Our shade sails are made of an HDPE fabric, “raschel” lock-stitch knit of monofilament and tape yarns. As such, they are water resistant in that they are not subject to mold and other water damage. You can leave a shade sail up in the rain without worrying that it might be ruined. However, shade fabric is water permeable, meaning that it will not prevent all water from passing through, especially if water is allowed to pool on top. The same breathability that allows warm air to rise through a shade sail allows small amounts of water to permeate the fabric. Layering and sloping can successfully improve water deflection and rain shed, because the rain will not pool and will run off at the low points.