What holds a zip in place

Short answer: A zip is sewn on through its own tape, not through the teeth. The tape is woven and stiffer than the fabric, which is why knits often need extra support. That keeps the edge straight and relieves the seam.

The tape carries the seam

Along both sides of the teeth sits a flat tape. That is where the needle goes through, so the width of the tape decides where the seam can lie. The teeth are metal or plastic and cannot be sewn through. The tape connects the zip to the garment, and all the pull ends up here.

How the tape is made

The tape is woven, so it is stable and hardly gives. That is an advantage for a zip that must hold its shape, but a drawback when it is sewn into a stretchy fabric. Fabric and tape behave very differently, and the sewing has to allow for that.

Knit and zip fit badly together

The fabric stretches, but the zip does not. Without stabilising, the edge waves and the zip ends up curved. Knit can give under the seam while the zip stays put. It is a known problem in clothes with a zip in soft material. See bamboo sweatpants for men.

That is why there is a tape inside

A strip of interfacing or a woven tape is laid behind the fabric before sewing. It keeps the edge straight and relieves the seam. Without it, the needle would stretch the fabric and it would lie in waves when the zip is closed. The small strip is easy to overlook but makes a big difference. See also bamboo tank top for men and bamboo undershirt for men.

Why the fabric waves

The fabric waves at the zip if it was stretched during sewing or lacks stabilising. Then the edge is longer than the zip, and the excess has to go somewhere. The result is small folds along the opening. It is not a fault in the zip but in how the fabric was joined to it.

Can a zip be replaced

Yes, but it takes time, because the whole edge has to be unpicked. You must remove the old zip without damaging the fabric and fit a new one with stabilising. That takes patience and practice. Many leave it to a tailor. If you are used to sewing, you can also do it yourself.

Why sliders break

Mostly from wear in the metal or because fabric has been caught. The slider is the small moving part, and it takes the most wear. Close the zip gently and keep the fabric clear, so you avoid bending or forcing it. That noticeably extends its life.

How to look after a zip

Pull the zip evenly and without hard tugs, and keep fabric away from the teeth. Close it before washing so the teeth do not catch on other clothes. A zip that runs stiffly can be eased with a little soap or wax. With some care it lasts longer and keeps closing cleanly.

Frequently asked questions

Why does the fabric wave at the zip

Because the fabric was stretched during sewing or lacks stabilising. Then the edge is longer than the zip, and the excess forms small waves along the opening.