Sanitary Drainage
Slope governs everything
Standard slopes: 1/4 inch per foot for 2-1/2 inch and smaller, 1/8 inch per foot for 3 through 6 inch, and 1/16 inch per foot for 8 inch and larger. Slope must be uniform - sags and backfall are violations even if average fall is correct.
Fittings and direction of flow
Flow direction dictates the fitting. Horizontal-to-horizontal changes of direction normally require a long-sweep fitting or a combination wye-and-eighth-bend; a sanitary tee is for a vertical-to-horizontal connection such as a fixture drain entering a vertical stack. Using a sanitary tee on its back for a horizontal change is a standard trap question, as is omitting a cleanout at the base of a stack.
Cleanouts, support, and testing
Provide cleanouts at the base of each stack, at intervals not exceeding 100 feet of horizontal run, and at required changes of direction; they must be accessible and the same size as the pipe they serve. Support horizontal piping at code intervals and provide proper bedding under buried pipe. Test drainage systems with a water test (typically a 10-foot head) or an approved air test before concealment.
Worked example: trap arm length
The trap arm may fall only about one pipe diameter before the vent takes over. For 1-1/2 inch pipe at 1/4 inch per foot: 1.5 ÷ 0.25 = 6 feet maximum. The same arithmetic gives about 8 feet for 2 inch, 12 feet for 3 inch, and 16 feet for 4 inch.