Drainage, Waste, and Vent Systems
The DWV system does three jobs: carry waste, keep sewer gas out of the building, and protect trap seals. Nearly every exam question traces back to one of those.
Drainage is sized by fixture units (f.u.), not gallons. A private bathroom group is roughly 5 to 6 f.u.; a water closet about 3 f.u. public and 2.5 to 3 private; a lavatory 1; a bathtub or shower 2; a kitchen sink or clothes washer 2. Slope is 1/4 in per foot for 2-1/2 in and smaller and 1/8 in per foot for 3 in and larger; steeper is legal, but the minimum is not negotiable under a slab. Use wyes and 45s for horizontal-to-vertical changes, never a santee on its back or on a horizontal drain, no reducing fittings in the direction of flow, no double trapping, no S-traps, and no crown venting.
Every trap needs a vent. The trap arm between trap weir and vent is limited by size (commonly 5 ft for 1-1/4 in, 6 ft for 1-1/2 in, 8 ft for 2 in) and must rise, not drop, to the vent. A vent must rise at least 6 in above the flood level rim before it offsets. Terminals generally finish at least 6 in above the roof (a foot or more under some codes), at least 10 ft horizontally from windows, doors, and air intakes, and 2 ft above anything within 10 ft.
Worked example: a 4 in building drain runs 60 ft at 1/8 in per foot. Fall = 60 x 0.125 = 7.5 in, so the downstream invert is 7-1/2 in lower than the upstream invert.