Hey Karl,

My inspector tells me I have too many drainage fixture units for a 3 inch building drain.  I am not sure what he means or how to fix it.


Question: What is a drainage fixture unit?


Answer:  2012 IPC Section 710.  A Drainage Fixture Unit (dfu) is a measure of the probable discharge into the drainage system by various types of plumbing fixtures.  Your tub/shower, toilet, laundry, Jacuzzi, mud sink, lavatory, all drain into the building drain.  The drain must be sized big enough to handle all of the water discharging to prevent it from backing up.  Each drain has a number or dfu associated with it.  Add all of the dfu’s in a house to determine the building drain size.  The maximum for a 3 inch building drain would be 36 at a pipe slope of 1/8 inch of fall per foot.