Short answer: A waterproof zipper is built to keep water out of the closure itself. The tape carries a polyurethane coating or a laminated film, the chain is either a coated coil or a moulded chain with a sealing profile, and the slider is shaped to close the gap. It works best when the garment also has a storm flap, taped seams and a water-repellent finish.
The term is used loosely in the trade. Some suppliers call any zip with a coated tape waterproof, even though water can still pass through the chain or the stitching. Buyers need to know which level of protection they are paying for, because the price difference between a water-repellent zip and a truly sealed zip is substantial.
Detailed Explanation
Water reaches a zipper by three routes: through the tape, through the chain and through the stitch holes. A coated tape blocks the first route. The coating is usually polyurethane on one or both faces, sometimes with a matte or glossy finish. It makes the tape stiff, which is why waterproof zips feel different from ordinary ones and why they need a slightly larger seam allowance.
The chain is the harder problem. A standard coil has gaps between the elements, and water driven by wind will pass through. A waterproof coil is coated after weaving so that the coil and tape form a continuous film, and the slider has a tighter channel to compress the two halves. A moulded waterproof chain goes further, using an interlocking profile with a lip that overlaps when closed, similar in principle to a food bag seal.
Stitch holes are the route most often forgotten. Every needle penetration makes a hole in the coating, so a waterproof zip that is sewn with ordinary thread and no sealing will leak along the stitch line. Factories address this with a smaller stitch length, a waterproof thread, seam tape over the stitching or a storm flap that covers the whole closure.
The result is a system, not a component. A jacket with a waterproof zip, no flap and untaped seams will still leak at the shoulders. A jacket with an ordinary zip and a well-designed double storm flap can keep out heavy rain. The specification has to be written for the whole closure, not just the zip.
Comparison
| Type | Water resistance | Flexibility | Typical use |
|---|---|---|---|
| Standard coil | None | High | Casual garments |
| Water-repellent coil | Light rain | High | Light outdoor, bags |
| Coated waterproof coil | Moderate to heavy | Medium | Shell jackets, dry bags |
| Moulded waterproof chain | Heavy, near sealed | Low | Marine, technical outerwear |
| Zip plus storm flap | Highest in practice | Depends on flap | Rainwear, expedition |
The table explains why the cheapest option often wins on light outdoor products. A water-repellent coil with a flap is more flexible, lighter and cheaper than a moulded sealed chain, and it performs well in everything short of sustained driving rain.
How to Choose
Start from the exposure. For urban jackets and bags, a water-repellent coil with a flap is sufficient. For hiking shells, choose a coated waterproof coil and specify taped seams. For marine, rescue and expedition gear, use a moulded waterproof chain and accept the stiffness.
Check the test method, not just the label. Ask for a hydrostatic head result on the tape and a spray test on the assembly, and confirm whether the test was done on the zip alone or on a sewn seam. A number quoted for the film is not the same as a number for the finished closure.
Confirm the slider and the care instructions. Waterproof sliders need periodic lubrication with a silicone product, and the coating degrades if washed at high temperature or dried in a tumble dryer. Donglong Zipper supplies coated waterproof chains with matched sliders and can provide the assembly test data that brands need for their technical file.
Common Mistakes
Buying a waterproof zip and sewing it with a standard stitch and no flap is the classic error. The second is using a coating that cracks at low temperature; cheap PU films become brittle below about minus 10 degrees Celsius and fail along the fold. The third is ignoring the seam allowance, because coated tape does not stretch and a tight seam will pucker.
Another mistake is dry cleaning a coated zip, which attacks the film. Finally, designers sometimes place a waterproof zip where it bends sharply, such as a curved pocket, forgetting that the stiff tape will not follow the curve cleanly.
FAQ Summary
- Coated tape blocks water through the tape, not through the chain.
- Coil and moulded chains need different sealing approaches.
- Stitch holes must be taped or covered by a flap.
- Match the level of protection to the exposure.
- Ask for test data on the sewn assembly, not the film alone.
Waterproofing is a chain of details. The zip is the most visible one, but the flap, the seam tape and the thread decide whether the closure actually keeps water out. Specify the whole system and test it as it will be worn.