Sanitary Drainage Systems
Gravity is the whole system
Sanitary drainage works by gravity, so slope, sizing, and fitting choice all serve to keep solids moving and air behind the flow. Size pipe from DFU tables using the total load on the section. Slope: commonly 1/4 in/ft for 2-inch and smaller pipe, 1/8 in/ft for 3-inch and larger, always toward the building drain. Flatter than allowed is a guaranteed exam failure; steeper than about 1/2 in/ft on a long horizontal run can separate liquids from solids.
Fittings and changes of direction
- Horizontal to vertical (in the direction of flow): long-sweep fittings.
- Vertical to horizontal: wye or combination with 1/8 bends.
- Sanitary tees belong on vertical-to-horizontal branch connections, not on their back for drainage.
- No reducing fittings in the direction of flow; reductions are made with bushings sized to keep the crown of the pipe even.
Cleanouts, stacks, and special conditions
Cleanouts at the base of each stack, at changes of direction greater than 45 degrees, at the building sewer, and at code-specified intervals on long runs. Stacks carry waste down while the vent stack beside them supplies air. Backwater valves protect fixtures below the level of the next upstream manhole. Fixtures on the lowest level may need to discharge to a sump and pump rather than by gravity.
Worked example: An 80-ft run of 4-inch building drain at 1/8 in/ft drops 80 × 0.125 = 10 in. If the same run needed 1/4 in/ft (2-inch pipe), the drop would be 20 in - a difference that changes the invert depth and sometimes forces a pump.
Trap: questions often show a sanitary tee laid on its back on a horizontal drain, or a 90-degree elbow where a long sweep is required.