01 / Roof Dimensions
Roof slope angle. Used to calculate effective catchment width.
Horizontal length of gutter from end to outlet/downpipe.
Horizontal distance from outer wall to roof ridge at apex.
02 / Rainfall & Location
Design rainfall: Use the 5-minute duration, 10-year ARI intensity for your location. Australian values from AS/NZS 3500.3 Appendix B. NZ values indicative — confirm with regional council data.
03 / Downpipe Configuration
Equal spacing assumed. Each downpipe services equal catchment area.
04 / Result
Recommended gutter cross-section and downpipe sizes
| Gutter Type | Min. Cross-Section (mm²) | Equivalent Width × Depth | Status |
|---|
Downpipe sizing — required outlet area per downpipe
| Downpipe DN | Outlet Area (mm²) | Max Catchment / DP | Status |
|---|
AS/NZS 3500.3:2021 / NZS 3500.3:2003 — scope and limitations. This tool applies the rational method (Q = C × I × A / 3600) for sizing roof drainage. It covers standard residential and commercial roof configurations. Flat roofs (pitch < 3°), large commercial roofs (> 200 m²), siphonic systems, and valleys and complex roof shapes require specific hydraulic engineering design. Minimum downpipe size is DN 75 for residential. Gutter fall must be minimum 1:500 (2 mm per metre) toward the outlet. All drainage work must comply with NZBC G13 and be installed by a licensed plumber or drainlayer.
Reference — Gutter Cross-Section Areas (AS/NZS 3500.3)
| Gutter Profile | Typical Size | Cross-Section (mm²) | Max Catchment (m²) |
|---|---|---|---|
| Quarter-round (100mm) | 100 × 75 mm | 3,750 | ~25 m² |
| Quarter-round (125mm) | 125 × 90 mm | 5,625 | ~35 m² |
| Square-line | 125 × 100 mm | 10,000 | ~65 m² |
| Square-line (large) | 150 × 125 mm | 15,000 | ~95 m² |
| OG (oval) — large | 175 × 125 mm | 17,500 | ~110 m² |
| Box gutter | 200 × 150 mm | 30,000 | ~190 m² |
| Box gutter — wide | 300 × 150 mm | 45,000 | ~280 m² |
Values at 150 mm/hr intensity, C = 0.9, per downpipe. Adjust proportionally for different intensities. Box gutters require continuous support and flashing per AS/NZS 3500.3.