Drainage, Waste, and Vent Systems
The three questions
Every DWV question reduces to: how much flow, how much slope, and how is it vented? Flow is measured in drainage fixture units (DFU) - a private water closet is 4, a lavatory 1, a shower or floor drain 2, a kitchen sink 2, a washing machine standpipe about 2. Add them and read the size from the tables for the building drain, branch, and stack.
Slope and fall
Standard slopes are ¼ in. per foot for 2½ in. and smaller, ⅛ in. per foot for 3 in. through 6 in., and flatter still for larger mains. Compute fall as slope × length: 60 ft of 3 in. pipe at ⅛ in./ft gives 7½ in. of fall.
Venting logic
Every trap must protect its 2–4 in. trap seal. Traps must be level and vented; S-traps, double trapping, and crown venting are prohibited. A trap arm is limited in length so the trap weir stays above the vent fitting - the limit shrinks as pipe size shrinks and slope increases. Vent terminals must clear the roof and stay away from windows, doors, and air intakes. Wet venting, common vents, and air admittance valves are permitted only within the code's specific limits, so read the qualifying language rather than assuming.
Cleanouts and special cases
Require cleanouts at the base of stacks, at aggregate turns greater than 45°, at long horizontal runs, and at the building sewer. Watch for backwater valve requirements where fixtures sit below the sewer grade, sump/ejector rules for below-grade drainage, and prohibitions on running building sewers through or under footings without approved protection.
Rule of thumb
If a fixture gurgles on the exam, the diagnosis is almost always inadequate venting or an over-length trap arm, not undersized drain pipe.