How to sew a zip fly

A zip fly was one of those sewing things that used to absolutely terrify me. I would even go so far as to avoid making a garment I really wanted to sew if it dared to have a zip fly! However, last year I decided to take the plunge and give one a go in a pair of shorts and, guess what, it wasn’t half as scary as I had envisaged.

Like most things in sewing, it’s just a process you need to follow step by step and I had to admit that in the end the whole process was really quite satisfying! So, if you’re feeling a bit nervous about sewing a zip fly too, I hope this tutorial will be helpful. I’m going to be sewing a zip fly into a pair of Dawn Jeans by Megan Neilsen. For the purpose of this tutorial, I’m assuming that we are sewing a pair of jeans. If you’re sewing something different, the same process can still apply but do check your pattern as we go along for any discrepancies.

Did you know....
I’ve recently discovered that for women, the zip fly is placed on the left side of the crotch for jeans and the right side for trousers/pants. For men, the zipper is always on the left (opening from the right). This swapping of sides did confuse me for a while so I thought I would just clarify before we begin.

You will need:

A fly pattern piece
A fly extension pattern piece
A jeans zipper
Regular thread
Topstitching thread
A zipper foot for your sewing machine
A topstitching or denim/ jeans needle (optional)

Cutting out

Cut one fly pattern piece right side up
Cut one fly extension piece right side up

TIP: Always follow the grain lines and cutting layout for your chosen sewing pattern.

Step 1: Neaten the edges of the crotch pattern pieces on both front legs. You can do this by using a serger, a zig zag stitch or pinking shears. You will likely be sewing over a notch snip here so it can be helpful to mark this again with a chalk pencil or fabric marker.

Step 2: Take your fly extension piece and fold it in half right sides together. Sew together the curved bottom edge at your required seam allowance.

Trim the seam allowance and turn the fly extension through to the right side. Poke out the point with a point turner, shape the curve and give it a good press.

Step Three: Finish the long straight edge of the fly extension and the curved edge of the fly using your preferred method. You do not want to trim any fabric here, simply neaten the edge of the fabric. Again, mark your notch with a fabric marker so as not to lose it.

Step 4: Take your fly extension piece and your zipper and pin the zip right side up, with the edge of the zipper tape against the edge of the finished fly extension piece. The zipper stop should match up with the notch on the edge of the extension piece.

TIP: If your zipper is longer than your fly extension, don’t worry, this will be sorted out when we add our waistband later. It’s more important to make sure that the zip stop matches the notch at this stage.

Step 5: Using a zipper foot on your sewing machine, sew this side of your zip in place at your required seam allowance. It can be helpful here to move the zipper pull out of the way as you are sewing.

Step 6: Place your right leg piece right side up and place the fly extension right side down on the crotch edge. Line up the top, edges and notches and pin in place. The right sides of your fabric should be facing, and the zipper should be sandwiched in the middle. Pin in place.

Step 7: Sew along the entire length of the fly extension using a zipper foot. Be sure to keep your required seam allowance from the crotch edge.

Clip into the seam allowance just below the fly extension. Do not clip further than your seam allowance.

Step 8: Turn your leg piece and fly extension over so that it is right side up. Switch to a topstitching thread on your sewing machine and sew a line of topstitching close to the edge of the zipper.

TIP: Rather than backstitching at the end of your topstitching, cut the thread and pull the end through to the back and knot. This gives a neater finish.

Step 9: Place your left leg piece right sides up in front of you. Place the FLY piece on top, right sides together lining up the edge of the fly with the edge of the crotch. Pin in place and sew at your required seam allowance.

Clip into the seam allowance just below the fly. Do not clip further than your seam allowance.

Step 10: Turn the fly to the back of the leg and press in place. You should not be able to see any of the fly from the right side. Switch to a topstitch thread and sew a line of topstitching close to the edge of the crotch.

Step 11: Place the two leg pieces, right sides together. Line up the crotch seam, pin in place and baste. This will keep the legs in place ready for the next step.

Step 12: Open up the jeans and from the right side, overlap the front over the zipper and fly extension. The folded edge of the fly should be in the centre. You will usually have a notched marked in the waistline to indicate where the fold should meet. Pop a pin in the top to keep everything in place.

Step 13: Flip the jeans over to the wrong side. Flip the fly extension out of the way so that the zipper is now visible. Pin the other (unsewn)edge of the zipper tape to the FLY ONLY. The edge of the zipper tape should be an even distance from the edge of the fly all the way down.

Step 14: Using a zip foot, sew the zip to the FLY ONLY. Sew two lines of stitching. Once close to the edge of the zipper teeth and one close to the edge of the zip tape.

Step 15 We can now unpick the basted crotch seam. Once unpicked, pin the right crotch to the right leg. Open out the fly extension and pin this to the right leg too. This will keep everything out of the way for the next steps.

Step 16: Using a contrasting thread, baste around the curved edge of the FLY from the inside of the jeans. Sew through the fly piece and the left leg piece. The basting stitches will act as a guide for topstitching the fly.

Step 17: Using topstitching thread and a regular foot, topstitch along your basting stitches from top to bottom. Sew another line of topstitching ¼” from your first line.

It’s important to go slowly here and sew as neatly and as accurately as possible since the topstitching will be the most visible part of the zip fly.

Unpick your original basting stitches and unpin the fly extension we were holding out of the way.

Step 18: Re-pin the crotch seam and sew together ‘properly’ this time. Snip into the curved edges being careful not to snip past the seam allowance. Press towards the left side.

Step 19: Turn the jeans around so that the right side is facing you and mark the zipper stop with a pin or tailor’s chalk. Using topstitching thread, sew a line of topstitching close to the crotch seam from the crotch upwards towards the fly. Make sure that your seam allowance is still pressed to the left on the underside.

When you reach the zipper stop mark, pivot the machine needle. Sew across ¼” and then sew back down the crotch creating a double line of topstitching.

Step 20: Optional. If you would like to, you can finish your topstitching with a couple of bar tacks. Bar tacks help to reinforce the stitching on areas that come under a lot of stress such as zippers and curved areas.

Your machine may be fitted with a bar tack option, or you can use a very narrow zigzag setting. Sew a bar tack at the top of the crotch top stitches (the ¼” where we pivoted) and another along the curve of the fly.

And that’s it, zip fly complete! You can now go on to sew the rest of your garment knowing the hard part is out of the way. Zip fly’s aren’t too difficult to sew but I think it’s important to slow down, take your time with this part and most of all enjoy the therapeutic process – it is therapeutic I promise!!