Drainage, Waste, and Vent Systems: Slope, Traps, and Air
The drain relies on gravity and air
Every drain must slope uniformly toward the building drain: commonly 1/4 in. per foot for 2 1/2 in. and smaller, with 1/8 in. per foot permitted for larger pipe. Every fixture needs a trap with a 2 in. minimum and 4 in. maximum seal, and the trap must be level and self-cleaning. Keep the trap weir close to the fixture - the fixture outlet to trap weir distance is limited (commonly 24 in.) - and give the trap arm a vent connection before the arm drops below the trap weir. If the vent opening sits lower than the weir, the trap siphons. That single principle explains most trap-arm length limits.
Vents
A vent protects the seal by admitting air. Vents must rise vertically (or at no more than a 45-degree offset in many codes) to at least 6 in. above the fixture's flood level rim before running horizontally, and the vent terminal must extend above the roof and away from operable openings. Vent sizing is by fixture-unit tables, and a drain may not be used as a vent nor a vent as a drain. Wet venting and combination waste-and-vent arrangements are allowed only within their specific limits. Studor (air admittance) valves are permitted only where the code allows, must be accessible, and never replace a required vent to open air.
Traps in exam questions
Double-trapping, S-traps, crown venting, and a vent that goes flat are all prohibited because they destroy the seal. Cleanouts are required at the base of stacks and at changes of direction and long runs.
Worked example
A 2 in. lavatory trap arm at 1/4 in./ft can travel about 8 ft if that length keeps the vent above the trap weir; at 1 1/2 in. the practical limit drops to roughly 6 ft. When in doubt, shorten the arm rather than enlarge the vent.