Source Documents
These are the council documents used to build the NCC calculator. If a link is broken, the document may have moved on the council website — contact NCC or check their infrastructure page.
Stormwater Standard Design Guidance v2
Code of Practice for Land Development and Subdivision Infrastructure
NCC Stormwater Information Page
Calculation Methodology
Method: Table Lookup (Acceptable Solutions)
NCC provides two acceptable solutions for residential stormwater detention. The calculator matches your roof area to the correct table row and returns the required detention volume, orifice diameter, and retention volume.
Design event: 10-year ARI (Annual Recurrence Interval)
Acceptable Solution No 1 (AS1) — Combined Tank
- Roof area: 15 - 30 m²
- Single combined retention + detention tank
- Orifice at 500mm above tank base
Acceptable Solution No 2 (AS2) — Separate Tanks
- Roof area: 31 - 400 m²
- Separate retention tank + detention tank required
- Orifice at 100mm above tank base
Retention Volume Formula
Rv = (Ar + Ai) × 10 litres
Where Ar = roof area (m²) and Ai = impervious area not connected to tank (m²)
Where Ar = roof area (m²) and Ai = impervious area not connected to tank (m²)
AS2 Sizing Table
| Roof Area (m²) | Max Impervious (m²) | Max Total Area (m²) | Orifice (mm) | Detention (L) |
|---|---|---|---|---|
| 31 - 50 | < 65 | < 500 | 17 | 3,000 |
| 51 - 100 | < 65 | < 500 | 13 | 4,000 |
| 101 - 150 | < 65 | < 500 | 15 | 6,000 |
| 151 - 200 | < 100 | < 500 | 20 | 7,000 |
| 201 - 250 | < 125 | < 600 | 22 | 8,000 |
| 251 - 300 | < 125 | < 600 | 30 | 9,000 |
| 301 - 350 | < 150 | < 750 | 28 | 10,000 |
| 351 - 400 | < 150 | < 1000 | 29 | 12,000 |
Outside these limits? Sites with roof area < 15m² or > 400m², or exceeding the impervious/total area limits, require site-specific design by a suitably qualified person (CPEng or equivalent).
Design Requirements
- Tank(s) must be above ground
- Only roof water to retention system
- Overflow pipe invert min 1.7m above tank base
- 10-year ARI design event
- Tank outlet above stormwater reticulation level
- Factory fitted min 50mm diameter outlet
- Non-potable pipes must be lilac coloured
- "Non-Potable Water" warning sign on tank
- 100mm uPVC overflow pipe
Submission
Required Documents
- Completed stormwater management worksheet
- Manufacturer's tank specifications
- Proposed design drawings
- Operation and Maintenance Manual
Contact
Last verified: March 2026 · NZ Detention Tank Calculator