Zippers come in different types, lengths and sizes for different applications. I buy continuous zipper by the metre and cut to size instead of buying fixed length. The ends of the zippers are then sewn closed with a ½ inch seam allowance, and/or melted together.
Zip Sizes
A size #3 zipper has 3mm teeth, good for ultralight stuff sacks. #5 is the standard you’ll find on most outdoor gear. #8 to #10 are the strongest (and bulkiest), and best for projects like bikepacking frame bags and structural zippers on backpacks that will take a lot of stress and dirt.
Zip Teeth
The two main zipper teeth types are coil and moulded tooth (vislon). Coil zips are better for curves, have a nice smooth opening pull and are widely available in many colours. Moulded tooth have a less smooth opening feel, but are better for situations where mud and grit would jam up and break the thinner teeth on coil zips (e.g. bike frame bags). They are not very good for curves as they are much less flexible.
Make sure to buy zipper sliders that match your zip teeth as they are not interchangeable. ‘Waterproof’ zips like YKK Aquaguard need a ‘reverse slider’ / ‘waterproof zip slider’, which are upside down to a normal coil slider. A normal slider will usually misleadingly work fine, but you might not realise your mistake until you finish your bag and see that your zip on the wrong-side of the bag!
Note however that you can use a reverse slider on a normal coil zip if you flip the zip upside down, so the teeth point inwards to the bag. This can give a nice aesthetic to your bag.
Cheap off-brand waterproof zips aren’t worth it in my experience and will delaminate within days. Buy genuine YKK Aquaguard from a reputable seller.
Zipper sewing techniques
Zipper Ends
Zippers don’t bend well around tight corners, even coil zippers, which can make your bag’s edges not look as crisp as you’d like, or worst case just mess up the shape of small bags. This can be avoided by sewing some fabric to the zipper that extends past the zipper a bit, and sewing this fabric into the seam allowance of your bag, not the zipper itself. You’ll want around half an inch of seam allowance to make sure the zipper doesn’t come apart, and triple stitch over the zipper. Be careful not to sew onto the teeth themselves if using Vislon zips or you’ll break a needle.
Zipper Seam Allowance
The amount of fabric you need (or don’t need) for the seam allowance for your zippers depends on how close you sew to the zipper teeth and the size of the zipper. In the example photo below the original fabric strips were 3 inch wide. When sewing the zipper while using a zipper presser foot the final panel width (centre zipper teeth to the edge) is ¼ inch shorter than the original fabric piece, which means you need to add a small amount of seam allowance. When using a standard width presser foot it comes out 1/8 inch longer than the original piece, which means you need to remove a small amount of fabric width. Assuming a #10 zip tape is 38 mm (1.5 inch) wide, divide by 4 (9.5 mm / 3⁄8 inch) to get the seam allowance you need to maintain the same panel width, which might not always be possible depending on your presser foot width.
If your fabric is prone to fraying, such as cotton canvas, allow extra seam allowance for your zippers and/or bind the edge before folding over and top stitching. You may need to add a bit more fabric to your pattern piece width to compensate for this.
I’ve frayed the fabric to demonstrate how the extra seam allowance will help give the bag a longer lifespan. Without the extra material, the frayed threads would already be almost at the stitch line. The majority of zipper failures I’ve experienced are from the seam allowance frying and the zip pulling out, not the zipper itself failing. Binding the raw edge with binding tape will also help increase longevity
Zipper Garage
Near the end of the zip you will get a small gap where the zip can’t reach the final teeth to close. This can let in a tiny amount of water, and perhaps isn’t the aesthetic you want. A zipper garage is a small flap of fabric that covers this and gives your zip a nice place to rest in. Zip garages give a professional finishing touch to many bags, such as featured on the TRVL 20L Travel Backpack sewing guide!
Sewing the zipper to the fabric
This is usually done with two lines of stitching. An initial line to hold the zipper in place, then a top stitch after folding the fabric to hold the zipper seam flat and add extra strength.
For the initial stitch, the zip needs sewing on to the fabric upside down. If using a waterproof zip, upside down means shiny tape touching the right side up side of the fabric, and teeth pointing upwards. For a normal coil zipper upside down means the teeth touch the fabric, and the smooth tape is facing up. For Vislon zips, it doesn’t matter which way up they go as the zipper is centered on the tape.
You can optionally bind the raw edge with grosgrain or similar, or run a zig zag stitch to capture the raw edge.
Then fold the fabric over along the seam line, top stitch, and admire your beautiful zip!
