Two slopes, split at pipe size
2018 IRC P3005.3 sets the minimum slope for horizontal drainage piping by diameter:
- 2-1/2 in diameter and smaller: 1/4 in per ft (2%)
- 3 in diameter and larger: 1/8 in per ft (1%)
So a 2 in drain needs 1/4 in per ft, and a 4 in drain may run at 1/8 in per ft.
The traps
- Applying 1/8 in per ft to 2 in branch drains is the common error. The 1/8 in figure starts at 3 in pipe.
- The 1/16 in per ft value appears only in Table P3005.4.2, where it is used for sizing building drains and sewers. It is not the P3005.3 minimum slope.
- 1/2 in per ft and 3/8 in per ft are not figures in P3005.3.
Turning slope into drop
Total drop = run length in ft × slope in inches per ft.
Worked example: a 2 in horizontal branch drain runs 20 ft at the minimum slope. A 2 in pipe is 2-1/2 in or smaller, so the slope is 1/4 in per ft.
20 ft × 1/4 in per ft = 5 in of drop.
Each wrong result comes from the wrong slope:
- 20 × 1/8 = 2-1/2 in wrongly uses the 3 in and larger slope.
- 20 × 1/16 = 1-1/4 in uses the table sizing value.
- 20 × 1/2 = 10 in uses 1/2 in per ft, which is not the code minimum.
Check the diameter first
Before any slope question, sort the pipe: at or below 2-1/2 in, use 1/4 in per ft; at 3 in or above, use 1/8 in per ft. Then multiply by the run if the question asks for the drop. Section P3005.3 also requires horizontal drainage piping to be installed in uniform alignment at uniform slopes.