Drainage, Waste, and Vent Systems
Slope, sizing, and cleanouts
- Slope is 1/4 in per ft for pipe 2 1/2 in and smaller and 1/8 in per ft for 3 in and larger.
- Sizing uses drainage fixture units (DFU) from the code table, accumulated along the run, then converted to pipe size for the building drain, building sewer, and branches.
- Cleanouts are required at the base of each stack, at the building sewer, at intervals not exceeding 100 ft in horizontal runs, and at changes of direction greater than 135 degrees. They must stay accessible.
- A backwater valve is required where a fixture drains below the level of the next upstream manhole - the classic DC basement. Drainage below the sewer requires a lift station.
Traps and vents
- Each fixture gets its own trap except where the code permits a common trap; trap seal depth is 2 to 4 in.
- S-traps, drum traps, bell traps, and traps with moving parts are prohibited.
- Trap arm length from the trap weir to the vent is limited by the code table; exceed it and the trap siphons.
- The vent terminal must extend at least 6 in above the roof surface and be located away from doors, windows, and air intakes.
- Wet venting is allowed only where the code's bathroom-group rules are met, and the wet vent must be sized for the total load it carries.
Worked example
A 3 in building drain runs 40 ft at the code minimum slope. Reasoning: 3 in uses 1/8 in per ft, so total fall is 40 times 1/8, or 5 in. The same run in 2 in pipe would need 1/4 in per ft, or 10 in of fall - which often decides whether the drain clears a slab at all. The slope check therefore comes before layout, not after.
Traps that cost marks
Using 1/4 in per ft on every size and running out of cover; omitting the backwater valve on basement fixtures; venting too far from the trap; and treating an air admittance valve as a substitute for a vent stack where the code requires a real vent.