What must be the configuration of a stack vent when extended above the first floor to receive plumbing fixture discharges?

The configuration of a stack vent when extended above the first floor must be full size in order to effectively receive the discharges from plumbing fixtures. This requirement ensures that the stack vent maintains adequate ventilation for the drainage system, allowing for proper air flow and preventing the potential for siphoning or trapping of wastewater gases. An adequately sized stack vent above the first floor prevents pressure fluctuations that can occur when multiple fixtures are discharged simultaneously, helping to protect against backups and ensuring the system can adequately vent without creating negative pressure.

Other sizes, such as two-inch or three-inch configurations, may be insufficient for the volume of air and gases that need to be vented, leading to inefficiencies in the plumbing system. Additionally, reducing the size of the vent to half the original size would compromise the system's ability to function properly, risking issues such as the introduction of odors or harmful gases into living spaces. Therefore, maintaining the vent at full size is essential for compliance with plumbing codes and for the overall effectiveness of the plumbing system.