Abstract: A waste sewage system for a building constructed to prevent sewage from backing up within the building. A valve is connected to an end of a clean-out conduit line of a waste sewage system for normally closing the same and openable in response to sewage effluent backing up within the clean-out conduit line. The valve comprises a tubular body portion having one end connectable to the end of a clean-out conduit line, a cover pivotally mounted on the body portion and normally closing the other end of the body portion, and sealing means carried by the body portion and cooperating with the cover to minimize escapement of noxious sewer gases.