Semi-circular flaps (butterfly damper) cover the exhaust stack when fan is off. When fan is on, flaps are forced out of the way to provide a clear path for air movement. The built-in gutter system is designed to prevent rain and snow from entering the stack. Made from galvanized Spiral pipe for strength and durability; available in most sizes. Weather hoods are pre-assembled for immediate installation. Available with or without Vanstone flange. Click here for basic methods of connecting.