A zip is the last thing put into a garment and the first thing a customer feels go wrong. It ripples, it gapes at the top, it will not close over the hip, or it shows a hard ridge down a smooth back.
A tray of zips sorted by length in different colours with a tailor's hand lifting one out
Almost none of those faults is a stitching fault. The zip was chosen after the garment was cut, the opening was made to suit the zip instead of the body, and the seam allowance at the opening was the same as everywhere else.
This guide covers how to choose the zip, how long it should be, where the opening starts, and how to set it so the cloth on both sides lies flat.
In this guide you will find:
- Why a zip ripples, gapes or bubbles
- The zip types and what each one is for
- A working chart for zip length and where the opening begins
- How to set a zip so it lies flat
- How the fabric changes the zip you should use
- How to rescue a zip that already ripples
Why a Zip Ripples, Gapes or Bubbles
Four reasons, and all four are decided before the zip is touched.
The Zip Was Chosen After the Garment Was Cut
The opening has to be planned in the pattern, with the right length, a wider allowance and a place for the top stop. Cut first and fit a zip afterwards and something has to give, usually the cloth around it.
The Opening Is Too Short to Pass the Body
A zip is only useful if the garment goes on. A side zip on a kurti or a salwar has to clear the hip, a back zip on a blouse has to clear the shoulder and the bust. If it does not, the customer forces it and the cloth around the zip is strained.
The Seam Allowance Is Too Narrow at the Zip
A zip needs room for the tape and for the stitching beside the teeth. On a narrow allowance the stitching crowds the seam and the zip pulls the cloth into a ridge.
The Tape Was Stretched While Sewing
A zip tape feeds through the machine slightly differently from the cloth, and one side ends up longer than the other. The two sides then sit at different heights, and the zip ripples however it was pressed.
Choosing the Zip
Three types cover almost everything a shop stitches:
Type | What it looks like | Best for | Watch for |
|---|
Invisible | Teeth turn to the back, only a thin seam shows | Blouse, kurti, dress, skirt | Needs a special foot; set before the seam is closed |
Conventional nylon coil | Visible coil teeth, plastic slider | Salwar, palazzo, cushion, bag | Teeth show; melts under a hot iron |
Metal | Visible metal teeth | Trousers, jeans, jackets | Heavier; can scratch fine cloth |
Open-end | Separates fully at the bottom | Coats, jackets, chef coat | Longer; needs a stiffer facing |
When in doubt, an invisible zip is the safest choice for anything meant to look smooth, and a conventional zip for anything that will be washed hard.
Zip Length and Where the Opening Begins
The length that matters is the usable length, from the top stop to the bottom stop. The overall tape is always longer, because it carries the tape ends. These are working figures in inches for a medium size, and they depend on the cut of the garment, so check the first one on a body.
Garment | Zip | Usable length | Opening starts |
|---|
Blouse, back opening | Invisible | 10 – 12 | Just below the neck seam |
Blouse, side opening | Invisible | 6 – 8 | At the armhole, running down the side seam |
Kurti, side opening | Invisible or nylon | 8 – 10 | About 4 inches below the armhole |
Salwar and palazzo | Nylon | 6 – 7 | At the waist seam, down the side |
Trouser fly | Metal or nylon | 6 – 8 | At the waistband seam, centre front |
Skirt or dress, back | Invisible | 14 – 22 | At the neck or waist seam |
In every case leave about half an inch between the top stop and the seam line it sits under. That is the room for the hook and eye, the facing or the waistband, and without it the slider has no place to rest.
The bust, waist and hip figures the opening has to clear are in our blouse cutting measurement chart and our kurti cutting measurement chart.
How to Set a Zip That Lies Flat
The back of a cream blouse on an ironing board with a zip set flat and a seam gauge held against the zip tape
The order matters more than the skill. Do it like this:
- Fuse a strip of lightweight interlining along the seam allowance where the zip will sit, a little longer than the opening. It stops the cloth stretching.
- Finish the raw edges of the opening before the zip goes in. Once the zip is in, the allowance is hard to reach.
- Sew the seam closed with a long machine stitch where the zip will be, then press it open flat.
- Place the zip face down on the pressed seam and pin it at the top stop, the middle and the bottom, so the tape cannot creep.
- Stitch with a zip foot, as close to the teeth as the foot allows, feeding both sides in the same direction.
- Sew the seam below the zip, then remove the long stitches and press from the wrong side over a pressing cloth.
How wide the allowance at the opening should be is in our seam allowance guide for tailors. Allow about an inch at a zip, wider than the rest of the seam.
How the Fabric Changes the Zip
- Crisp cottons and poplin – any zip works, and a lightweight interlining is usually enough.
- Soft cottons, georgette and chiffon – use a lighter nylon or invisible zip and a firmer interlining, or the cloth ripples around the teeth.
- Silk and satin – test on a scrap first. A heavy zip pulls fine cloth, and satin marks show where the stitches were unpicked.
- Heavy twill, canvas and denim – a metal zip, and grade the seam allowance inside so the stitching is not through six layers.
- Stretch fabric – the zip is stiff and the cloth is not. Use a stay tape along the opening and stretch the cloth slightly to match the zip, never the other way round.
- Velvet and napped cloth – baste the zip by hand. A machine foot will creep the pile.
Which cloth suits which garment in the first place is covered in our guide to which fabric for which garment.
Rescuing a Zip That Already Ripples
- Unpick one side only, re-press the cloth flat, and re-sew it, matching the height on the other side with a mark at the top stop and the waist seam.
- If the ripple is in the cloth beside the zip, fuse a narrow strip behind it and re-press. Often this is all that is needed.
- If the zip will not pass the hip, do not shorten the zip – lengthen the opening, which usually means moving the top stop up.
- If the top gapes, the zip is set too low and the garment is pulling on the hook. Move the zip up half an inch and re-set the hook and eye.
- If the zip itself is wavy, replace it. A tape that has been stretched in the pack will never lie flat.
Zips in Tailoring CAD
In tailoring CAD the opening length, the wider allowance at the zip, and the notch for the top and bottom stops are part of the pattern. They print on every piece in every size, and the opening grades with the garment. Nobody measures down from a seam with a tape, and the second kurti is set exactly like the first.
Our tailoring CAD software page shows how the marks come off the plotter ready to sew.
Questions Tailors Ask
How long should a zip be?
As long as the opening needs to pass the widest part of the body, plus about half an inch. Measure the usable length from the top stop to the bottom stop, not the overall tape.
What is the difference between an invisible and a normal zip?
An invisible zip has its teeth on the back, so only a thin seam shows. A normal zip has its teeth on the front, so they are always visible. Invisible zips need a special foot and are set before the seam is closed.
How much seam allowance does a zip need?
About an inch at the opening, wider than the rest of the seam. A narrow allowance crowds the stitching and pulls the cloth into a ridge beside the teeth.
Why does my zip ripple?
Usually the tape was stretched while sewing, the allowance was too narrow, or nothing was fused behind the zip to stop the cloth stretching. Fuse a strip first, and feed both sides in the same direction.
Can I shorten a zip?
Yes, by sewing a new bottom stop and cutting the end off, or by trimming from the top and re-making the stop. It is easier to buy the right length than to shorten a long one.
Can I iron a nylon zip?
On a low heat with a pressing cloth. A hot iron melts nylon teeth. Metal zips can take more heat, but still use a cloth so the teeth do not mark the fabric.
Plan the Opening Before You Cut
A zip that lies flat is the result of three decisions made at the pattern stage: the right length, the right place and the right allowance. Once those are right, the stitching is the easy part.
Book a free demo – on-site or online, in your language, with no commitment. Bring a garment whose zip never lay flat and we will plan the opening on screen.