Section 702.1/Drainage System Pipe Sizing
IPC 702 covers sanitary drainage pipe sizing using drainage fixture units (DFU), establishing minimum pipe diameters for building drains, branches, and stacks.
Drainage pipe sizing is based on the total drainage fixture units (DFU) connected to each section of pipe. Table 710.1(1) provides the maximum DFU load for horizontal drainage branches and building drains based on pipe diameter and slope. Table 710.1(2) provides the maximum DFU load for vertical stacks. Minimum pipe sizes are also limited by the largest fixture drain connected: no drainage pipe can be smaller than the fixture drain it serves.
Why this section exists
Undersized drainage pipes cause backups, slow drainage, and eventual blockages. The DFU system assigns a relative load value to each fixture type based on its peak discharge rate and frequency of use. By summing the DFU loads and referencing the sizing tables, the designer ensures that each pipe section has adequate capacity for the connected fixtures under worst-case simultaneous discharge conditions.
What plan reviewers look for
Plan reviewers trace the drainage system from each fixture to the building drain, checking pipe sizes against the DFU tables. They verify that pipe sizes increase (never decrease) in the direction of flow, that the building drain slope is adequate (typically 1/4 inch per foot for pipes 3 inches and larger), and that cleanouts are provided at the required intervals.
Common violations
Related IPC requirements
Table 710.1(1) and (2) provide the DFU sizing tables. Section 708 covers cleanout requirements. Section 903 covers the vent system that works in conjunction with the drainage system. Section 704 covers fixture drain sizing.