The unit of measurement that indicates the impermeability of technical fabrics is called the water column.
To determine the value, a cylinder 10 cm in diameter is placed on the fabric to be tested for 24 hours and filled with water. Depending on the filling level at which the first drops of water penetrate the fabric within 24 hours, the water column value is determined in millimeters.

That means for a water column of 1,000 mm, you can put a 1 meter high pipe filled with water on a fabric for 24 hours before the water penetrates through the fabric.