Bonus-room toilet drain over the garage
What's wrong here?
- The horizontal drain runs uphill toward the stack instead of falling toward it
- A cleanout is required at the upper end of this horizontal drain
- The hangers are too far apart for PVC drain pipe
- A horizontal drain may not enter a vertical stack through a sanitary tee
Show the answer and the code reference
The answer
The 3 in. drain is back-pitched. It rises from the closet bend toward the stack, so waste flows the wrong way and sits in the pipe.
Correct choice: 1. The horizontal drain runs uphill toward the stack instead of falling toward it
UPC 708.1 requires horizontal drainage piping to run in practical alignment at a uniform slope of at least 1/4 in. per ft toward the point of disposal. Here the closet bend end hangs lower than the stack end, as the long left strap, the short right strap, the gap under the joist and the bubble pushed toward the stack all show. Solids settle at the low end next to the closet bend, and paper and waste build up until the line stops. A drain that is dead level or back-pitched fails the rough-in inspection regardless of pipe size.
How to fix it
Re-hang the run so it falls at least 1/4 in. per ft from the closet bend to the stack; over this roughly 4 ft run that is about 1 in. of fall. If the joist depth will not allow it, lower the tee on the stack rather than raising the toilet end.
Code references
UPC 2018 708.1 — Grade of Horizontal Drainage Piping
Horizontal drainage piping is to be run in practical alignment at a uniform slope of not less than 1/4 in. per ft (2 percent) toward the point of disposal. Pipe 4 in. and larger may be run at not less than 1/8 in. per ft where it is impractical otherwise and the Authority Having Jurisdiction approves it first.
Drills citing 708.1UPC 2018 706.2 — Horizontal to Vertical
Horizontal drainage lines connecting with a vertical stack may enter through 45-degree wye branches, combination wye and 1/8 bend branches, sanitary tee or sanitary tapped tee branches, or other approved fittings of equivalent sweep.
Drills citing 706.2
Minnesota
Minnesota adopts the 2018 Uniform Plumbing Code with Minnesota amendments as the Minnesota Plumbing Code (Minn. Rules ch. 4714, effective Dec. 17, 2021). No Minnesota amendment to 708.1 is known; check ch. 4714 before relying on it. A back-pitched drain fails under any slope rule.
About the Minnesota Plumbing CodeWhy the other choices are wrong
- A cleanout is required at the upper end of this horizontal drain. UPC 707.4 exception 3 drops the cleanout requirement for piping above the lowest floor, except the building drain and its horizontal branches (Minnesota's amended 707.4 is reported to also keep cleanouts for kitchen sinks and urinals). A toilet branch in a garage ceiling is none of those.
- The hangers are too far apart for PVC drain pipe. UPC Table 313.3 allows horizontal ABS and PVC to be supported every 4 ft. These hangers are under 3 ft apart, and the closet bend and the stack carry the ends.
- A horizontal drain may not enter a vertical stack through a sanitary tee. UPC 706.2 lists the sanitary tee for horizontal-to-vertical connections. The tee is only prohibited where flow turns from vertical to horizontal or between two horizontal lines.