General Regulations and Drainage System Design
General regulations
Plumbing must be installed without weakening structural members, protected from freezing, and separated from electrical hazards. Piping must be supported and aligned; changes in direction use fittings, not bent pipe. Backwater valves are required where the building drain is below the nearest upstream manhole or where the code official determines backflow is possible. Cleanouts are required at the base of stacks, at the building drain, at long horizontal runs, and at specified direction changes.
Drainage design
Drainage flows by gravity. The code prescribes minimum slope by pipe size: 1/4 inch per foot for 2 1/2 inches and smaller, 1/8 inch per foot for 3- through 6-inch pipe, and 1/16 inch per foot for 8-inch and larger. Fixture units from the drainage table are added to find the drainage load; the building drain and sewer are then sized from the DFU tables. Horizontal branches must be sized for the fixtures served, and the drain must remain accessible for cleaning.
Worked example
A 60-foot run of 4-inch building drain at 1/8 inch per foot falls 60 x 0.125 = 7.5 inches. If the design instead calls for 1/16 inch per foot on that 4-inch line, it does not meet the minimum slope for that size, even though the fall would be 3.75 inches.
Traps
Using 1/16 inch per foot on 3- or 4-inch pipe is a common error. Another is adding DFU values without checking whether the fixture is private or public, or ignoring the increase for continuous flow. Do not bury a cleanout or install a horizontal drain with a backfall.