May 18, 2023
From the 2021 UPC Illustrated Training Manual, Chapter 4, PLUMBING FIXTURES AND FIXTURE FITTINGS
402.6.3 Securing Floor-Mounted, Back-Outlet Water Closet Bowls. Floor-mounted, back-outlet water closet bowls shall be set level with an angle of 90 degrees (1.57 rad) between the floor and wall at the centerline of the fixture outlet. The floor and wall shall have a flat mounting surface not less than 5 inches (127 mm) to the right and left of the fixture outlet centerline. The fixture shall be secured to the wall outlet flange or drainage connection and the floor by corrosion-resistant screws or bolts. The closet flange shall be secured to a firm base.
Where floor-mounted, back-outlet water closets are used, the soil pipe shall be not less than 3 inches (80 mm) in diameter. Offset, eccentric, or reducing closet flanges shall not be used.
The code takes special care here to give instructions on installing floor-mounted, back-outlet water closets. These fixtures can be difficult to install properly because there are two mounting surfaces, the floor and the wall. Both have to be flat and create a 90-degree angle to be properly set. The closet has to mount rigidly to the back outlet and to the floor (see Figure 402.6.3).
Note: While properly designed offset, eccentric or reducing closet flanges are permitted for floor outlet water closets, because of the geometry involved they are not allowed for floor-mounted, rear-discharge water closets.
The 2021 Uniform Plumbing Code Illustrated Training Manual is available for purchase here.
(This is not to be considered the official position of IAPMO, nor is it an official interpretation of the Codes.)
Last modified: May 11, 2023