Why does my waterproof zipper leak at the seam?

Q: Why does my waterproof zipper leak at the seam? A leak at the seam is usually a seam-sealing issue rather than a zipper issue, but the tape and the film both matter. A controlled water test that locates the leak before any repair is far more useful than a general report that the zipper leaks. Identify the leak point with a controlled water test and mark it. Separate the joint: does water enter along the stitch line, at the tape edge, or through the chain? Inspect the seam seal tape for gaps, bubbles or a missed section. Check that the stitch holes were sealed and that the seal width covers them. Test a piece of the same zipper alone to establish whether the chain itself passes water. A leak through the stitch line is a sealing process issue, while a leak through the chain or tape is a zipper issue. Keep the test pressure and duration consistent, because comparing results from different methods leads to arguments rather than fixes. Photograph the marked leak point before any rework.