The Broan-NuTone® 634 Steel Roof Cap is an easy-to-install vent for exhaust fans (range hoods, bathroom, rec room, home gym, etc.). This cap provides reliable roof top ventilation and features both a back draft damper and bird screen, preventing outside air from entering the home and birds or small pests from entering duct work. Compatible with up to 8" ducting and 3-1/4" x 10" ducting. Not suitable for dryer venting.