IPC · 2024 International Plumbing Code
§1003 · Trap-arm layout
Effective in IPC 2024.
Plain-English explainer
1003 limits the horizontal developed length of the fixture drain between the trap weir and the inner edge of the vent, with Table 1003.1 setting the maximum for each trap size — a 1¼ in trap at ¼ in/ft fall gets at most about 30 in of developed length — so the trap seal is not lost to self-siphonage or induced siphonage from the vent stack. The drafter cites it whenever a permit runs a fixture branch to a vent stack.
Why inspectors flag this
A trap-arm that exceeds the table limit for its size is the canonical 1003 red-tag; the drafter lifts the rule so the branch is re-laid or the vent stack is moved closer before the rough-in walk-through.
As quoted in the code: The horizontal developed length of the fixture drain between the trap weir and the inner edge of the vent is limited by Table 1003.1 — for a 1¼ in trap the fall is one-quarter inch per foot and the developed length is not to exceed the table value (for a 1¼ in trap no more than about 30 in, scaled for size) so the trap seal is not lost to self-siphonage or to induced siphonage from the vent stack.
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