Most shade sails are not waterproof unless you are using a waterproof fabric, however, one of the reasons we suggest varying the heights of your posts/attachment points is to encourage water run-off. When your sails are sloped and tightened correctly, the majority of the water will run off to the side of the sail, with only a fine mist making it through the mesh. Heavy rain is not an issue if the sail has a significant slope and is very tight. If you would like to view our waterproof shade sails you can do so here.