Sanitary Drainage, Vents, and Venting Systems
Sanitary drainage and stack design
Soil and waste stacks carry discharge vertically; horizontal branches connect fixtures to stacks. Stack sizing depends on fixture units and the number of branch intervals. A stack must be vented, and the vent must extend outdoors above the roof. Drainage fittings must direct flow smoothly; sanitary tees are used on vertical-to-horizontal changes, while wyes and combination fittings are used on horizontal-to-horizontal changes. Horizontal drains need cleanouts at required intervals and at the base of stacks.
Venting systems
Every trap needs a vent. Common vent types include individual, common, wet, circuit, island, and stack vents. A wet vent may serve a bathroom group when sized and limited by the code; it cannot serve fixtures from different floors in the same way. An air admittance valve is allowed only where the code permits it and must be accessible. Vent terminals must be outside, above the roof, and kept away from windows, doors, and air intakes.
Worked example
A bathroom group has a water closet, lavatory, and tub. A 2-inch wet vent serving the group may be permitted if the fixture units and length limits are met, but adding a kitchen sink on the same wet vent is not allowed. The sink needs its own vent or a properly designed circuit vent.
Traps
Do not use a stack as a vent without a vent connection. Do not terminate a vent in an attic or crawl space. Do not assume an AAV can replace a required roof vent. Check that the vent is at least two pipe diameters from the trap weir and that the trap arm is within its length limit.