Catchment area (roof drainage)
A catchment area is the plan area of roof draining to a given gutter or downpipe; with rainfall intensity it sizes the drainage under AS/NZS 3500.3 and the NCC.
Ask Chalkline about this →A catchment area is the plan area of roof (or site) that drains to a given gutter, downpipe, or stormwater outlet. It is the key input that, together with the local rainfall intensity, sizes the drainage under AS/NZS 3500.3 and NCC Part 7.4.
The number that matters is the plan area, the horizontal footprint the rain falls on, not the sloping surface area of the roof. A pitched roof catches rain over its plan projection, so a steeply pitched roof and a flat roof of the same plan dimensions collect the same volume of water. Each gutter and downpipe serves a portion of the roof, and that portion is its catchment.
Sizing works like this: catchment area multiplied by the design rainfall intensity (for the relevant storm and location) gives the flow the gutter and downpipe must carry. Bigger catchment, or higher rainfall intensity, means a bigger gutter, more downpipes, or larger outlets. Get the catchment per downpipe wrong (too much roof to one pipe) and the gutter overtops in heavy rain.
For a builder the practical points are to divide the roof into sensible catchments with enough downpipes so no single one is overloaded, to use the plan area (not the raked area) in the calculation, and to design to the right rainfall intensity for the location and storm event. Pair the sizing with an overflow provision so that if an outlet blocks, water sheds clear of the building rather than backing into the eaves.
Also known as: Roof catchment, drainage catchment.
Category: Roofing / Drainage.
Related
See also
References
- AS/NZS 3500.3 Stormwater drainage, Standards Australia (verified 2026-06-01)
Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.