NZ Drainage Pipe Grade Requirements
All drainage systems in NZ must comply with AS/NZS 3500 (Plumbing and Drainage) as adopted by the NZ Building Code.
Minimum Grades by Pipe Size
| Pipe diameter | Minimum grade | Minimum fall/m |
|---|---|---|
| 40–50mm (basin, shower) | 1:40 | 25mm/m |
| 65–80mm | 1:60 | 16.7mm/m |
| 100mm (main drain) | 1:80 | 12.5mm/m |
| 150mm (sewer) | 1:150 | 6.7mm/m |
Why Grade Matters
- Too flat → solids settle, blockages, odours
- Too steep → water outruns solids (self-siphonage), trap seal loss
- Sweet spot for 100mm: 1:60 to 1:80
Fixture Units (NZ AS/NZS 3500)
| Fixture | Fixture units |
|---|---|
| Hand basin | 1 |
| Shower | 2 |
| Bath | 2 |
| WC | 4 |
| Kitchen sink | 2 |
| Dishwasher | 2 |
All drainage work must be done by a licensed drainlayer.