Also called: Water column · Waterproofness · HH
In the hydrostatic pressure test (ISO 811), water pressure on one side of the fabric is increased until drops appear on the other side. The result is given as the height of the equivalent water column, usually in millimetres or centimetres (or as pressure in kPa). A higher value means the fabric resists water under more pressure — from heavy rain, wind or kneeling and sitting on wet ground.
Tent and shelter specifications normally give separate values for the roof or fly, the walls and the groundsheet, because the groundsheet is loaded far more. Seams matter as much as fabric: state whether seams must be taped or sealed, and whether the value applies before or after ageing or washing.
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