Plumbing Fixtures, Fixture Tables, and Fixture Unit Sizing
Fixture selection and installation
The UPC requires plumbing fixtures to be approved, durable, smooth, nonabsorbent, and accessible for cleaning. Fixtures must be securely fastened, level, and sealed to the drainage system. Minimum fixture counts depend on occupancy; use the UPC minimum facilities table for the occupancy rather than guessing from a similar building. Accessible fixtures must comply with the referenced accessibility standard adopted by the code, including clear floor space and mounting heights.
Reading fixture tables
- Drainage fixture units (DFU) express the probable drainage load of a fixture. Add the DFU values for fixtures connected to the same drainage branch or stack, then use the drainage sizing tables.
- Water supply fixture units (WSFU) express demand on the supply system. Supply sizing also uses available pressure, developed length, elevation, and diversity; WSFU values are not interchangeable with DFU.
- Fixture unit tables are organized by fixture type and sometimes by type of water closet (tank versus flushometer). Use the row that matches the actual fixture, not the row for a similar fixture.
Worked example
A private bathroom group has a tank-type water closet, one lavatory, and a bathtub/shower. Using common UPC fixture-unit values: water closet = 3 DFU, lavatory = 1 DFU, bathtub/shower = 2 DFU. Total drainage load = 6 DFU for the group. The trap and trap arm for each fixture still must be sized individually; the group total is used for the branch or stack. Do not add the group total to the individual fixture totals again-that double-counts the same fixtures.
Traps that cost marks
- Using WSFU to size a drain or DFU to size a water line.
- Adding a bathroom-group value and then adding each fixture again.
- Choosing a fixture-unit row for a flushometer when the job has tank water closets.
- Ignoring occupancy minimums and accessibility clearances.
- Forgetting floor drains, laundry trays, or indirect wastes in the count.