Section 904.1/Vent Pipe Sizing
UPC 904 covers vent pipe sizing based on drain size, fixture units connected, developed length, and vent type.
Vent pipes must be sized using UPC Table 7-5 (or the applicable sizing table) based on the size of the drain served, the total drainage fixture units connected, and the developed length of the vent pipe. The minimum vent size is generally half the diameter of the drain served, but not less than 1.25 inches. Individual vents for a single fixture are sized based on the fixture drain size and the developed length of the vent. Common vents and vent stacks serving multiple fixtures are sized for the total fixture units and the total developed length from the lowest connection to the vent terminal.
Why this section exists
Vent pipes protect trap seals by equalizing pressure in the drainage system. When water flows through a drain, it creates positive and negative pressure waves that can siphon or blow out trap seals, allowing sewer gas to enter the building. An undersized vent cannot supply enough air to equalize the pressure, causing trap seal loss. The sizing tables are derived from pneumatic flow calculations that ensure the vent can supply air at the rate needed for the connected drainage load.
What plan reviewers look for
Plan reviewers check the plumbing riser diagram for vent sizes at each segment. They verify the vent size against the sizing table for the drain size, fixture units, and developed length. They check that the minimum vent size (1.25 inches) is maintained. For vent stacks, they verify the total fixture units and the developed length from the lowest connection to the roof terminal.