Sanitary Drainage, Slope and Drainage Sizing
Sanitary Drainage
The path and the parts
Drainage moves fixture drain → branch → stack → building drain → building sewer. Each component has a fitting rule: horizontal-to-vertical and vertical-to-horizontal changes use wyes and combination fittings with long-radius sweeps, not sanitary tees except in specific allowed positions. Fixture unit tables assign DFU by fixture type and occupancy; add them along the branch and stack to size the pipe.
Rules that show up in questions
Every fixture is trapped and vented, and the trap-arm length from trap weir to vent is limited by pipe size. Cleanouts are required at the base of each stack, at horizontal changes of direction over 45°, near the foundation on the building drain, and at long runs. Backwater valves protect fixtures below the elevation of the next upstream manhole. Sleeves, structural crossings and freeze protection also apply.
Traps
- Using a sanitary tee where a wye is required for a vertical-to-horizontal change.
- Combining fixture discharges in a way that creates a prohibited wet vent or double trap.
- Forgetting the cleanout at the base of the stack.
- Assuming the building drain may be smaller than the sewer it connects to.
- Adding DFU from the wrong occupancy column.
Worked example
A bathroom group of water closet (4 DFU), lavatory (1) and tub (2) totals 7 DFU. A 3 in. branch is more than adequate; branch size comes from the horizontal-branch DFU table, not from fixture count alone. Then check the trap-arm length of the 1-1/2 in. lavatory drain against the vent table before locating the vent.
Rule of thumb: drainage goes downhill only, and fittings must sweep with the flow - never a tee on its back.
Slope and Drainage Sizing
Slope basics
Gravity is the only pump. Standard slope is 1/4 in. per foot for 2-1/2 in. and smaller pipe and 1/8 in. per foot for 3 in. and larger, including the building drain and sewer. Pipe must never be back-graded, and long runs must be checked against the fall actually available - a common field failure and a common exam question.
Sizing
Drainage sizing uses the DFU tables for horizontal branches, stacks, and building drains and sewers, read at the correct slope. Some tables give different capacities for 1/8 vs. 1/4 in. per foot, so read the slope column. Vent sizing has its own table based on the DFU served and the developed length of the vent.
Traps
- Using 1/16 in. per foot "to gain fall." Not a recognized slope.
- Sizing the drain from fixture count instead of the DFU table.
- Making a change in pipe size without the proper fitting.
- Not checking that the building drain invert stays above the sewer invert at the property line.
- Exceeding the maximum developed length for a given size at a given slope.
Worked example
A 60 ft building drain at 1/4 in. per foot needs 60 × 0.25 = 15 in. of fall. If the sewer invert is 4 ft below the basement slab and the run to the property line is 140 ft total, you have 48 in. of fall against 35 in. required - so the constraint is the elevation of the municipal sewer, not the pipe size.
Rule of thumb: fall in inches = length in feet × slope in inches per foot. Do that multiplication before you promise a fixture a drain.