Plumbing: Fixture Units, Traps, and Venting
Drainage sizing starts with fixture units
Drainage pipe is sized by drainage fixture units (DFUs), not by pipe diameter guesswork. Each fixture type has an assigned DFU value, and those values are summed per branch and per stack. A trap in exam questions is mixing supply fixture units with drainage fixture units; they are different scales and are never interchangeable. Also watch for fixture-unit reductions when a group of fixtures is unlikely to be used simultaneously, but only apply reductions where the code explicitly permits them.
Traps and venting protect the water seal
Every fixture except a few listed exceptions needs a trap, and every trap needs protection from siphonage and back pressure. Understand the difference between a vent stack, a stack vent, a branch vent, and an individual vent. The critical concept is the trap seal: it must remain intact, so the vent must connect within the prescribed distance from the trap and rise above the fixture's flood level rim before turning horizontally. Questions often describe a vent that runs horizontally below the fixture, which is a violation.
Slope, size, and cleanouts
Gravity drainage needs slope. Know typical minimum slopes for small versus large pipe and why slope decreases as diameter increases. Cleanouts must be placed at changes of direction and at the base of stacks, and must remain accessible.
Worked rule of thumb
Sum DFUs for a branch: a water closet at 4 DFUs, a lavatory at 1, and a floor drain at 2 gives 7 DFUs. Use the table to find the minimum pipe size for that total, then verify the slope. Do not size from the largest single fixture; size from the total.