Stack Air Admittance Valves for Sanitary Drainage Systems (herein referred to as "device") are devices used as vent terminals for stacks in plumbing drainage systems. These devices shall not be used to relieve back pressure, but only to allow air to enter the system. When these devices are installed in a building, there shall be at least one (1) open vent terminal which extends to atmosphere outside of the building serving the building drain on which these devices are installed. These devices are designed to be installed on stacks where branches on multiple floors are connected. These devices consist of a one-way valve designed to allow air to enter the plumbing drainage system when a pressure less than atmospheric develops. The device closes and seals by gravity under zero (0) differential pressure (static or no flow condition) and under positive pressure. These devices prevent sewer gases from entering a building. The device consists of a hooded or shielded body which contains a movable guided diaphragm which seats and seals air flow when closed and allows air to enter when open.