The cover material is stretched between tracks or rollers on the edge of the coping and a motor-driven pulley system to roll it up on a barrel roller. The manufacturer sell and service them. They protect, save, and insulate, and, best of all, they keep dirt and debris out of the water. Also you have to cleanup the debris from top of it before removing the cover. They are made of close woven mesh, mesh being made of plastic strips and reinforced with steel or other wires running in crisscross patterns within the fabric.
Installation is a series of hooks and eyes on the cover and deck that secure the unit in place.
The cover does not touch water, it lets rain through but not leaves and debris. It is designed for security. Water evaporation can take place through these cover. Spa Covers Spa covers are same material as pool covers, but due to small size of spas creative designs and materials are coming up as spa covers. Spa covers retain water heat, keep out dirt, leaves and debris.