Drainage Systems: Sizing, Slope, Fixtures, and Cleanouts
Drainage Systems: Sizing, Slope, Fixtures, and Cleanouts
The exam wants gravity drainage that is self-cleaning, correctly sized and cleanable. Most missed items are slope and cleanout errors.
Slope
- Pipe 2½ in. and smaller: 1/4 in. per ft.
- Pipe 3 in. and larger: 1/8 in. per ft.
- Keep slope uniform; no sags and no double-back offsets where solids can settle.
Sizing
Add the DFU of every fixture on the branch, then enter the code's table for that slope and pipe size. DFU values vary by fixture type and by private versus public use; a tank-type water closet contributes far more than a lavatory. Never reduce a drain in the direction of flow, and never size by the pipe that happens to be on the truck.
Cleanouts
Provide cleanouts at the base of each stack, at the end of horizontal runs, at aggregate changes of direction greater than 45°, and at the intervals the code sets for long runs. Openings must be the code size, accessible, and sealed with gas- and water-tight covers where required. Under-floor cleanouts need access through a wall or floor plate; slab cleanouts must be brought flush and stay accessible.
Worked example. A 24 ft horizontal branch of 2 in. pipe must fall at 1/4 in. per ft: 24 × 0.25 = 6 in., or 0.50 ft. If the upstream invert is at 100.00 ft, the downstream invert is at 99.50 ft. Switching to 3 in. pipe does not let you relax to 1/8 in. per ft unless the whole downstream line is 3 in.
Traps. Using 1/8 in. per ft on small pipe; hiding a cleanout behind drywall; forgetting a cleanout where two 45s make an aggregate 90° turn; and reducing pipe size downstream.