| Primary value | 1 : 60 for 100 mm dia (1 : 80 for 150 mm · 1 : 100 for 230 mm and above) |
| Applies to | Soil pipes carrying WC discharge (100 mm and above) · Waste pipes from kitchen, bath and washbasin (75 mm and above) · Sewer lateral connecting building to municipal sewer or septic tank |
| Exceptions | 100 mm dia soil pipe → 1 : 60 |
| 150 mm dia soil/sewer → 1 : 80 | |
| 230 mm and larger → 1 : 100 | |
| Self-cleansing velocity → ≥ 0.75 m/s for soil pipes | |
| Maximum velocity (avoid pipe wear) → 3.0 m/s | |
| Measured as | Same as SWD: fall ÷ run. 1:60 = 16.7 mm/m of run = 1.67%. Maintained over the entire length from fixture to manhole or septic-tank inlet. |
| Source | NBC 2016 — Part 9, Section 2, Cl. 5.6.4 ✓ Verified |
16 related items across IS codes, knowledge articles, design rules, maps and tools
Soil pipes carry solids and need a steeper gradient than storm water — at 1:60 a 100 mm pipe carries water fast enough to keep solids suspended. Lay it flatter and faecal matter settles, breeds smell and eventually clogs. Lay it steeper and water races ahead of solids, leaving them stranded — a worse outcome.
Plumbing contractors set 100 mm soil pipes at 1:50 to 1:60 and 150 mm sewer mains at 1:80. A common site mistake is connecting a 100 mm WC discharge to a long horizontal run with insufficient fall — within a year you can hear gurgling traps and notice slow flushes.