Q: What does hydrostatic head mean for zipper tape?
Hydrostatic head is the height of a water column that a fabric or tape can support before water passes through, measured by a test such as ISO 811. It is reported in millimeters: 1,000 mm roughly corresponds to light rain resistance, 5,000 mm to steady rain, 10,000 mm to sustained rain and 20,000 mm or more to severe or prolonged exposure.
For zipper tape, the number describes the coated or laminated barrier only. It says nothing about the chain, the slider gap or the stitch holes, which is why a high tape rating can still produce a leaking garment. Coating weight, film thickness, adhesion and the presence of pinholes all influence the result, and the value drops after flexing or abrasion.
Use the figure as a starting specification, then validate the whole assembly. Ask for the test method, the sample conditioning and whether the result is for the tape or the finished zipper. For a rain shell, a tape in the 10,000 to 20,000 mm range combined with seam sealing is a realistic target; for a fashion jacket that only needs shower resistance, 3,000 to 5,000 mm is often sufficient.