Sanitary drainage, venting, and traps
Drainage principles
Sanitary drainage relies on gravity, slope, and venting. Typical slope is 1/4 in./ft for 2 1/2 in. and smaller pipe, 1/8 in./ft for 3 in. through 6 in. pipe, and 1/16 in./ft for larger pipe, subject to the adopted code. Drainage fixture units are summed by branch, stack, and building drain. Changes of direction require approved fittings; cleanouts must remain accessible. Trap seals are normally 2 to 4 in. deep. Each fixture needs a trap, protected by a vent sized and located within the allowed trap-arm distance. Vents terminate outdoors, clear of openings and air intakes, and must not be obstructed by frost closure.
Venting and traps
A vent protects the trap seal from siphonage and backpressure. Common venting, wet venting, and air admittance valves are permitted only where the code allows. Prohibited traps include S-traps, drum traps, bell traps, and traps with moving parts. A trap arm must be long enough to connect but short enough that the vent protects the seal. Floor drains need trap primers where evaporation would break the seal.
Worked example
A 3 in. branch runs 40 ft at 1/8 in./ft. Total fall = 40 x 0.125 = 5 in. If the upstream invert is 10 ft 0 in. (120 in.) above datum, the downstream invert is 115 in., or 9 ft 7 in. That fall must be checked against the connection elevation and the required cover. A 2 in. branch at 1/4 in./ft over the same length would fall 10 in., which may not fit.
Traps
Using 1/4 in./ft for a 4 in. drain when the code allows 1/8; flat or backfalling vent connections; missing cleanouts; S-traps; unvented floor drains; AAVs in inaccessible or return-air spaces.