Trouser fly fastening – zip

The most common fly fastening is a zip. This post is if you can’t get your head around some other instructions (and hopefully all my photos from different angles makes it clearer) or if you need to replace a zip in your jeans. I’ll do another post if you want to change a pattern to a button fly. A button fly is just as easy, it’s pretty similar but obviously a little different.

Men’s fly opening is always left over right. Traditionally ladies is right over left but I have noticed in recent years with the popularity of boyfriend jeans, that a lot of ladies jeans fly openings are left over right.

Step one – Attach the facing

Stitch from the base of the opening a couple of inches. If you’re replacing a zip, then you don’t need to do this step but you need to unpick the zip and zip guard completely.

This is just a sample so I’ve probably sewn further than you would on a garment

I recommend using a fusible interfacing on the fly facing, this stops the facing from distorting when you attach the zip later. If you’re replacing a zip, this step won’t apply to you. Make sure the facing is completely free from any stitching.

Attach the fused fly facing from top to bottom of opening.

I like to understitch the facing and then press back, this gives a nice crisp finish to the edge. If you’re making a pair of jeans, then you’ll topstitch instead. To get a nice finish, first press the facing/garment seam open and completely flat. Then fold the facing back and press again making sure the seam is directly on the fold.

Step two – Attach the zip

Press the seam allowance back on the other side of the opening towards the inside. Line the zip up against this pressed edge and attach using a zipper foot close to the edge of the fold. If the zip is too long, that’s fine as the bottom of the zip can be cut off and a stitch over the teeth can prevent the zip pull from being pulled off.

I’ve kept it quite simple for this post but on actual trousers, you want to off-set the folded back seam allowance/zip. If you look at your own jeans you’ll see it can be off-set under the facing by about 1cm

Close the zip and line up the the side with the facing on over the top. Pin or tack right on the edge to hold in place. On the inside, pin and then stitch the free side of the zip to the facing ONLY. Make sure this line of stitching is NOT attached to the front of the garment.

Step three – Attach zip guard

Attach the zip guard by top stitching directly on top of the first line of stitching holding the zip in place.

If you’re doing tailored trousers, the zip and zip guard are not attached with a top stitch. But by flipping the garment over to wrong side and attaching with right sides of both zip and garment facing each other. Hopefully that makes sense?!

Step four – Top stitching

Now that both sides of the zip are attached, the top stitching on the front of the garment through the facing to hold it in place can be done. Make sure the zip guard isn’t in the way. It’s a good idea to pin it back so there’s no risk of it being in the way.

To get a nice neat top stitch you can either mark the line by tailor tacks, a tracing wheel, making a paper template or free hand drawing it. If you draw it, use something like a frixion marker (these are fantastic because the moment the iron touches it, it comes off) or chalk; and not anything that will forever mark the fabric, like pencil.

Finally lay the zip guard flat and either do a bar tack or a small top stitch from front of garment over the top stitching to hold the zip guard in place. Obviously not the whole way up, just across the bottom, enough to stop it from flapping about.


My photos are a sample I produced in class so I did the stitching in a contrast colour to make it more noticeable for the students. I always use interfacing on the facing and zip guard to give it a bit of structure. If your zip is cut off at the top, that’s fine because the top of the fly will be sewn into a waistband so all the raw edges are contained and the zip has no way of coming off.

In my next post I’ll go through making a button fly. Hope you enjoyed reading.

Rosie xx

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