Drainage Systems and Drainage Piping
The governing idea
Drainage is gravity-driven and self-scouring. Every requirement - slope, fitting type, venting, cleanouts - exists to move solids, protect trap seals, and keep the pipe clear. Understand the path: fixture → trap → trap arm → fixture branch → branch/stack → building drain → building sewer.
Slope and fittings
Commonly, 2½-inch and smaller drains run at 1/4 in/ft, 3-inch through 6-inch at 1/8 in/ft, with flatter slopes for very large pipe. Horizontal-to-horizontal direction changes use long-sweep fittings or a combination wye and 1/8 bend - never a sanitary tee laid on its back, and never a double sanitary tee for back-to-back water closets (use a double fixture fitting). Vertical-to-horizontal changes below fixtures must avoid creating an S-trap effect.
Cleanouts and protection
Provide cleanouts at the base of stacks, at the end of horizontal runs, at intervals (commonly every 100 ft), and at aggregate changes of direction beyond the code's limit (often 135°). Backwater valves protect fixtures below the next upstream manhole. Floor drains and equipment drains need trap primers.
Worked example
A 3-inch building drain runs 60 ft. At 1/8 in/ft, total fall = 60 × 0.125 = 7.5 in. If the same run were mistakenly installed at 1/4 in/ft, fall would be 15 in - often enough to break the connection at the sewer or create a shallow bury.
Traps the exam sets
Using a sanitary tee on its back; exceeding trap-arm length; omitting a cleanout at the base of a stack; installing a horizontal drain without adequate slope; and forgetting that a building drain ends at the building wall while the building sewer begins outside it.