Sanitary Drainage, Traps, and Vents
Slope, size, and cleanouts
Gravity drainage depends on consistent slope: about 1/4 inch per foot for 2-1/2-inch and smaller pipe, 1/8 inch per foot for 3- and 4-inch, and 1/16 inch per foot for larger sizes. Drain size is set from fixture units and the fixture's own minimum branch size. Remember that smaller pipe needs more slope, not less - a classic answer-distractor. Cleanouts are required at the base of stacks, at the ends of horizontal runs, at changes of direction beyond the angle the code allows, and at intervals along long horizontal runs.
Traps
Each fixture gets its own trap, with a water seal of roughly 2 to 4 inches. Trap arms have a maximum developed length based on pipe size, and no trap may be double-trapped, no fixture double-trapped, and no trap relying on a mechanical device alone where the code requires a seal. Trap arms must be protected from siphonage and backpressure.
Venting
The vent system equalizes pressure so seals survive. Know the stack vent, vent stack, common vent, wet vent, circuit vent, and air admittance valve arrangements. Vent terminals extend above the roof by the required amount, and vent openings are kept away from windows, doors, and air intakes. Every trap needs a vent, unless it is specifically permitted to be unvented.
Worked example: a 3-inch drain run 40 feet at 1/8 inch per foot falls 5 inches - compute fall as slope × length, and convert units carefully before choosing an answer.
Trap: assuming the longest trap arm is always acceptable; arm length is tied to trap arm size.