How to sew a jeans fly

A jeans fly is a very popular type of closure used in trousers and skirts. It consist of a zipper concealed under the opening. It is attached on one side under a tab and the other side is attached to a facing, usually in a "J" shape. This is why the jeans fly pretty much always features a bold "J" shaped topstitch. Today I will show you how to sew a real one, like the ones from commercial jeans! Let's do this...

Step 1: The jeans fly components. From the left: the facing, the fly shield and the metal zipper. Technically, you can use a regular plastic zipper but the classic choice is a metal one. I recommend using a zipper with a small puller to avoid bulk. The one pictured below is not a good a good example!

To help you sew thick fabrics such as denim, use a bigger needle size, such as 16/100 or any needles labelled for jeans, denim or topstitches. 

If you wish to create the parts from scratch, draft them accordingly to these measurements:

FACING
- Length = Zipper length + 20mm (3/4in) + 3mm (1/8in) + 1 seam allowance width.

- Width = Desired with + 1 seam allowance width.
*The curved edge is drafted on the left side for a woman, and on the right side for a man.

FLY SHIELD
- Length= Zipper length + 20mm (3/4 in) + 3mm (1/8in)  + 2 seam allowances width.
- Width = 2x the facing width.

Step 2: Fuse the facing and the shield.

Step 3: On the garment, mark the bottom of the fly opening along the centre front seam. If you pattern doesn't have one, the marking position from the top edge will be calculated like so...

Zipper length + 3mm (1/8in) + 10mm (3/8in) + 1 seam allowance width.

Step 4: Neaten the centre front seams. Click here for more info on how to neaten fabric with a sewing machine.

Step 5: Pin the centre front panels together along the centre front seam, right sides facing, and stitch from the bottom edge to the marking.

Step 6: Clip through the seam allowance, at the marking level, on the right (women) or left (men) panel only. We do this so we can later place both seam allowances on one side.

Step 7: Neaten the curved edge of the fly facing.

Step 8: Pin the fly facing along the left (women) or right (men) panel, right side facing. The facing is meant to be 10mm (3/8in) longer than the actual fly opening. On the picture below, the green pin is placed at the bottom of the fly opening

Step 9: Then stitch from the top edge and stop at the marking level, namely at 10mm (3/8in) before the end of the facing. To remove some bulk in the seam, trim the seam allowance along the facing only.

Step 10: Lift the facing and topstitch closely along the seam. Topstitch on the facing only, from the top edge to the marking level.

Step 11: Fold the facing back inside the garment and press. If you wish to, you can topstitch along the finished edge as well!

Step 12: Fold the fly shield lengthwise and stitch along the bottom edge. Then, trim the seam allowance.

Step 13: Turn the fly shield inside out, brig out the corner and press. Then, neaten the raw edges together.

Step 14: Pin the left side of the metal zipper along the edge of the fly shield. The top stopper must be 3mm (1/8in) below the top seam allowance. For instance, if you are working with a 10mm (3/8in) seam allowance, the top stopper will be at 13mm (1/2in) from the top edge.

Step 15: Pin the fly shield along the right panel, right sides facing. Again, the fly shield is meant to be 10mm (3/8in) longer than the actual fly opening. On the picture below, the horizontal pin shows the end op the opening.

Then, stitch from the top edge to the bottom of the fly opening. To do so, ensure to sew at 6mm (1/4in) from the edge.

Step 16: Lift the fly shield and topstitch closely along the seam. On the picture below, I traced a line to show you the centre front seam line. I used a 12mm (1/2in) seam allowance and stitched at only 6mm (1/4in) therefore, the actual seam line is at 6mm (1/4in) from the seam. We do this to ensure that the zipper is well concealed under the facing.

Step 17: Place the front panels on top of each other. The side with the facing is placed on the top, with its finished edge aligned on the centre front. In this case, it is 6mm (1/4in) away from the fly shield. Then, pin both panels together.

Step 18: Flip the garment wrong side up and lift the fly shield away. Then, lift the facing and pin the other side of the zipper on the facing only. Grab the facing and stitch along the zipper tape. Ensure to not grab the front panels while doing so.

Step 19: We are almost done! Now it's time to make the famous "J" topstitch. Do make a perfect one, we will work in two times. First, lay the front panels right side up. Then, chalk a straight line from the top edge to about 25mm (1in) above the fly opening end. Bear in mind that the topstitch must be within the fly facing.

Step 20: Before you topstitch, keep the fly shield away from the facing. Pin in down under the right (women) or left panel (men) but ensure to not twist the zipper.

Also, check that both centre front seam allowances are placed toward the facing side, below the shield. Neat!

Step 21: Proceed with the straight topstitch, following the chalk line made previously. To make it bold, use the jeans or topstitch thread. It is much thicker and will make your jeans more authentic! To end the stitch, do not backstitch.

Step 22: To resume the "J" topstitch, bring the fly shield back under the facing and create a curve that will end at the bottom of the fly opening. Curves can be quite difficult to make so help yourself by tracing it with a chalk beforehand!

If you wish to make a double topstitch, start with the outer one and ensure to end it slightly below the end fly opening. To make the inner topstitch, proceed the same way but end it right at the end of fly opening.

Step 23: Strengthen the topstitch with some bar-tacks. Some sewing machines has already a preset to make them, but if you do not have it, just use a narrow and closely set zig zag. Make a test first on a scrap of fabric. Once all set, make two bar-tacks of about 10mm (3/8in) long. One is placed where the topstitch was resumed, just before the curve, and another one at the bottom end.

If you made a double topstitch, connect each of them with a perpendicular bar-tack, always where the stitches were resumed and at the bottom end.

...And you are done!

I hope you liked this tutorial! If you have any questions, leave it in the comment section below.