The right finishes make projects go more smoothly, look more professional, and give you an upper hand when it comes to impressing friends with your vast sewing knowledge! Making a flat felled (or flat fell) seam is a detail with a place in history as well as a place in the world of professional seam finishes. You can find references to the flat felled seam technique in vintage as well as hand sewing (once the only way to sew anything). And, if you look down right now at the inside seam of your jeans, you’ll see a trademark flat felled seam.

Within the flat felled seam family there’s the French seam, mock French seam, Hong Kong finish, overcast, overlock, and coverstitch. Each of these make the wrong side of your seam look as good as the right side. Some can only be made with a specialty machine, but the standard flat felled seam can be completed with your regular sewing machine. It takes a bit more time than a standard seam, and, as we mention so often in our tutorials, requires accuracy.

For more on other types of seam finishing, we have a great four-part series that starts with the most popular and basic options, then goes on to cover French, Hong Kong, and more.

Okay… back to your favorite pair of jeans. The flat felled seam is usually on the inside seam as well as the crotch seam (where there’s the most stress). Flat felled seams are known for being strong, durable, and comfortable for the wearer. They’re also ideal for anything that will be washed repeatedly. Although this seam works well in denim, it’s not usually recommended for fabrics any heavier; the seam would be much too bulky. We have a solution for this… keep reading through to the last section of the tutorial.

If you’re not a jeans-wearin’ kind of person, you may have noticed flat felled seams on men’s clothing, such as dress shirts and even boxer shorts. It’s also a go-to choice in children’s clothing. We’re willing to bet it’s that durability aspect of the flat felled seam that makes it so popular on garments for men and kids!

Other types of projects perfect for a flat felled seam finish include various types of bags, active wear, skirts or dresses, garments made of sheer fabrics, or reversible garments. From a design standpoint, creating a flat felled seam is a great way to finish a garment (or other type of project) while adding distinctive detail and texture. In reality, you can use a flat felled seam anywhere it calls for a straight or vertical seam. 

Plus, depending on the fabric type, you can use a variety of threads and thread colors in the final steps of sewing a flat felled seam for a customized look that adds a touch of embellishment to the hard-working seam.

The tools and machines you need to sew a flat felled seam

You need the appropriate needle for your fabric type. For example, in the case of a flat felled seam on jeans, you should use a denim needle.

The appropriate thread for construction is important too, and how you want your final seam to look will dictate the thread you use. In our jeans example, you’d need a stronger thread in a heavier weight. Look for Dual Duty XP Plus Jeans thread from Coats, which comes in that exact red-orange topstitching color that can be so hard to match. And since we’re sewing jeans in our head right now, you might also want to look for Coats’ Denim thread. It’s an all-purpose weight, but comes in a perfect blended denim blue color.

Besides a regular or topstitch thread, you could also use a rayon for shine, or a metallic for a touch of bling.

The sewing machine foot you use is also an important element. In our example below, we use a standard presser foot. Our Sew4Home studio machines are supplied by Janome, and their standard presser foot has a little black button on the back that is used to lock the foot in place when going over an uneven (or bulky) seam. It works like a charm for this technique.

Some manufacturers have a foot specifically designed to sew flat felled seams, called a felling (or lap seam) foot. This type of foot is designed with a groove in the base that rides over the raised fabric in the seam. You can also use the foot to roll the fabric over and sew it in place all in one pass. Check with your local sewing machine retailer for information about this type of foot and to find out if it is available for your make and model of machine.

Other important tools include sharp scissors for trimming the seam, such as appliqué scissors or a similar shorter blade scissor.

You may find marking your seam allowances is helpful. especially when just learning the technique. If so, you’ll need a fabric marking pen or pencil and a clear ruler.

We find a seam gauge to be handy for folding the raw edge of the flat felled seam to ensure our seam is consistent from the first line of stitching.

Finally, since a flat felled seam can be finished with two lines of parallel stitching, you can use a twin needle to sew two evenly spaced rows of stitching at once.

In regards to machines, as we mentioned above, your standard sewing machine is just fine. However, you can use a specialty machine for flat felled seams, such as a serger with a felling guide. You can also create a similar seam with the Janome CoverPro 1000CPX machine.

In commercial production, there are actual felling machines that only sew flat felled seams. That’s how they make all those pairs of jeans so fast!

A few other notes about seam allowance and direction

Seam allowance is a critical part of this process. The type of project you’re sewing dictates the seam allowance used. In garment construction, a ⅝” seam allowance is standard, whereas in home décor projects, bags, etc., a ½” seam allowance is most common.

You need to be very careful about seam allowance, because with flat felled seams, one side of the seam is trimmed away to ⅛” or ¼”. If you’re following a pattern or tutorial, the seam allowance and the amount to trim will be indicated. If you’re incorporating flat felled seams into your own project, be sure to test first on scraps so you can determine your trim amount.

You also need to think about the direction of any flat felled seam before you sew it. In a garment, you always want the side seams to fold toward the back. This means you have to sew and trim each side of a garment differently.

Speaking of directions, a flat felled seam can be done on the inside or outside. It all depends on how you start the technique: right sides together or wrong sides together. We illustrate both below.

Once you’ve sewn flat felled seams, you are likely to need to cross them with another piece (waistband, lining, etc.), which means you need to be prepared to sew over the “hump” of the flat felled seam. For this, you can use the little black button trick we mentioned above (provided you have a Janome), or use what’s called a hump jumper (shown below) that lifts up the fabric behind the presser foot, leveling it so it moves more easily and smoothly under the foot.

Advanced seamstresses can learn to sew a zipper into a flat felled seam or insert a sleeve (or armscye) with a flat felled seam. You can find information on how to achieve these more complex techniques in sewing books and on the Internet. We’re sticking to straight seams for now.

Sewing an outside flat felled seam

In our examples below, we’ve used a light colored polka dot fabric so you can see the right side versus the wrong side. We also used various thread colors for the same reason. No style points… just clarity!

1. Place your fabric edges WRONG sides together. Pin in place as needed.
2. Set up your sewing machine for a standard straight stitch.
3. Sew a straight seam, with the appropriate seam allowance. We used ⅝” in our example.
4. Open out the fabric and press both sides of the seam allowance in one direction. (See our notes above on seam direction.)
   
5. Trim the lower side (the underneath side) of the seam allowance. We trimmed ours to ¼”.
   
6. Here’s where the super accuracy comes into play! Fold the fabric so it is again wrong sides together. Lay it on your work surface so the side of the fabric with the uncut seam allowance is on the bottom. Fold in this uncut seam allowance edge to meet the cut seam allowance edge. It’s helpful to do this step right on your ironing board if possible. Press in place.
   
7. Pressing well is super important. To help with accuracy, you can use a seam gauge to check that the folded-in edge to the cut is the same width as from the line of stitching to the cut.
   
8. Open the fabric flat, then press the seam flat in the same direction you originally pressed, which means that cut edge is again underneath.
   
   NOTE: Depending on your fabric type, you can use a temporary adhesive to hold the layers in place.
9. Edgestitch along the folded edge (We used purple thread for this step).  Remember, since we’re doing an outside flat felled seam, we are working on the right side of the fabric.
   NOTE: Since you’re sewing through a few layers here, you may want to adjust your stitch length depending on the fabric type.
10. Congratulations! You’ve sewn an outside flat felled seam.
    
11. In case you’re wondering (you were wondering, weren’t you?), here’s what the wrong side looks like.
    

Sewing an inside flat felled seam

1. Place your fabric edges RIGHT sides together. Pin in place as needed.
   
2. Following similar steps as for an outside flat felled seam, press the seam allowance together and to one side, trim away the lower seam allowance, fold back the raw edge on the upper seam allowance so it aligns with the cut edge, and re-press in place to one side.
   
3. Edgestitch along the fold.
   
4. Anything look a little different? Beside the flat seam being on the wrong side of the fabric, you only have one line of stitching on the right side.
   
5. You can leave it this way, or you can add another line of stitching as embellishment.
6. Simply sew another line of edgestitching on the right side of the fabric. (We used teal thread for this step.)
   
7. Below is a picture of the outside flat felled seam (bottom) and inside flat felled seam (top). Can you see a difference? Not really, right?
   
8. The difference is where the bulk of the fabric sits. On inside flat felled seam, it’s on the inside. On the outside flat felled seam, it’s on the outside. So how do you know which one to use? It’s partly personal choice, and partly determined by the type of fabric.

Mock flat felled seam option

For a quicker version of the flat felled seam, there is what’s called a mock flat felled seam. This also works well on heavier fabrics that otherwise would be too bulky for the traditional flat fell seam (remember.. we promised you a solution for heavier weight fabrics). In the end, you have a similar looking finish from the outside.

1. Place the fabric right sides together.
2. Set up your sewing machine for a standard straight stitch.
3. Sew a straight seam, with the appropriate seam allowance.
4. Here’s the difference: finish the edges of the seam allowance with a zig zag or overcast stitch on your sewing machine, or with an overcast edge using a serger. We opted to use a serger.
5. Press the seam allowances to one side, as in the above steps (remembering direction as you did above as well).
   
6. Using a straight stitch, sew two parallel lines on the right side of your project. There aren’t any hard-and-fast rules for determining exactly where to sew the two lines. A good rule of thumb is to first sew along the “well of the seam,” just as you would when edgestitching. Then, move the needle to a left position, and use the foot itself as a guide to sew an even distance from the first line of stitching. This would also be an ideal time to use a twin needle!
   
7. Here’s what the mock flat felled seam looks like from the back (top) and the front (bottom).
   

A final word about specialty threads

If you’ve decided you want your flat felled seam to have a little pizazz, you can use a specialty thread, such as those mentioned above. It is recommended you use a standard, all-purpose thread to construct the seam (to make the first seam). Then, re-thread your machine with your chosen specialty thread and sew the edgestitching as well as an optional additional line of stitching. This works especially well for the inside flat felled seam and mock flat felled seam techniques.

Sample Creation and Instructional Outline: Jodi Kelly