Vents and Venting Systems
Purpose first
A vent does two jobs: it lets air into the drainage system so flow doesn't siphon trap seals, and it lets sewer gas escape above the roof. Every trap must be vented, and the vent must connect so that it cannot become a drain.
Vocabulary that gets tested
Individual vent, common vent, wet vent (a drain serving as a vent for another fixture), circuit vent, relief vent, yoke vent, stack vent (top of a soil stack), vent stack (parallel vent for a drainage stack), and air admittance valve (one-way, must be accessible and installed per listing).
Rules of thumb that appear in questions
- A vent must rise vertically (typically at least 6 in) above the fixture's flood-level rim before any horizontal offset.
- Vents slope back toward the drain, not away from it.
- Vent terminals are commonly required to extend at least 6 in above the roof and to be kept a specified distance from windows, doors, and property lines (10 ft is a common figure - confirm in the adopted code).
- In cold climates, increase the vent through the roof to resist frost closure.
- Vent size follows the drainage load and developed length; oversizing is not a substitute for proper routing.
Worked example
A lavatory is wet-venting a water closet. Check the wet-vent rules: the WC must be the downstream fixture, the wet-vented section must be vertical or horizontal with a limit on length, and the total fixture units on the wet vent are capped. If any of those fail, run a separate vent.
Traps
Connecting a vent below the trap weir; using a drain as a vent beyond wet-vent limits; venting a trap with an unlisted AAV in an inaccessible location; and forgetting that an offset vent still has to rise above the flood-level rim first.