Removing the old zip
First you need to unpick the old zip. Take out the bar tack at the bottom of the zip which holds the inner facing. This gives access to the bottom of the zip.
Then unpick the top side of the zip (the side with the button hole) up to the waistband but don’t unpick any part of the waistband. The waistband is very thick and difficult to sew on a domestic machine.
Then cut away top of the old zip as close to the waistband as you can. At the bottom of the zip snip it away without damaging the top stitching (to the right). Then repeat these steps on the opposite side of the zip. This will fully remove the old zip leaving the zip placket attached to the top.
Once that is done it should look like the picture below.
Fitting the new zip
TIP – Metal tooth zips are the best for trousers and jeans because they are harder wearing then plastic ones. Ideal for something that will be used a lot.
To add the new zip you start with the underneath side (the side with the button). Turn the top edge of the zip tape under close to the top of the teeth to make the zip go as close to the waistband as possible. I used a Wonder Clip to show you, as illustrated by the photo below.
Put the zip foot on your machine. For this first bit of stitching you want the needle to the right and the foot to the left. Stitch about ¼” from the teeth from the top down to the bottom. Don’t worry if you can’t stitch too close to the top at this stage, there will be a second row of stitches, which will catch the top later.
Note – As you may spot in the photos the zip we used was a little too long. This doesn’t matter as the bottom of the zip will just tuck away below the placket.
Now still working on the same side, you want to fold the jeans over the zip placket and pin into place as per the image. Sew down that side of the zip right from the waistband to the bottom, sewing quite close to the edge of the fabric.
This is one side of your zip completed.
Now we are going to do the other side of the zip. Start by doing up the button on the waistband, then looking inside the jeans turn back the zip placket so you can see the zip and pin the zip to the buttonhole side of the trousers.
Make sure you have pinned the zip straight before you start stitching. Undo the waistband button, open the zip all the way down. Then fold the tape at the top under close to the waistband like you did on the other side. Machine stitch from the waistband down to the curved top stitching at the bottom twice, once ¼” from the teeth and once on the edge of the tape to hold it really securely. These two lines of stitches will be visible on the outside of the trousers so make sure you colour match your thread well.
The final stage is to do the zip up and the button at the waistband. Flap the placket over the zip on the inside and pin in place at the bottom. Then replace the bar tack that holds the placket in place – the one you removed at the very beginning. Then you are finished and have a new fully functioning zip in place.
To bar tack, make half an inch of straight stitch backwards and forward a few times, or you can do it with a small zig zag very close stitched. Bar tacks are used on stress points to attach two pieces of fabric together in a very small area. For example you will usually find them at the top and bottom of pockets, or on belt loops or the top of pleats.