Minimum slope, IPC 2021 Table 704.1
Horizontal drainage pipe slope is set by pipe size in three bands:
- 2½ inches or smaller: 1/4 inch per foot.
- 3 to 6 inches: 1/8 inch per foot.
- 8 inches or larger: 1/16 inch per foot.
Larger pipe is allowed a flatter slope. A 2-inch branch takes 1/4 inch per foot; giving it 1/8 inch is the common error. A 4-inch building drain takes 1/8 inch per foot. A 10-inch building sewer takes 1/16 inch per foot.
The 2½-inch trap: 2½-inch pipe belongs to the smallest band at 1/4 inch per foot, not the 3- to 6-inch band. The 1/8 band starts at 3 inches.
Working out total fall
Total fall is run length times slope:
- 3-inch drain, 60 feet: 3-inch is in the 3- to 6-inch band, so 60 × 1/8 = 7.5 inches. Using 1/16 gives 3.75 inches and using 1/4 gives 15 inches, both from the wrong band.
- 2½-inch drain, 40 feet: 40 × 1/4 = 10 inches. Using 1/8 gives 5 inches, the error of treating 2½-inch pipe as part of the 3- to 6-inch group.
Cleanout clearance, IPC 2021 Chapter 7
Clearance in front of a cleanout opening also depends on pipe size, split at the same 6-inch and 8-inch line:
- Piping 6 inches and smaller: at least 18 inches of clearance in front of the opening. A cleanout on 4-inch piping needs 18 inches.
- Piping 8 inches and larger: at least 36 inches, measured perpendicular to the opening. A cleanout on an 8-inch building drain needs 36 inches.
The trap is applying 18 inches to an 8-inch drain; 18 inches stops at 6-inch pipe.
One line to remember
Pipe size decides both numbers. For slope the breaks are 2½ inches and smaller, 3 to 6 inches, and 8 inches and larger. For cleanout clearance the break is 6 inches and smaller versus 8 inches and larger.