Used for under deck patio seating area. Goal was to keep the area dry. It is impermeable to water. But we have an issue with the sail then sagging under the weight of the water instead of the running off. It is pitched in a downward slope almost 8 inches over the 7 foot width which should be enough to let rain run off but we had to rig it up with wire and a weight to assist in drainage. Does keep area underneath dry though.