Sanitary Drainage, Waste, and Venting
Slope carries the solids
Gravity drainage depends on self-scouring velocity: 1/4 inch per foot for pipe 2-1/2 inches and smaller (commonly quoted through 3 inch) and 1/8 inch per foot for 4 inch and larger. Steeper is not automatically better on long runs. Drainage fixture units (DFU) and trap sizes come from tables; the building drain runs to the building sewer at the foundation.
Every trap needs a vent
Except for listed exceptions, each fixture needs a trap, and each trap needs a vent. The trap arm between the trap weir and the vent fitting is limited by pipe size - about 5 feet for 1-1/4 inch, 6 feet for 1-1/2 inch, 8 feet for 2 inch, 12 feet for 3 inch, 16 feet for 4 inch. Exceed it and the trap siphons; that is an S-trap. Vent pipe rises vertically or at no more than 45 degrees from vertical until it clears the flood level rim of the highest served fixture. Vent terminals sit at least 6 inches above the roof and away from openings and air intakes. Wet venting, circuit venting, and air admittance valves all carry fixture-count and configuration limits.
Cleanouts
Provide cleanouts at the base of stacks, at the building sewer, and at changes of direction greater than 45 degrees; they must stay accessible.
Traps
Sanitary tees used on their back on a horizontal drain, double sanitary tees on horizontals, crown venting, and piping a vent into a drain below a fixture.
Worked example
A 2-inch lavatory drain with a 2-inch trap arm can run 8 feet to the vent. At 10 feet, upsize the trap arm or add a vent - do not simply stretch the arm.