A flat-felled seam is sewn with a 1.5 cm (5/8 in) allowance, then one allowance is trimmed to 6 mm (1/4 in), the wider one is folded over it and pressed flat, and the fold is edgestitched through all layers. Every raw edge ends up enclosed, the seam lies flat, and two parallel rows of stitching show on one side. It is the seam on the inside leg of jeans and the side seams of work shirts, and it needs no serger.

What is a flat-felled seam and where do you see it?

A flat-felled seam is a self-finished seam: the allowances wrap around each other and are stitched down, so there is nothing left to fray. It is strong because the seam is held by two lines of stitching and four layers of fabric, and it is flat because nothing stands up on the inside. That is why it is used on jeans, denim jackets, shirts, pajamas, sportswear and anything unlined that gets washed often.

Looked at from the right side, a felled seam shows a plain seam line and a second row of topstitching about 7 mm (1/4 in) away. On jeans the fold itself is on the outside, which gives the slightly raised ridge you can feel on the inseam. On a dress shirt the fold is usually on the inside, so the outside shows two flat rows of stitching and the inside shows the enclosed fold.

What do you need to sew a flat-felled seam?

  • A pattern with a seam allowance of at least 1.5 cm (5/8 in). Narrower than that and the finished fell becomes too narrow to hold; 1 cm (3/8 in) allowances need widening by 5 mm (1/4 in) when you cut.
  • A medium to heavy woven fabric: denim, chambray, cotton twill, poplin, linen. Light fabrics work too, but the fell shows through.
  • An all-purpose thread for the seam, and optionally a thicker topstitching thread for the visible rows on denim, with a 90/14 or 100/16 jeans needle.
  • Sharp scissors, an iron, and either a seam gauge or a strip of masking tape on the machine bed as a stitching guide.
  • A felling foot is optional. It folds the allowance for you, but most sewists get an equally neat result with an ordinary foot and careful pressing.

How do you sew a flat-felled seam step by step?

This is the jeans version, with the fold on the outside. For a shirt with the fold inside, start by placing the pieces right sides together in step 1 and do everything else the same way.

  1. Sew the seam wrong sides together at 1.5 cm (5/8 in) with a stitch length of 3 mm. The allowances are now on the right side of the garment. Press the seam as sewn to set the stitches.
  2. Decide which way the fell will fold (see the next section), then trim the allowance that will be underneath to 6 mm (1/4 in). Cut evenly: this edge sets the width of the finished fell.
  3. Press both allowances to the chosen side, the wide one over the narrow one.
  4. Fold the raw edge of the wide allowance under so it meets the seam line and covers the trimmed edge. The fold is about 7 mm (1/4 in) wide. Press it well; on denim, a shot of steam and the weight of the iron held for a few seconds help. Pin or clip every 8–10 cm (3–4 in), pins at a right angle to the fold.
  5. Edgestitch the fold 1–2 mm (1/16 in) from its edge, through all layers, with a stitch length of 3 mm (3.5 mm with topstitching thread). Sew with the fold on top so you can see the edge, and let the feed dogs move the fabric rather than pushing it.
  6. Press again from the right side with a pressing cloth. If you want the look of jeans, add a second row of topstitching on the seam line side, 7 mm (1/4 in) from the first.

With a felling foot, steps 4 and 5 happen together: the trimmed and folded edge feeds through the foot's scroll, which turns it under as you sew. Feed the start of the seam in by hand and keep the fabric from pulling to either side.

Which way should a flat-felled seam fold?

Consistency matters more than the direction itself. On side seams and sleeve seams, fold toward the back of the garment. On a shirt yoke, fold toward the yoke. On jeans, the inseam usually folds toward the back and the center seam toward the left side of the body as worn. Whatever you decide, do the same on the left and right halves of the garment, or the two sides will look different.

Where two felled seams cross, as at the crotch of trousers or the underarm of a shirt, you have eight layers of fabric. Trim the allowances inside the fell to 3 mm (1/8 in) in the crossing area, press hard, and walk the machine over the bump with the handwheel. Hammering the crossing flat with a rubber mallet is a legitimate denim trick.

Can you sew a flat-felled seam on a curve?

Yes, on a gentle curve such as a shirt armhole or a trouser crotch, but it takes a narrower fell. Trim the under allowance to 5 mm (3/16 in), fold the wide one over at 6 mm, and clip the fold at 2 cm (3/4 in) intervals on concave sections so it can open out. Press the fold over a tailor's ham so the curve keeps its shape. On a tight curve, consider the mock flat-felled seam instead: sew right sides together, serge or zigzag the allowances together, press them to one side and topstitch twice from the outside. It looks the same from the right side and is far easier on curves, though the inside is not enclosed.

Common mistakes with flat-felled seams

  • Trimming both allowances. Only the one that goes underneath is trimmed. Trim both and the fold has nothing to cover.
  • A fell that wanders in width. The cause is uneven trimming in step 2 or an unpressed fold. Measure the trimmed edge every few centimeters and press before pinning.
  • Tunneling or ripples along the fold. The stitch is too tight for the layers, or the fabric was pushed through. Lengthen the stitch to 3–3.5 mm, loosen the upper tension slightly, and test on a scrap of four layers first.
  • Skipped stitches on the thick crossings. Use a jeans needle and go over the crossing one stitch at a time with the handwheel.
  • Folding the wrong way on one side. Mark the direction on each pattern piece with chalk before you sew.

Sewly's patterns use 1.5 cm (5/8 in) of seam allowance on woven fabric by default, which is the width a flat-felled seam needs, and you can ask the sewing guide which seams of your pattern are good candidates for felling.

When should you use a flat-felled seam instead of a French seam or a serged seam?

Choose a flat-felled seam when the garment will be worn hard and washed often, when the fabric is opaque and medium to heavy, when the seam is straight or gently curved, and when visible topstitching suits the style. For sheer and lightweight fabrics, the French seam encloses the edges with no topstitching and no bulk showing through; the comparison of French and flat-felled seams goes into where each belongs. For quick construction on knits or on seams that will be hidden by a lining, a serged or zigzagged edge is faster; the guide to finishing seams without a serger covers the alternatives.