How to read the slope tables
The first table gives the minimum fall for horizontal drainage piping by nominal pipe size. Under the International Plumbing Code 2021, Table 704.1 has three rows:
- 2-1/2 inches or less: 1/4 in/ft
- 3 to 6 inches: 1/8 in/ft
- 8 inches or larger: 1/16 in/ft
The Uniform Plumbing Code takes a simpler line: 1/4 in/ft for all horizontal drains, with 1/8 in/ft allowed for 4 inch and larger pipe where 1/4 in/ft is impractical and the authority having jurisdiction approves it first. Which column applies depends on the code your state or city has adopted.
The total-drop table answers the practical question on site: how much lower must the far end be? Read across from the run length to the slope column. For example, a 32 ft run of 3 inch pipe at the IPC minimum drops 4".
Why the slope matters
A drain has to move water fast enough to carry solids and scour the pipe, but a gravity drain is never meant to run full. Larger pipes carry the same flow at a lower depth and need less fall to keep it moving, which is why the minimum slope decreases as the size increases. Too little slope leads to standing water, buildup and slow drains; a belly (a sag in an otherwise sloped pipe) has the same effect.
Laying out the fall
- Set the low end first (the connection to the stack, building drain or sewer) and work up toward the fixtures.
- Use a level marked for slope, a laser or a string line with measured drops; a 4 ft level with a 1" shim under one end reads 1/4 in/ft.
- Support the pipe at the spacing your code requires so it cannot sag between hangers.
- Check the slope again after backfilling or after other trades have worked nearby.
Common drain slope mistakes
- Applying the IPC 1/8 in/ft allowance for 3 inch pipe in a UPC jurisdiction.
- Measuring the run horizontally on the plan when the pipe takes a longer path.
- Running out of height: on long runs, check the total drop against the depth available before starting.
Figures are from the 2021 IPC and 2021 UPC. Many jurisdictions adopt these codes with amendments; confirm the edition in force locally.