Roof Cap with Built-In Damper for 3 in. or 4 in. Round Duct in BlackSelect the Broan 636 Roof Cap to provide reliable rooftop ventilation. Features a black enamel finish and is designed for use with 3 in. or 4 in. round ducts. Includes a built in back draft damper, and protective bird screen to prevent small pests from entering duct. Designed for easy installation Compatible with either a 3 in. or 4 in. round duct Manufactured of steel with baked black enamel finish for added durability and resilience Includes built