Sanitary Fixtures and Drainage
Core concepts
Know the standard fixtures and their drain and trap sizes: water closet, lavatory, bathtub, shower, floor drain, sink, urinal, and washing machine standpipe. Understand fixture units, trap arm length limits, vent distance from the trap, and the requirement that a trap arm connects to a vent before it drops more than one pipe diameter. Understand backflow protection: air gaps, vacuum breakers, and backwater valves for fixtures below the sewer invert.
Traps in questions
Watch for questions where a fixture is installed without a trap, or where a trap is double-trapped or double-trapped in series - both are prohibited. Another trap is confusing a floor drain trap primer with a trap itself. Expect items on the water closet's integral trap, the required water seal depth, and the need for a backwater valve on fixtures below the flood level rim of the lowest upstream manhole. Also be ready for questions on accessibility clearances and fixture spacing.
Worked example
Rule of thumb: the water seal of a trap should be at least 2 inches and no more than 4 inches deep. For a lavatory with a 1-1/4 inch trap arm, keep the trap arm short and vent it within the code distance so the seal is not siphoned; if the fixture is below sewer level, fit a backwater valve.