Sanitary Drainage
Sanitary Drainage. This domain tests gravity drainage: slope, fixture units, pipe sizing, fittings, cleanouts, changes of direction, and protection from backflow. Expect questions on drainage fixture units (DFU), building drains, building sewers, traps, and the difference between drainage and venting rules.
Core rules
- Drainage must flow by gravity at code slope. Common rules: 2½ in and smaller at ¼ in per ft, 3 in to 6 in at ⅛ in per ft, and larger sizes at a lesser approved slope. Always verify the adopted table.
- Size drains by DFU and the code tables, not by the number of fixtures alone. A fixture's trap size, branch length, and drainage load all matter.
- Use approved drainage fittings. Changes of direction must use long-radius fittings or wyes as required; sanitary tees are limited on horizontal drains.
- Provide cleanouts at required locations, including the building sewer, bases of stacks, and changes of direction. Protect fixtures subject to backflow with an approved backwater valve where required.
Worked example
A 3 in building drain runs 30 ft at ⅛ in per ft. Total fall is 30 × 0.125 = 3.75 in. If the upstream invert is 24 in below the floor, the downstream invert is 24 − 3.75 = 20.25 in. The reasoning: slope is a rate per foot, so multiply before subtracting. Check that the resulting depth still gives required cover and grade.
Common traps
Do not use ¼ in per ft for every pipe size. Do not assume a larger drain always fixes a backflow problem. Do not confuse DFU with fixture count. Do not bury a cleanout.