Sanitary Drainage and Venting
Sanitary Drainage and Venting
This domain tests gravity drainage, fixture-unit sizing, slope, venting, cleanouts, and testing. The goal is to carry waste without clogging, protect trap seals from siphonage, and allow air to move through the system.
Core rules
- Horizontal drainage pipe must slope uniformly toward the drain. Common code slopes are 1/4 inch per foot for smaller pipe and 1/8 inch per foot for larger pipe, but confirm the adopted table for the size used.
- Drainage fixture units (DFU) are added by branch and stack. Use the adopted table to select pipe size; do not size by the fixture outlet alone.
- Every trap must be vented, and the trap arm length from trap to vent is limited. A vent must rise vertically or at approved slope and terminate outdoors, clear of openings.
- Cleanouts are required at the building drain, at changes of direction, and at long runs. Test drainage systems with water or air before concealment.
Worked example
A 2-inch branch runs 18 feet horizontally to a stack. The required slope is 1/4 inch per foot, so fall is 18 x 0.25 = 4.5 inches. If the upstream invert must be at elevation 100.00 feet, the downstream invert is 100.00 - 0.375 feet = 99.625 feet. Check that the trap arm length to its vent is within the table limit; if not, move the vent closer.
Traps
Do not install flat or back-sloped pipe. Do not exceed trap-arm length or rely on an unvented trap. Do not connect a vent to a drain in a way that creates a wet vent where prohibited. Do not omit cleanouts at direction changes.