Drain Falls — Minimum Grades
Drain Falls — Minimum Grades
Too flat and it blocks, too steep and the water outruns the solids. The minimum grades for sanitary drainage under AS/NZS 3500.2.
Minimum grade by pipe size
| DN100 | 1.65% — 1:60 (16.5 mm fall per metre) |
| DN150 | 1.00% — 1:100 |
| DN65 branch | 2.50% — 1:40 |
In practice
| Over a 10 m run (DN100) | 165 mm of fall at the minimum grade |
| Where the depth allows | 2.0–2.5% gives better self-cleaning and fewer blockage callbacks |
Reference only — grades, venting and pipe sizing must follow AS/NZS 3500.2 and your plumbing regulator. Drainage is licensed plumbing work in every state.
Sources: AS/NZS 3500.2 (sanitary plumbing & drainage) · ABCB / NCC Plumbing Code
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