Drain, Waste, and Vent Systems
The core of the trade
DWV questions reward understanding the why: every rule exists to keep the trap seal, 2 to 4 inches of water, intact.
Slope and fittings. Horizontal drainage runs at 1/4 in per foot for 2 in and smaller, and 1/8 in per foot for 3 in and larger. Use wyes with long-sweep or 1/8 bends. Never install a sanitary tee on its back, a double sanitary tee for back-to-back water closets, an S-trap, a double trap, or a crown vent.
Trap arms. Each fixture gets its own trap, and the distance from trap weir to vent is limited by trap size, roughly 5 ft for 1-1/4 in, 6 ft for 1-1/2 in, 8 ft for 2 in, 12 ft for 3 in, and 16 ft for 4 in in the table. The arm is measured to the vent fitting, not the stack, and it may fall no more than one pipe diameter before the vent.
Vents. Size the vent by the fixture drain it serves. A stack vent runs full size through the roof and terminates at least 6 in above the roof surface, clear of windows and air intakes per the code table. Air admittance valves are permitted only where the adopted code allows and must remain accessible.
Drains and cleanouts. Cleanouts at the base of each stack, at the building drain, at each aggregate 135 degrees of horizontal change, and every 100 ft of horizontal run. Backwater valves protect fixtures below the next upstream manhole.
Worked example. A 2-in lavatory drain runs at 1/4 in per ft with a maximum trap arm of 8 ft and a vent of at least 1-1/4 in. If the fixture sits 10 ft from the stack, that arm is too long; add a revent or relocate the fixture. That is the classic exam trap.