Waterproofness is an essential factor for functional clothing and canvas materials. Impregnated and coated materials should allow very little water to pass through them, if at all. They are subject to the EN 20811 standard. A "water column" can be used to test how waterproof these materials are. It determines the water pressure at which a material begins to allow water to penetrate it. The higher the value, therefore, the better the material will protect against rain.