TradesHow this works
A horizontal drain has to fall enough to move water and solids, but not so much that the water outruns the solids. The IPC sets a minimum slope by pipe size, steeper for small pipe and flatter for large pipe, and everyday practice targets a quarter inch per foot on typical residential lines.
In plan mode, enter the pipe size and run and the tool gives the fall at the target and the code minimum, plus the over-steep line to stay under. In check mode, enter a drop you measured and it tells you whether that slope is in range, too flat to drain, or steep enough to leave solids behind.
Common questions
What slope does a drain pipe need?
The IPC minimum depends on pipe size: a quarter inch per foot for pipe 2.5 inches and smaller, an eighth of an inch per foot for 3 to 6 inch pipe, and a sixteenth per foot for 8 inch and larger. For typical residential drains, a quarter inch per foot is the everyday target even where code allows an eighth, because it drains reliably.
Can a drain be too steep?
Yes. Past about half an inch per foot on a horizontal drain, the liquid can run out from under the solids and leave them behind in the pipe, which leads to clogs over time. If you need to lose a lot of height, the better move is to drop vertically and then run the horizontal section at a normal grade, rather than running the whole line steep.
How much fall is a quarter inch per foot over my run?
It is a quarter inch of drop for every foot of length, so multiply the run by 0.25. A 40 foot run at a quarter inch per foot falls 10 inches end to end. The tool does this for your pipe size and run, shows the code minimum and the target, and flags a slope that is too flat or too steep.
Sources: IPC Table 704.1 minimum slope of horizontal drainage piping (2021) · Over-steep self-scouring guidance (industry practice). Engine version 1.0.0. Risk tier 1.