Drainage Systems: Sizing, Slope, and Layout
Gravity drainage fundamentals
Drainage systems carry waste by gravity. Pipes must slope uniformly toward the point of discharge, and the slope depends on pipe size. Smaller pipes need steeper slopes to maintain velocity. Drainage fittings must be of the drainage type, with smooth interior surfaces and directional flow. Sanitary tees and wyes are used for drainage; vent fittings are not interchangeable with drainage fittings.
Sizing and fixture units
Drainage sizing is based on fixture units, which reflect the load a fixture imposes. The building drain, building sewer, and branches are sized from tables using the total fixture units and the slope. Candidates must know that a larger pipe at a given slope carries more fixture units, and that reducing pipe size in the direction of flow is generally prohibited.
Layout rules
Changes of direction require approved fittings. Horizontal to vertical transitions use a fitting that directs flow downward. Vertical to horizontal transitions need a long-radius or combination fitting. Cleanouts are required at the base of stacks and at specified intervals. Support spacing depends on pipe material.
Traps in questions
Watch for questions about prohibited fittings, improper slope, and undersized drains. Also watch for drainage connections that bypass a trap.
Worked example
A 4-inch building drain at 1/8 inch per foot carries more fixture units than a 4-inch drain at 1/4 inch per foot? No; steeper slope increases capacity, so the 1/4-inch slope carries at least as much. Always check the table rather than guessing.
Rule of thumb
Flow moves downhill, fittings must direct flow smoothly, and pipe size must never decrease in the direction of flow.