Where the fixture drains connect at different levels, the vent shall connect as a vertical extension of the vertical drain. The vertical drain pipe connecting the two fixture drains shall be considered the vent for the lower fixture drain, and shall be sized in accordance with Table 911.3. The upper fixture shall not be a water closet.
TABLE 911.3
COMMON VENT SIZES
| PIPE SIZE (inches) | MAXIMUM DISCHARGE FROM UPPER FIXTURE DRAIN (dfu) |
| 11/2 | 1 |
| 2 | 4 |
| 21/2 to 3 | 6 |
For SI: 1 inch = 25.4 mm.