Sanitary Drainage, Venting, and Storm Drainage
Drainage Principles
Sanitary drainage relies on gravity, proper slope, and adequate venting. Horizontal branches slope toward the stack (commonly 1/4 inch per foot for small pipe). Questions test slope, fixture unit loading, and the prohibition on horizontal runs that are flat or reverse-sloped.
Venting
Every trap needs a vent to protect its seal. Vent types include individual, common, wet, and circuit vents. Traps in questions include an unvented trap, a vent that is too far from the trap, and a wet vent used where not permitted. Vent terminals must be located away from windows and air intakes.
Storm Drainage
Storm drainage handles rainwater and must be separate from sanitary drainage unless combined systems are approved. Questions test roof drain sizing, leader sizing, and the prohibition on connecting storm drainage to the sanitary system where separate systems are required.
Rule of Thumb
If a fixture gurgles or drains slowly, suspect venting before suspecting a clog. If a question describes cross-connection between storm and sanitary, the answer is separation.
Worked Example
A lavatory drains slowly and gurgles. The trap arm is within slope limits but the vent is missing. Adding a proper vent restores the trap seal and the gurgling stops.