Plumbing and drainage — Stormwater drainage
Sizing of gutters, downpipes and stormwater drainage.
Overview
AS/NZS 3500.3 sizes the roof drainage system. It uses rainfall intensity (location-specific, from BoM data), catchment area and gutter/downpipe geometry to size eaves gutters, box gutters and downpipes. Box gutters in particular have strict overflow and sump requirements.
Key Requirements
- Calculate rainfall intensity for the location (1-in-20 or 1-in-100 year ARI).
- Size eaves gutters and downpipes for catchment area.
- Box gutters require sumps, overflow devices and minimum dimensions.
- Downpipe spacing — generally one downpipe per 12 m run of gutter as a starting point.
How it's used in the NCC
Called up by Part 7.4 of the Housing Provisions for gutters and downpipes.
Practical Notes
- A simple rule of thumb: one downpipe per ~100 m² of roof catchment, but check the calculation for steep roofs and tropical areas.
- Box gutters fail most often — undersized sumps, missing overflow, no proper fall.
Where to obtain
The full text of AS/NZS 3500.3 is published by Standards Australia and must be purchased — typical price A$150–400 per part. Always work from the current edition referenced in the NCC, not an older one.