Only Coolaroo Shade Sails feature Coolaroo's unique Commercial Grade high density polyethylene knitted fabric specially designed to breathe and keep you cooler and more comfortable in hot weather. This state of the art material contains 300 grams per square meter of fabric and blocks up to 90% of the sun’s harmful UV rays yet remains totally unaffected by moisture and natural temperature extremes. And because Coolaroo fabric is knitted, not woven, it won't tear or fray. The perimeter of each sail is designed with a gentle curve inwards towards the center of the sail to control fabric tension. Shade sails have reinforced webbing around the perimeter and a stainless steel "Delta" ring at each corner. Therefore, when tension is placed on the shade sail, the fabric will not wrinkle. Shade sails do not sag or flap in the wind. The material is not water proof.