IPC · 2024 International Plumbing Code
§712 · Slope of horizontal drains
Effective in IPC 2024.
Plain-English explainer
712 sets the slope rule for horizontal sanitary drains: not less than one-quarter inch per foot (¼ in/ft) for nominal pipe sizes of 3 in and smaller, and not less than one-eighth inch per foot (⅛ in/ft) for nominal pipe sizes of 4 in and larger, so solids are carried along with the liquid flow. The drafter cites it whenever a permit lists a building drain or a horizontal branch.
Why inspectors flag this
A 4 in building drain at ⅛ in/ft that drops below the slope table, or a 3 in branch at less than ¼ in/ft, is the canonical 712 failure; the drafter surfaces the rule so the trench grade matches before the inspector walks the rough-in.
As quoted in the code: Horizontal sanitary drains are installed at a uniform slope of not less than one-quarter inch per foot (¼ in/ft) for nominal pipe sizes of 3 in and smaller and not less than one-eighth inch per foot (⅛ in/ft) for nominal pipe sizes of 4 in and larger, so solids are carried along with the liquid flow.
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