearing and perfect for overalls and rompers. They are comfortable because the bulk of the seam is on the outside.
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What is a Flat Felled Seam
A flat felled seam is a simple seam designed to encase raw edges to prevent them from fraying as well as providing an attractive and extremely durable seam.
You can see from this photo below that the inside of the item has a clean finish with no raw edges or finishing visible. This makes it a great seam for people and especially kids who are irritated by serged seams.
Flat felled seams are a seam and seam finish all in one! If you don't have a serger then this can be a great alternative to create simple flat open seams.
When to Use a Flat Felled Seam
The flat felled seam is ideal for denim and cotton fabrics but may be too bulky for heavier material. If you look at your denim jeans or jackets, you will see lots of felled seams.
It is more difficult to sew around curves and is best used on straight seams if you are a beginner.
Flat Felled Seam - Video
Here is a video I made to show you how easy it is to sew a flat felled seam. Subscribe to my YouTube channel for weekly sewing videos.
Relax and press play below >>
How to Sew a Flat Felled Seam
Step 1 - Alter Seam Allowance
Before you start, check your seam allowance width. It should be at least ½ inch (12mm) wide. Any less than this will result in a really narrow seam that will be hard to sew.
Do a test of the flat felled seam on a scrap of fabric and adjust the seam allowance if necessary.
Step 2 - Stitch the Seam
Place your fabric pieces with wrong sides together and stitch along the normal seam line.
Press the seam open.
Step 3 - Trim
Trim one side of the seam to a width of ⅛ inch (3mm).
Step 4 - Press and Fold
Press the wider (untrimmed) edge over by ¼ inch (6mm).
Fold the wider, pressed edge over to encase the trimmed edge.
Make sure the pressed and folded edge is the same width all along the seam. It can look strange if it goes in and out.
Step 5 - Outside Stitching
Stitch along the open edge of the seam 1/16 (1.5mm) to ⅛ inch (3mm) from the edge. You want to stitch as close as you can without going off track.
Beginners: Use pins or hand baste with a large running stitch to secure the seam before you sew. Cotton normally irons a crisp edge but slippery fabrics or thin fabrics like silk and polyester will be difficult to sew without some hand basting first.
You can use your regular all-purpose sewing foot, but for really accurate and neat stitching, an edge stitching foot can really help.
See how it has a guide in the center that will run along the edge of the seam. You will need to move your needle slightly to the left rather than leaving it in the center as you would for normal sewing.
Further Reading: How to use an edge stitching foot
Flat Felled Seam around a Curve
Flat felled seams are often sewn around curves in the center crotch area of pants and trousers. These give a durable seam that won't unexpectedly rip and cause undue embarrassment.
If you are using the flat felled seam around a curved edge, then clip the trimmed edge of the seam before encasing the seam as this will give more elasticity to the seam and allow you to turn a corner. There is an example of this in my video.
When clipping, put tiny snips every ¼ inch (6mm) making sure you do not cut the stitching line. If you accidentally snip the stitching line, make sure you go back with your machine and reinforce it.
Further Reading: Clipping Sewing
However, it is worth noting that the flat felled seam is best suited to the straight path of your seam line or gently curved seams.
Alternatives to a Flat Felled Seam
While slightly different in the look and construction, topstitching, French seams and edge stitching can also be used to strengthen seams and add a decorative edge.
Flat Felled Seam vs Topstitch
Topstitching is mainly decorative and does not always encase the seams like flat felled seams. Rather than joining pieces of fabric flat, topstitch is often used on the edge of 2 pieces of fabric sewn together and then turned the right way out. It can also be used on pocket edges and may be done in 1 or 2 rows. When done in 2 rows, it looks very similar to a flat felled seam on the outside.
Flat Felled Seam vs French Seam
French stitching will encase the edges but there is no stitching visible on the outside. It is also a durable seam and is often used on sheer fabrics and pillowcases. Like the flat felled seam, it is best for straight seams with little or no curve.
Flat Felled Seam vs Edge Stitch
Edge stitch has a decorative row of stitching on the outside like flat felled seams but has a raw or serged seam underneath. Because you don't need to hide the raw edges, it can be used on thicker fabrics. Often there will only be one row of stitching instead of 2.
Further Reading
Topstitch 
Edgestitch French
Flat Felled Seam - In Conclusion
Run your fingers along the completed article and see what a smooth finish you have on the inside and a tough edge to the outside. This is a rugged, but smart seam, and a winner for sports and outdoor wear!
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