Drain slope conventions (fall per foot)
A drain has to fall enough to carry solids but not so much that water races ahead and leaves them behind. The conventions below (1/4 in per foot for pipe up to 2-1/2 in, 1/8 in per foot for 3 in and larger) are widely used labeled planning conventions — your local plumbing code governs. Use them in the drain slope tool; fall = length × slope.
| Drain size | Minimum slope | Example |
|---|---|---|
| Pipe up to 2-1/2 in | 0.250 in per ft (1/4 in) | 10 in of fall over 40 ft |
| Pipe 3 in and larger | 0.125 in per ft (1/8 in) | 5 in of fall over 40 ft |
Labeled planning conventions — too flat clogs, too steep leaves solids behind. Fall = length × slope. Your local plumbing code governs.