The Round Back Roof Jack is used for ventilating bathroom fans or other duct applications. In addition, the round design is aerodynamic to withstand wind speeds up to 200 mph. It allows for maximum air to flow while alleviating back pressure. The Round Back Roof Jack is engineered to stop insects and wildlife from entering the duct. It is also designed to prevent rain or snow from entering. These vents have an all-aluminum construction. To clarify, this means they are durable, lightweight, and will not rust. The Roof Jack's tail pipe is approximately 2 inches long and is corrugated at the bottom to fit existing duct work. This allows for a quick and easy installation. The RBV-3-C4-TP is the Round Back Roof Jack with a 3 inch inner diameter and 4 inch tall collar. In addition, it also has a 2 inch tailpipe that can be used for duct ventilation. It is mainly used to ventilating bathroom fans and range hoods. Color: Mill.