Vents and Venting
Purpose first
The vent system exists to protect trap seals against siphonage and back pressure, and to let sewer gas escape above the roof. Any venting question can usually be answered by asking which trap seal is being protected and whether the vent can be blocked by waste flow.
The vocabulary
Know individual vents, common vents, wet vents, circuit vents, stack vents (the extension of a soil or waste stack through the roof), vent stacks, relief vents, yoke vents, and island/loop vents. An air admittance valve (AAV) is permitted only where the adopted code allows, must be accessible, and cannot substitute for every vent type.
Rules that show up in questions
A vent must rise vertically at least 6 inches above the flood-level rim of the highest served fixture before offsetting horizontally. Vents cannot drain or serve as waste piping. A vent passing through the roof terminates well above the roof surface (commonly at least 6 inches; some jurisdictions require 12 inches - check local amendments). Wet venting is limited, generally to a bathroom group on a properly sized wet vent. Circuit venting requires a relief vent and limits the number of fixtures. Vent size is based on fixture units and developed length, and a vent is never smaller than roughly half the drain it serves, with 1-1/4 inch as the practical minimum.
Worked example
A 3-inch fixture drain runs 10 feet to a stack. The installer proposes a 1-1/2 inch individual vent at the trap and 12 feet of horizontal offset before rising. Two problems: the trap arm is beyond its 12-foot limit only if slope is inadequate - verify with diameter ÷ slope - and the vent must first rise 6 inches above the fixture's flood-level rim before it offsets.