| Primary value | 1 : 100 minimum slope (100 mm fall over 10 m run) |
| Applies to | Storm water drains (open or covered) collecting roof and surface runoff · Site drainage from compound to municipal storm-sewer connection · Internal terrace gutters and parapet rainwater channels |
| Exceptions | 150 mm dia drain (minimum) → 1 : 100 |
| Larger drains (300+ mm) → 1 : 150 acceptable | |
| Self-cleansing velocity → ≥ 0.6 m/s (NBC 2016) | |
| Open / channel drain (Manning n = 0.013) → Slope per Manning's equation | |
| Combined with sewer (no longer code-permitted) → 1 : 60 | |
| Measured as | Slope is fall ÷ run, expressed as a ratio (1 : N) or percentage. 1:100 means 1 m fall over 100 m horizontal distance — equivalent to ~10 mm/m or 1%. |
| Source | NBC 2016 — Part 9, Section 2, Cl. 5.6 ✓ Verified |
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Self-cleansing gradient prevents grit and silt from settling in the pipe — once sediment builds up, the drain blocks during the next monsoon and the building floods. 1:100 keeps water velocity above ~0.6 m/s in a 150 mm pipe carrying typical Indian rainfall intensity (50 mm/h).
Most Indian sites lay SWD pipes at 1:80 or 1:100 — 100 mm fall per 8–10 m of run is easy to set with a string-line. Tighter slopes (1:60) are reserved for pipe lengths under 5 m where the extra fall is cheaper than the digging.