Downpipe Size in Perth (2026)
In Perth downpipe sizing follows the roof area per pipe and the Mediterranean winter rainfall intensity, with the sandy soils aiding soakwell discharge.
How we estimate this
Downpipe sizing follows AS/NZS 3500.3 and depends on the roof area draining to each pipe and your local rainfall intensity, not just the roof size. As a rough guide, one standard 90 to 100mm downpipe handles a modest suburban catchment, and large roofs need several.
Pricing reviewed: October 2026.
What a downpipe size costs in Perth
Sizing downpipes for a Perth roof comes from the roof area draining to each pipe and the intensity of a Mediterranean winter rain event, since the downpipe must carry the peak flow its catchment sheds, and the plumbing code turns that area and the local rainfall rate into a diameter and a number. Perth's wet winters and dry summers mean the design intensity is set by the winter storm peak, so the roof is divided into catchments and each downpipe sized for its share.
A distinctive Perth factor is that the sandy, free-draining coastal-plain soils suit soakwells, so downpipes often discharge into underground soakwells that let the stormwater soak away on the block rather than run to the street, and the downpipe and soakwell are sized together for the catchment. A single-storey double-brick home has a conventional roof whose area sets the pipe count. The dry summer is no load on the system. The isolation of the WA market rarely affects a standard pipe, labour sits near the national average, and the roof plumber sizes and installs them to the plumbing code.
Downpipes are roof plumbing sized and installed by a licensed roof plumber to the plumbing code, connecting to the stormwater system or a soakwell, in Western Australia under WA Building Services. For a Perth roof, the clearest way to size downpipes is by the roof area per pipe and the winter rainfall intensity, with the sandy soils letting them discharge to soakwells on the block.
Compare against the national picture: Downpipe Size in Australia →
Downpipe Size in other cities
Frequently asked questions
How are downpipes sized in Perth?
By the roof area draining to each pipe and the intensity of a Mediterranean winter rain event, which the plumbing code turns into a diameter and number, with the design intensity set by the winter storm peak in Perth.
Do Perth downpipes discharge to soakwells?
Often. The sandy, free-draining coastal-plain soils suit soakwells, so Perth downpipes frequently discharge into underground soakwells that let stormwater soak away on the block, with the downpipe and soakwell sized together for the catchment.
Does Perth's dry summer change downpipe sizing?
No. The design intensity is set by the winter storm peak, not the dry summer, so a Perth roof is split into catchments and each downpipe sized for its share of the winter rainfall.
Do I need a licensed trade for downpipes in Perth?
Yes. Downpipes are roof plumbing sized and installed by a licensed roof plumber to the plumbing code, connecting to the stormwater system or a soakwell, in Western Australia under WA Building Services.