The Broan-NuTone® 636 Steel Roof Cap is an easy-to-install vent for exhaust fans (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 3" and 4" ducting. Not suitable for dryer venting.