drainage stack instead of having separate drainage and vent stacks The Philadelphia system successfully utilizes “S” traps for fixture traps located above the floor and within a specified distance from the stack. However, because the IPC prohibits the use of S traps, the Single Stack Vent System in the IPC utilizes an oversized vertical pipe from its connection at the end of the fixture trap’s fixture drain (also known by some tradesmen as a ‘trap arm’) to the horizontal drainage piping of the system. This is the same concept utilized in combination waste and vent systems for fixtures that are located above the floor. The oversized vertical pipe eliminates any possibility of siphoning of a fixture trap as the waste volume entering the vertical pipe could not possibly block the venting air space in the vertical pipe. In a Single Stack Vent System the drainage stack serves as both a single-stack drainage and vent system. The drainage stack and branch piping are considered as vents for the drainage system as a whole. Pipe sizing in a single stack drainage system is larger than in a conventional one, however a significant cost saving is achieved by the reduction of the vent piping needed. This venting system serves as a viable alternative to the more traditional systems that are being used. The length of trap arms is limited and the vertical drop from the fixtures is oversized also. Fixture connections that do not meet the requirement for a single stack system must be conventionally vented. The length of the trap arm is limited to reduce any suction buildup, and the stack is oversized to limit the internal air pressure and vacuum build up. Stacks greater than two branch intervals in height are prohibited from receiving the discharge of horizontal branches on the lower two floors. The separate stack serving the lower two floors, is required to connect to the building drain at a distance of not less than10 pipe diameters downstream from the base of the connection of the single stack vented system. This proven method has been laboratory tested to determine sizing and installation requirements that provide proper venting to the drainage system. The size of a single stack drainage stack is based upon the height and the total number of dfu connected to the stack. Table 917.2provides the stack dfu limitations according to stack height. The height of the stack is from the connection to the building drain to the uppermost drainage connection to the stack. Drainage stacks must be the same size from the base to the termination of the stack to the outdoors. The requirement for the stack vent to be the same size as the drainage stack allows the stack vent to be of any length on its path to its termination point outdoors.