Drainage, Waste, and Vent Systems
The Core Relationships
DWV design rests on three linked ideas: adequate slope, a trap for every fixture, and a vent that protects each trap seal. Slope must be uniform from the fixture to the stack or building drain. Every trap must be vented, and the trap arm must be short enough and sloped so the trap weir stays above the vent connection; the larger the trap arm, the longer it may be, but the limit is a code table, not a judgment call.
What Questions Target
- Venting methods: individual, common, wet, circuit, and island venting, plus where air admittance valves are permitted and where they are prohibited.
- Changes in direction: a horizontal drain turning to vertical, or a vertical stack turning horizontal, may need a long-sweep fitting or two 45s rather than a short-radius 90.
- Cleanouts: required at the base of stacks, at the building drain, at the upstream end of long runs, and at changes of direction beyond specified angles.
- Sizing by drainage fixture units and pipe capacity, and the prohibition on reducing pipe size in the direction of flow.
- Prohibited connections: downspouts and sump pumps to the sanitary system, and fixture drains to a vent.
Worked Example
A branch 40 feet long installed at 1/4 inch per foot drops 40 x 0.25 = 10 inches. If the invert at the fixture is at elevation 98.40 feet, the invert at the stack is 98.40 - 0.83 = 97.57 feet. Check that the resulting depth keeps the pipe below frost or above structure, and confirm the trap arm length is within the table for that trap size before committing.