Vents and Venting Systems
Why vents exist
A trap seal lasts only if the pressure on both sides stays equal. Venting admits air behind the flow, prevents siphonage and back pressure, and lets sewer gas escape above the roof. Every trap needs a vented trap arm, and the vent must rise vertically (or at no more than 45 degrees from vertical) until it is above the flood level rim of the highest fixture served.
Vent types to recognize
Individual vent, common vent, wet vent, circuit vent, stack vent, relief vent, yoke vent, island (loop) vent, and air admittance valves where the AHJ permits them. Wet venting is limited: only certain fixture combinations (typically a bathroom group) may be wet vented, and the wet-vented section must be sized as both drain and vent. Air admittance valves must be accessible, installed per listing, and are never a substitute for the required vent at a sump or in a location without air.
Distances, terminations, and sizing
- Trap arm length is limited by pipe diameter; a crown vent (vent taken before the trap arm has any length) is prohibited.
- Vent terminal: commonly at least 6 inches above the roof, increased where snow is expected, and at a horizontal distance from or above doors, windows, and air intakes.
- Vents are sized from DFU and developed length; an undersized or overlong vent acts like no vent at all.
Traps the exam sets: wet venting a kitchen sink through a bathroom group, using a flat or horizontal dry vent, forgetting that a vent must be increased when the developed length exceeds the table, and running a vent terminal under a window.
Rule of thumb: if a question shows a trap with no vent or a vent that drops before it rises, the answer is almost always "add or correct the vent."