STORMWATER DRAINAGE
Downpipe and gutter sizing for Victorian rainfall
Downpipe and gutter sizing comes down to two things: how much roof drains to each downpipe, and how hard it rains where the property is. Get either wrong and the gutter overflows in a storm.
The catchment area principle
Every downpipe carries the runoff from a slice of roof, its catchment area. The bigger that area, the more water arrives at the downpipe in a storm. So a large roof draining to a single point needs either a larger downpipe or more downpipes spread across it. You size and space them so no one downpipe is asked to take more than it can clear.
Rainfall intensity drives it
The same roof in a high-rainfall location has to shed more water in a given burst than it would in a drier one. That is why sizing is tied to the local rainfall intensity for the area, not a single national figure. The wetter and more intense the local storms, the more downpipe capacity the same catchment area needs.
Gutter fall and overflow
The gutter has to fall toward the downpipes so water runs to them rather than sitting and backing up. The system also needs an overflow provision, a controlled way for water to escape if the downpipes are overwhelmed or blocked, so it spills clear of the building rather than back into the eaves. A flat gutter with no overflow is a leak into the roof space waiting for the next big storm.
Why undersized downpipes overflow
When the catchment area is too large for the downpipe, the gutter fills faster than the pipe can clear it. The water rises to the gutter edge and spills over, usually back against the fascia and into the building. The fix is not a deeper gutter, it is matching downpipe size and number to the catchment area and the local rainfall. Photograph the roof areas, the downpipe positions and sizes, and the overflow provision. Elemetric keeps that with the job.
Common questions
What determines how many downpipes a roof needs?
The catchment area each downpipe carries and the local rainfall intensity. A bigger roof area draining to one point needs either a larger downpipe or more of them, and a higher-rainfall location needs more capacity than a drier one for the same roof.
Why do downpipes overflow?
Usually because the catchment area feeding them is too large for the downpipe size, or there is not enough fall in the gutter, or the overflow provision was left out. In a heavy storm the gutter fills faster than the downpipes can clear it and it spills back over the edge.
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Download on the App Store →General information for licensed tradespeople, not legal or regulatory advice. The licensed plumber remains solely responsible for compliance. Refer to the current AS/NZS 3500 standards and the Building and Plumbing Commission (formerly the VBA) for authoritative requirements.