The picture shows the cover picture of the instructions.

Sewing fly with zipper in classic trousers #Z04

In the following article and the step-by-step video tutorial, I explain how to sew a zipper fly in the classic way. The processing variant differs from the instructions for jeans and chinos. Here I have closed the leg seams and crotch seam before sewing the fly with zipper.

This time I also decided to sew a classic “women’s slit”, which is also rather rare these days. Ladies’ fly means that I open and close the zipper of the finished trousers with my left hand. If you want to work the slit the other way round, you must follow the steps in these instructions in mirror image.

Have fun and good luck!

Video tutorial:

1. This is required:

The picture shows the required parts.
This is required.

Fabric pattern pieces:

  • the two trouser legs where the leg seams are already closed (my front trousers here in the example have pleats and are lined. I have therefore already cut back the right front edge on the side where the fly stitching will be later. I have also finished the edges of the fly, crotch and seat seams).
  • Fly facing (ZB) for the right-hand side
  • Fly underlap (ZU) for the left side

Pattern pieces from interlining:

  • Fly facing (EZB)
  • Fly underlap (EZU)

Made of paper:

  • Template for fly stitching (P-SZ)

Tools and aids:

  • Zipper (the length is determined by the pattern and the sewing instructions)
  • Hand shears
  • Chalk or marker pen
  • Pins or clips

2. Prepare fly underlap and facing

The picture shows how the interlining is ironed on.
The interlining is ironed on.

I start by ironing the interfacing onto the wrong side of the fly facing and fly bottom.

The picture shows how to prepare the slot underlay.
The fly underlap is prepared.

I lay the slit bottom right sides together and sew the slanted bottom edge with a seam width of 10 mm.

Then I turn the bottom of the slit, press out the seam and cut back the excess seam allowance.

The picture shows the finished parts.
Both parts are serged.

I finish the open cut edges with the overlock machine. The open edges on the long side of the slit underlap are serged together, on the slit facing the rounded back edge and the front edge up to the notch.

3. Sew on the fly facing

Now, I overlock the front edge of the right advantage with the slit facing. If you have not already done so in the previous steps, you must first cut back the front edge at the center front and neaten it.

The picture shows how the slotted cover is inserted.
The slit cover is pinned in place.

At this edge, I now lay the top of the fly piece flush right sides together on the front trousers and pin it in place. I mark the end of the slit on the clip. Until then, I topstitch the pieces together with a straight stitch at a seam width of 10 mm. I secure the slit end with a forward and backward stitch.

The picture shows how the seam allowance is cut in.
The seam allowance is cut in.

I cut the seam allowance at the slit end until just before the last stitch.

The picture shows how to stitch down the slit cover.
The fly piece is topstitched down.

Before turning the slit facing to the inside, you can press the seam allowance over or topstitch it down on the facing side with a 2 mm gap to the seam.

The picture shows how to iron the fly piece.
The slit facing is ironed to the inside.

Now I turn the slash facing inwards and iron it in place. I lay the lower cut edge of the facing to the cut edge of the front trousers. Depending on the design, the slit can also be topstitched through and through at the front edge – in this case it is not necessary to topstitch it down first.

The picture shows how the slit stitching is marked.
The slit stitching is marked.

I place the slit template on the center front and flush with the top of the front piece and mark the course of the slit.

4. Sew on the zipper

The picture shows the zipper.
The zipper is cushioned.

I now sew the zipper to the left front piece. Before that, I iron it over or steam it down.

The measurement of the zipper length required is noted on the pattern for the front trousers; you will also find it in the instructions. You can also determine it by measuring. The measurement corresponds to the length at the front center from the clip to the top edge minus 15 mm.

Shorten zipper

If you don’t have a zipper of the specified length, you can easily shorten a slightly longer plastic spiral zipper yourself. If you have a matching zipper, continue here.

The picture shows how the zipper length is marked.
The required length is marked.

I first mark the required length at the top, measured from the upper end of the zipper.

The picture shows how the zipper is locked.
The zipper is locked.

I then strap across the marked end. You can either sew back and forth several times or use a wide zigzag stitch.

I cut off the excess tape approx. 1.5-2 cm below this.

The picture shows how the spiral is cut away.
The zipper spiral is shortened.

I also cut the spiral back to 5 mm below the locking mechanism.

The picture shows how the zipper is attached to the fly.
The zipper is pinned to the front.

I mark 8 mm from the front cutting edge and 15 mm from the top edge. At this mark, I place the zipper right sides together (i.e. with the slider on the right side of the fabric) on the front piece and pin it in place.

The picture shows how the zipper is stitched in place.
The zipper is sewn in place.

Then I topstitch the zipper tape narrow-edged.

The picture shows how the zipper is stitched down.
The zipper is stitched down.

I then fold the zipper over so that the right side is visible. The seam is then topstitched narrow-edged to the front.

The picture shows how the front center is marked.
The front center is marked.

I mark the center front parallel to the inseam. In my example, the distance is 7 mm.

5. Connect the front pieces

For the next steps in sewing the fly with zipper, the two trouser legs are now joined together.

The picture shows the markings of the step curve
The beginning and end of the inseam are marked.

First, I turn the right trouser leg to the left side. Then I mark the height of the snap mark on the back of the trousers in the crotch curve.

I mark the end of the slit on the front piece.

The crotch curve is sewn together between these two markings with a seam width of 10 mm.

The picture shows how the trouser legs are pushed into each other.
The trouser legs are pushed into each other.

To do this, I pull the left trouser leg into the right one. At the center back and center front and the crotch curve, I place both pieces exactly on top of each other and pin them in place.

The picture shows how the inner leg seams meet.
The inner leg seams meet.

The inner leg seams also meet exactly.

The picture shows how the inseam for the fly with zipper is sewn together.
The inseam is closed.

Now the seam is sewn together from the rear marking to the exact end of the slit with a seam width of 10 mm. You can exert a slight pull in the area of the seat seam, as this area will be subject to a lot of stress later on.

The upper part of the seat seam is not closed until the waistband has been sewn on.

The picture shows how the inseam is ironed.
The seam allowance of the inseam is stretched.

I stretch the seam allowance of the crotch seam when ironing out so that the pants fit better in this area later.

6. Stitch the zipper to the facing

The picture shows the fly with zipper and slip.
The zipper is sewn to the facing.

In the next step, I stitch the zipper to the right facing.

The picture shows how the slip is folded.
The slip is folded to the side.

To do this, I fold the facing to the side and make sure that the front center meets. Then I pin both front pieces together at the waistband edge.

The picture shows how the zipper is attached to the facing.
The zipper tape is pinned in place.

I pin the zipper tape smoothly to the fly piece.

The picture shows how the zipper is stitched onto the facing.
The zipper is stitched in place.

I then topstitch it narrow-edged.

The picture shows the inside view of the fly with zipper.
Interior view.

This is how the fly with zipper now looks from the back.

7. Topstitch the fly with the zipper

The fly facing is now topstitched under the front trousers.

The picture shows how the facing is pinned to the fly with the zipper.
The slip is pinned in place.

To do this, I pin the slit facing and front piece together. Place the hand measure under the two layers so that I only join them together when pinning.

The picture shows how the fly with zipper is stitched.
The fly is quilted.

You can place the template on top of the stitching and sew past it or exactly on the marked markings.

I start by opening the zipper so that the layers lie on top of each other as smoothly as possible.

The picture shows the end of the slit.
The slot end is locked.

You can secure the end of the slot with a machine bolt. I will show a hand stitch here. So I leave some thread hanging and thread it into a sewing needle.

The picture shows how the slit seam is locked.
The slit seam is locked with a hand stitch.

I stitch the thread to the back and come out of the fabric a few millimeters above it.

Then I put the needle back on the reverse side just behind it and repeat the steps several times until I have reached the end of the inseam.

The picture shows the point stitch.
The point stitch.

This is the so-called point stitch.

The picture shows how the end of the fly with zipper is sewn by hand.
The slot is locked by hand.

I then sew a few more stitches diagonally down to the slash stitching. The bartack is then invisible with matching colored yarn.

There are many options for designing the fly and the short inseam. The slit can be single or double stitched. This also applies to the short inseam below the slit, which can also be left unstitched. You can work with a cross or lengthwise bartack at the end of the slit. However, you only do the latter after you have attached the slit underlay.

8. Connect the fly underlap

The picture shows the slotted bottom step.
The slot underlay is connected.

From the inside, I place the slit underlap over the slit facing. Pin the serged long edge to the seam allowance of the left front piece.

The picture shows the pinned slip.
The document protrudes a few millimeters.

The facing can protrude a few millimeters beyond the seam allowance.

The picture shows how the slit underlap is attached to the facing.
The slit underlay is pinned to the facing.

On the opposite side of the break, I pin the bottom of the slit to the bottom of the slit facing.

The picture shows how to sew on the slit underlap.
The slit bottom is sewn on.

Then sew the bottom of the slit to the seam allowance. I use a zipper foot for this so that I can topstitch close to the seam.

The picture shows how the slit underlap is sewn to the facing.
The slit bottom is sewn to the facing.

I then join the underlap and fly facing with a short seam or bar tack.

fly bartack

As described above, the slot end can be secured with a latch. I use a narrow zigzag stitch with a stitch width of 1.5 and a stitch length of 0.4. I try out the stitch on a scrap piece before sewing it onto the pants.

9. Finished fly with zipper

The picture shows the finished fly with zipper.
Finished fly with zipper.

This is what the finished fly with zipper looks like from the outside.

The picture shows the inside view of the fly with zipper.
Interior view.

Here you can see the finished inside view.

Here you will find the link to the next step:

If you’re not quite ready yet and perhaps want to start at the beginning, you’ll find some links here: