§710 · Drainage system sizing

Why inspectors flag this

A horizontal branch undersized for the DFU load, or a stack that runs above half-full, is the typical 710 fails; the drafter lifts the rule so the pipe schedule matches the fixture group before the rough-in.

As quoted in the code: Drain, branch, and stack sizes are selected by the drainage fixture unit (DFU) load they carry, using Table 710.1(1) for fixtures and Table 710.1(2) for sizing horizontal branches and stacks so the system never runs above half-full at design flow.