Sanitary Drainage, Vents, and Storm Drainage
Drainage
Sanitary drainage moves by gravity. Horizontal pipe is graded at a quarter inch per foot for 2.5-inch and smaller, and an eighth inch per foot for 3-inch and larger. Stacks run vertically and are supported at the base and at each floor. Horizontal-to-vertical changes use a wye or a combination wye and 45; a quarter bend laid on its side is not drainage fittings. No drainage line may reduce in size in the direction of flow. Cleanouts go at the base of stacks, at the building drain, and at the intervals and direction changes the code sets.
Vents
Every trap needs protection. A vent must rise vertically at least 6 inches above the flood level rim of the highest fixture it serves before being offset, and the trap arm between trap weir and vent connection is limited by pipe size; longer arms must be re-vented. Crown venting is prohibited. Wet venting and air admittance valves are permitted only within the code restrictions.
Storm drainage
Rain leaders and roof drains carry storm water in a system separate from sanitary drainage. Size from roof area and local rainfall intensity: gallons per minute equals roof area in square feet times intensity in inches per hour, divided by 96.
Worked example
A contractor proposes a 3-inch building drain, 60 feet long, at a quarter inch per foot. The drop would be 15 inches, but 3-inch pipe requires only an eighth inch per foot, so the permitted drop is 7.5 inches. The steeper grade is legal but drops the invert unnecessarily and can complicate the sewer connection.
Traps that cost marks
Grading 3-inch and larger pipe at a quarter inch, taking a vent within two pipe diameters of the trap weir, and connecting storm water to the sanitary system.