Sanitary Drainage Systems and Drainage Design
The sizing logic
Drainage is sized by drainage fixture units, slope, and pipe diameter, and the rules cascade from fixture drain to branch to stack to building drain. A fundamental rule: the pipe may not decrease in size downstream in the direction of flow. Slope is typically 1/4 in/ft for 2-1/2 in and smaller, and 1/8 in/ft for 3 in and larger, with larger slopes prohibited where they would leave solids behind or siphon traps.
Fittings and direction changes
- Horizontal to vertical: use a sanitary tee or long-turn fitting as allowed.
- Vertical to horizontal: use a long-turn or combination fitting.
- Horizontal to horizontal: use a wye or combination; 90° short-turn fittings are prohibited in drainage.
- Cleanouts at the base of stacks, at changes of direction over 45°, at the building drain, and at intervals on long runs (commonly 100 ft for smaller pipe).
Common traps
- Reducing pipe size downstream because the fixture load drops.
- Using a sanitary tee on its back or in the wrong orientation.
- Forgetting that the building drain ends at the wall and the building sewer begins outside.
- Ignoring backwater valve requirements where the sewer can surcharge.
- Installing a cleanout in a concealed or inaccessible location.
Rule of thumb
When in doubt on direction changes: horizontal-to-vertical is the permissive case; horizontal-to-horizontal is the restrictive one.
Worked example: A branch carrying 6 dfu at 1/4 in/ft requires a larger diameter than 12 dfu at 1/8 in/ft only when the tables say so - always read the dfu/slope/diameter table rather than extrapolating from load alone.