Drainage, Waste, and Vent Systems
This domain tests gravity drainage and venting: slope, sizing, traps, trap arms, vents, cleanouts, and testing. Most failures here come from measuring the wrong distance or venting a trap incorrectly.
Slope and sizing
Horizontal drainage slopes at 1/4 in per ft for 2-1/2 in and smaller, 1/8 in per ft for 3 in through 6 in, and 1/16 in per ft for 8 in and larger. Size drains and stacks from drainage fixture unit tables, and remember branch limits differ from stack limits.
Traps and trap arms
Each fixture trap holds a 2 in to 4 in seal. The trap arm - the pipe from the trap weir to its vent - is limited by trap size: 1-1/4 in = 2 ft 6 in, 1-1/2 in = 3 ft 6 in, 2 in = 5 ft, 3 in = 6 ft, 4 in = 10 ft. Every trap needs a vent that rises at least 6 in above the fixture's flood level rim before it offsets. S-traps, crown vents, and flat or back-graded pipe are prohibited.
Cleanouts and testing
Provide cleanouts at the base of each stack, at intervals not exceeding 100 ft in horizontal drains, and at aggregate direction changes over 135 degrees. Test the system before it is covered, using the water or air test the code specifies.
Worked example
A 2 in shower trap arm runs 4 ft horizontally, then 2 ft to the vent. Developed length from weir to vent is 6 ft, but the limit for a 2 in trap is 5 ft, so the vent must be relocated closer or a different layout used. Reasoning: measure from the trap weir, include fittings and vertical offsets, and compare against the limit for the trap size - upsizing only the arm does not extend the limit.
Traps
Measuring only horizontal pipe, ignoring fittings, undersizing vents, and accepting a wet vent beyond its allowed fixtures are the usual point-losers.