Hydrostatic Head (Tents) – a waterproof rating for technical fabrics measured in mm. The higher the rating the more waterproof the fabric is. A rating of 1500mm is deemed waterproof, but on a tent, look for a rating of 3000 or higher. Measured by the height of water column a fabric weave can take before it begins to “sweat”. “Polycotton” tents will not have a Hydrostatic Head level, as they are not treated for waterproofing.