The Broan Roof Vent Cap is a high capacity roof cap for ventilation products with CFM levels up to 500 CFM. This roof cap is made out of a quality steel baked enamel finish and features a built-in backdraft damper and bird screen. Broan Roof Cap comes in two different sizes, the large size fits up to 8in. round ducts while the small size fits up to 3-4in. ducts. Small size compatible with 3in. or 4in. round ductsLarge size compatible with 6in. round ducts