Vents and Venting
Why vents exist
A vent equalizes pressure so trap seals are not siphoned. Every trap must be protected by a vent sized and installed per the tables. The most-tested rule: a vent must rise vertically from the fixture drain to at least 6 inches above the flood level rim of the fixture before any horizontal offset. A vent that drops below that line can fill with waste and become a drain.
The vocabulary
Individual vent - one trap, one vent. Common vent - two traps on the same level connected at a fitting. Wet vent - a drain that also serves as a vent for another fixture, permitted only in the arrangements the code describes (typically one or two bathroom groups). Stack vent - the extension of a soil or waste stack above the highest fixture. Vent stack - a separate vertical vent serving a stack. Relief vent - added to tall stacks with many branch intervals. Circuit vent - vents a row of fixtures on one horizontal branch. Air admittance valve - allowed only where listed and where the code official and local amendments permit it.
Termination and sizing
Vents terminate outdoors, above the roof, at the code minimum height plus enough to clear expected snow in Michigan; a vent may not terminate under a window or within the code distance of an air intake. Vent size is set by dfu and developed length in the tables; the minimum is 1¼ inch, and no vent may be smaller than required for the drain it serves.
Rule of thumb
For quick checks, a vent is rarely smaller than one-half the size of the drain it serves and never less than 1¼ inches. Confirm every answer against the vent tables.