Drainage, Waste, and Vent Systems
Slope, sizing, and fixture units
Drainage pipe is sized in drainage fixture units (DFU) from the code tables, not by guesswork. Slope rules are a classic exam item: 2½ inch and smaller pipe at ¼ inch per foot, 3 inch through 6 inch at ⅛ inch per foot, and 8 inch and larger at 1/16 inch per foot. A building drain carries waste to the building sewer; the building sewer runs to the public sewer or private disposal system.
Traps and venting
Every fixture except those listed as exempt needs a trap with a 2- to 4-inch water seal, and each trap must be vented within the allowable trap-arm distance - about 8 feet for a 2-inch trap arm at ¼ inch per foot. Know individual, common, wet, circuit, and stack venting, plus air admittance valves where permitted. A vent stack and stack vent serve different functions. Vent terminals must clear roofs and openings per code.
Cleanouts and special wastes
Cleanouts are required at the base of each stack, at the end of each building drain, at changes of direction greater than 45 degrees, and at intervals not exceeding 100 feet on horizontal runs. Grease interceptors serve food service; acid neutralization serves labs; backwater valves protect fixtures below the level of the next upstream manhole; indirect waste receptors and air gaps separate potable from contaminated drainage.
Traps in exam questions
Wrong-slope answers, trap arms exceeded in length, missing cleanouts, and vents terminated in attics or too close to operable windows. Worked example: a horizontal 4-inch building drain run of 90 feet falls 90 × ⅛ inch = 11.25 inches, and it still needs cleanouts at the ends and at any turn greater than 45 degrees.