Drainage, Waste, and Venting
Domain overview
Drainage, Waste, and Venting covers the DWV system: fixture drains, building drains and sewers, stacks, traps, vents, cleanouts, and testing. The exam tests slope, sizing, venting, and the reasons behind the rules.
Core rules
- Horizontal drainage pipe must slope at least 1/4 inch per foot for 2-1/2 inch and smaller, and at least 1/8 inch per foot for 3 inch and larger, unless the adopted code says otherwise.
- Each trap needs a vent, and the vent must rise vertically or at an approved angle before it can run horizontally above the fixture flood level.
- Cleanouts are required at the base of stacks, at changes of direction, and at the building sewer, and they must remain accessible.
- Vent terminals must extend above the roof and be located away from windows, doors, and air intakes; in cold climates, protect vents from frost closure.
- Drainage and vent pipe must be supported and tested; concealed DWV work needs inspection before cover.
Worked example
A 3 inch building drain runs 20 feet to the stack. At the minimum slope of 1/8 inch per foot, the fall is 20 x 0.125 = 2.5 inches. If the installer uses 1/4 inch per foot, the fall is 5 inches, which is not a code failure by itself but may create a deeper trench and extra fittings. The key check is that the pipe never runs flat or backward. A 2 inch drain on the same 20 foot run needs at least 5 inches of fall because its minimum is 1/4 inch per foot.
Traps
Using 1/8 inch per foot on 2 inch pipe is a failure. A flat vent, a vent below the fixture flood level, or a missing cleanout are frequent inspection rejections. Do not assume a roof vent is acceptable next to a skylight that opens.