| Primary value | 1:100 (1%) |
| Applies to | Soil pipes · Waste pipes · Internal drainage runs |
| Exceptions | Long horizontal runs (recommended) → 1:80 to 1:60 |
| Vent pipes → no slope required | |
| Measured as | Vertical fall per unit horizontal length of the drainage pipe. |
| Source | NBC 2016 — Part 9 ⚠ Clause pending verification |
1:100 is the minimum slope that maintains self-cleansing velocity (≥0.6 m/s) for typical residential waste flow. Below this, solids settle in the pipe and cause blockages.
Plumbing designers usually specify 1:80 to give a buffer over the minimum. Long horizontal runs in basements often need 1:60 to maintain cleansing flow.