Vents and Venting Systems
Purpose and types
A vent equalizes pressure in the drainage system so trap seals survive. Know these types: individual, common, wet, circuit, stack, relief, and air admittance valves (where allowed). The stack vent is the extension of the soil stack above the highest branch; the vent stack is a separate vertical vent. Every trap needs a vent unless it is specifically exempted by a special venting arrangement.
Trap arm distance
The trap arm length - from the trap weir to the vent connection - is limited by pipe diameter and slope. Typical table values: 1-1/4 in = 5 ft, 1-1/2 in = 6 ft, 2 in = 8 ft, 3 in = 12 ft, 4 in = 16 ft. Always check the adopted code table, since slope changes these values.
Termination
Vents extend above the roof, clear of windows, doors, and air intakes - commonly 10 ft horizontally or 2 ft above an opening. Consider frost closure in cold climates (increase size before penetrating the roof).
Common traps
- Venting a trap with a vent that is downstream of another fixture's drain (wet venting without meeting the rules).
- Forgetting that vent size cannot be reduced in the direction of flow.
- Running a vent horizontally below the flood-level rim of the fixture it serves.
- Using an AAV where the code requires a true vent to atmosphere.
- Terminating a vent inside a building or below a window.
Rule of thumb
If the trap arm is too long for its diameter, the trap will siphon - shorten the arm or upsize the pipe.
Worked example: A 2 in trap arm with a 10 ft developed length exceeds the 8 ft limit and must be shortened or the vent relocated closer to the trap.