For a lavatory, what is the Drainage Fixture Units at a 1-1/4 inch trap size?

• A. 1 DFU
• B. 2 DFU
• C. 3 DFU
• D. 4 DFU

Answer: (A)

Drainage Fixture Units (DFUs) are a sizing tool that represents the probable drainage load a fixture imposes on the system, used to size piping and vents. Each fixture type has a DFU value in a code table, and that value guides how large the drainage system must be.

A lavatory is assigned a DFU value of 1 in these tables, even when the trap is the common 1-1/4 inch size. The trap size is a separate, physical specification, but it does not increase the DFU for this fixture. So, for a lavatory with a 1-1/4 inch trap, the DFU is 1. This is why this option is the correct choice.