Sanitary drainage and venting systems
Domain 6 - Sanitary drainage and venting systems
This domain is the core of the trade: slope, fixture units, pipe sizing, traps and trap arms, venting methods, and cleanouts. Expect questions where one small change - an added fixture, one extra foot of trap arm - flips the answer.
Slope and sizing
- Horizontal drainage slope: ¼ in./ft for 2½ in. pipe and smaller; ⅛ in./ft permitted for 3 in. and larger. Use the slope the table assumes: a 4 in. building drain carries 216 fixture units at ⅛ in./ft and 250 at ¼ in./ft.
- Fixture units are assigned per fixture type; convert the total to pipe size using the building drain and stack tables, and check the number of branches.
- Direction changes: horizontal to vertical uses a wye or combination; vertical to horizontal uses a long-turn or combination; sanitary tees serve vertical-to-horizontal stack connections and vent takeoffs.
Traps and venting
- Each fixture gets its own trap with a 2–4 in. seal. Double trapping, S-traps, bell traps, and crown vents are prohibited.
- Trap arm length is limited by size and slope - roughly 5 ft for 1¼ in., 6 ft for 1½ in., 8 ft for 2 in., 12 ft for 3 in. at ¼ in./ft. Beyond that the fixture is unvented.
- Vents: individual, common, wet, and stack venting each carry conditions. A vent terminal extends at least 6 in. above the roof (and above expected snow) and is kept away from windows and air intakes.
- Cleanouts are required at the base of each stack, at the building drain, at direction changes greater than 45°, and at intervals not exceeding 100 ft, sized to the pipe.
Worked example
A shower drain is 11 ft developed from the stack, on 2 in. pipe at ¼ in./ft. Reasoning: the allowable trap arm for 2 in. at ¼ in./ft is 8 ft, so the fixture is not vented and the installation fails. Fixes include relocating the vent connection closer with a re-vent to a nearby vent, using a circuit vent, or, where the code allows, an air-admittance valve inside the wall. Do not simply upsize the pipe and assume the distance doubles; verify the trap-arm table for the size and slope actually installed.
Where marks are lost
- Reading the fixture-unit table at ¼ in./ft while installing ⅛ in./ft on a 3 in. line.
- Measuring the trap arm to the vent stack rather than to the actual vent connection.
- Sizing a vent without checking the minimum ratio to the drain it serves.