Drain Slope: The Minimum Fall by Pipe Size
Drain Slope: The Minimum Fall by Pipe Size
How much fall a horizontal drain needs so it self-cleans, and an honest answer on the maximum that everyone argues about.
Minimum slope, IPC Table 704.1
| 2-1/2 in and smaller | 1/4 in per ft (1:48) |
| 3 in to 6 in | 1/8 in per ft (1:96) |
| 8 in and larger | 1/16 in per ft (1:192) |
Cleanouts
| Horizontal drain inside the building | At intervals not exceeding 100 ft |
| Building sewer smaller than 8 in | At intervals not exceeding 100 ft |
| Building sewer 8 in and larger | Manholes, within 200 ft of the junction then every 400 ft |
| At direction changes | Required at each change greater than 45 degrees |
| Building drain vs building sewer | Same slope table. The distinction is location and pipe material, not fall. |
Can a drain be TOO steep?
- There is no numeric maximum slope in the IPC.
- The workshop claim is that liquid outruns solids on a steep run and leaves them behind. It is widely repeated and genuinely disputed, and no code section sets a figure.
- So: hit the minimum, keep it consistent, and do not let anyone tell you a steep run fails inspection on a number. Ask them which section.
These are model-code figures from the IPC. Plumbing code adoption is state and often local, a good number of jurisdictions use the UPC instead, and amendments to slope and cleanout tables are common. Confirm against the plumbing code your jurisdiction has actually adopted before you set a fall.
Sources: iccsafe.org
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