Q: Do waterproof zippers need seam sealing?
Yes, in almost every case where the garment is expected to keep water out. Stitching through a coated or laminated tape creates a row of needle holes, and each hole is a path for water. The zipper seam therefore needs a sealing tape applied over the stitch line, usually a heat-activated tape on the inside face, or a bonded tape on the outside for a cleaner look.
The approach depends on construction. On a three-layer laminate, an internal seam tape is standard. On a two-layer garment with a free-hanging liner, the zipper seam may be covered by a storm flap instead, which sheds water but is not a true seal. The slider end, the top stop and the bottom box also need attention, often with a zipper garage, a drain channel or a molded end seal.
Test the finished seam, not just the tape. A common failure is a well-sealed zipper seam that leaks at the point where the sealing tape crosses the slider channel. Specify the sealing tape width, application temperature and pressure, and verify with a rain test or a hydrostatic test on the assembled panel.