Why Do Backpack and Tent Zippers Fail?

There are several kinds of zippers used on backpacks, tents, and outdoor apparel. Vislon zippers are made from molded plastic and are commonly found on apparel. Coil zippers have two coils that look like teeth, which are sewn to separate pieces of zipper tape. Coil zippers are mainly used on backpacks and tents because they are flexible and can flow along a curved seam or shape. A metal slider is used to force the two separate sides of the teeth together or apart.

Coil zippers are malleable, meaning they can bend into shapes, such as the curve of a rainbow tent door or a backpack pocket. When tiny debris such as sand becomes lodged in the coil, it will deform slightly when zipping or unzipping to accommodate the debris without jamming or pulling apart. This malleability or “self-healing” characteristic is the main reason coil zippers are used for outdoor applications such as backpacks and tents. But malleability has its limit, and coil zippers can fail in a variety of ways.

A Coil zipper, the type most commonly found on backpacks and tents.
A Coil zipper, the type most commonly found on backpacks and tents.

There are many zipper brands, but YKK is the most popular and very widely used for backpacks and tents. Below I discuss the best applications for different zipper sizes, different coil zipper types, how to maintain them, and how they can fail.

Zipper Sizes

If you look on the end or the back of the zipper slider nearest you, you will probably see a number; this indicates the size of the zipper. The numbers (#3, #5, #8, and so on) are based on a rough estimate of the width of the two sides of the coil when the teeth are locked together and the zipper is closed.

Sizes have changed over the years so an older #5 zipper may say “#5C” on the slider, whereas a new one might say “#5CN” or #5NO”. While all of these are very similar in size, they are not the same, and therefore not compatible with one another. But don’t fret, understanding these eccentricities only comes into play when swapping out a slider, which I’ll explain later.

#3 zippers work well for inner tents where fabric tension is low.
#3 zippers work well for inner tents where fabric tension is low.

It’s far more important to know that the most common sizes of zippers found on backpacks and tents are #3, #4.5, #5, #8, and #10. The bigger the number, the heavier the zipper will be. Low tension closures, such as the bug netting on tent doors, can often get away with #3 coil zippers, while higher tension closures, such as tent fly doors, often need a heavier zipper such as a #5 or #8. Problems arise when manufacturers use small zippers for high-tension applications.

A #5 water-resistant zipper on the Durston X-Mid 2P.
A #5 water-resistant zipper on the rainfly of the Durston X-Mid 2P.

Separating vs Non-separating Zippers

Separating zippers are usually found on apparel and sometimes on mummy bags. They allow the two sides of a jacket or sleeping bag to separate completely.

Non-separating zippers, on the other hand, are sewn together at one or both ends so they don’t come apart completely. They’re often used for tent fly doorways,

Waterproof and Water-resistant Zippers

YKK makes a water-resistant Uretek or Aquaguard coil zipper. The zipper tape is waterproof on the non-coil side of this zipper, with the two edges of the glossy, waterproof material coming flush together when the zipper is closed. Some manufacturers advertise these zippers as waterproof while others say they’re water-resistant. Because grains of sand, other debris, worn-out sliders, and high tension can easily create a gap between the two waterproof sides, I think it’s safe to assume these zippers are highly water-resistant and not waterproof. Tents such as the Mountain Laurel Designs Duomid and the Durston Gear X-Mid 1P use them.

Waterproof zippers do exist but they are not usually found on tents or sleeping bags. An example is YKK’s Proseal zipper which is used for drysuits and diving suits since it is both airtight and watertight.

Coil zippers are often found on backpack pockets such as this pocket on the Hanchor Marl
Coil zippers are often found on backpack pockets such as this pocket on the Hanchor Marl Backpack.

How to clean and maintain zippers

Zippers have a tendency to trap sand and other tiny debris between the teeth so it is a good idea to clean zippers regularly, especially when backpacking in sandy areas. Sand can jam zippers to the point that the slider won’t move back and forth along the coil. It can also keep the teeth from coming together completely so they come apart under moderate tension.

Over time, debris caught in the coil can wear down the metal slider. I know it’s counterintuitive, but the metal of the slider will grind down long before the plastic of the coil. This is due to the static nature of the slider vs the malleable nature of the coil. For this reason, tents and backpacks with multiple sliders on each zipper will last longer than those with only one slider. The coil will wear out eventually, with grit grinding up the teeth and fraying the thread that attaches the coil to the tape, but this will happen long after the slider wears out.

If you want to make your sliders and coil zippers last, clean your zippers after trips using Gear Aid Zipper Cleaner and Lubricant or soap, water, and a toothbrush. The benefit to using Gear Aid Zipper Cleaner and Lubricant is that it adds a protective finish to the coil that resists debris. Other dry lubricants such as silicone spray also help zippers slide smoothly and resist grit. Never use a wet lube such as olive oil, bacon fat, or WD-40 on your zipper. It will attract dirt, making things worse rather than better. Much worse.

If and when your tent zippers start separating every time you try and escape from the bugs, you will need to replace the slider. First, contact the manufacturer to see if they will send you a new slider. If they won’t, check the end or the back of the slider to identify the size and brand of your slider and order a few extras from a company like Ripstop By The Roll. Then, clip the coil, slide the old slider off, slide the new one on, and then repair the broken coil with a zipper stop, which can also be found where you get your sliders.

This #5 zipper is the right choice for this DCF tent fly where fabric tension will be high
This #5 zipper is the right choice for this DCF tent fly where fabric tension will be high.

How long should a coil zipper last?

The longevity of a coil zipper varies widely but is determined by the number of days the user used it, the types of environments in which it has been used (mountains vs beach vs desert), and the size of the zipper chosen for the particular application.

So, in theory, a tent manufactured with proper zippers (#3 for the inner and #5 for the fly), used primarily in non-sandy alpine environments, and cleaned regularly, won’t need a slider replacement for up to five years. The same tent used in filthy desert environments could need a slider replacement once a year. If this tent is made with a high tension fabric such as DCF, a slider replacement will be more frequent.

Take-aways

There are several types of zippers, but coil zippers are usually used on backpacks and tents.

  1. Tent and backpack manufacturers don’t always use the right zipper for the job. Be wary of #3 zippers on tent fly doors, but embrace their use on netting.
  2. Cleaning zippers will increase their lifespan.
  3. Sandy environments decrease the lifespan of a zipper.

Finally, when a zipper fails on a tent or backpack that doesn’t mean the product is dead. You can replace the slider and keep using it.

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