Getting your mind into the gutter and what's up with downpipes

Image of a gutter

Achieving compliance through the Acceptable Construction Practice (ACP) of NCC Volume Two.

The NCC has a number of requirements to ensure rainwater that is collected or concentrated by a building is disposed of in a way that prevents water entering a building or damaging other properties. Compliance can be achieved through various means but one method in particular, which was revised for NCC 2016, is the Acceptable Construction Practice (ACP) in Part 3.5.2 of Volume Two.

So what's in the ACP?

This ACP, entitled ‘Gutters and downpipes’,  provides a prescriptive DTS Solution for Class 1 and 10 buildings for eaves gutter design, acceptable overflow measures, and downpipes appropriate to different gutter designs. It can be applied not only through Volume Two but also Volume Three, the PCA, through its reference in Part D1.

How are eaves gutters designed?

Eaves gutters are designed based on two parameters: the roof catchment area size (M2;) and the rainfall intensity for the location (20 year 5 minute duration (mm/h)). These parameters are used to determine the appropriate eaves gutter type for the roof, ranging from Type A to F. They are further defined by shape and cross-sectional area.


Once an eaves gutter type has been chosen, selecting the downpipe is simple. The acceptable downpipe types are based on the chosen gutter type. 

Downpipe location is important as each downpipe can serve no more than 12m of gutter length. They should also be located as close as possible to valley gutters.

Overflow from eaves gutters

Overflow measures are part of the design process of eaves gutters and downpipes. They are provided to allow rainfall to be removed from the roof based on a 5 minute duration rainfall intensity with an average recurrence interval of once in 100 years.  The required overflow volumes are calculated in isolation to the gutters and downpipe design. This ensures that no rainwater will enter the building in a 100 year rainfall event, even if the downpipes are blocked. For eaves gutters, this is achieved by two overflow measure options: continuous and dedicated. Continuous measures operate over the length of the gutter while dedicated measures account for overflow in a given location along the gutter.

The two options can be used separately or together to meet the required overflow volume of a 100 year rainfall event.

The requirement to provide overflow measures does not apply to eaves gutters fixed to a veranda or an eave greater than 450 mm wide (with no lining or raked with a lining sloping away from the building).

Want to know more?

We are scoping additional guidance material to help practitioners understand and apply the eaves gutter and downpipes provisions. Keep an eye on our website for further updates.

Note: Each State’s and Territory’s legislation adopts the NCC subject to variation or deletion of some of its provisions, or the addition of extra provisions. This article provides information on the national provisions of the NCC only. For advice any State or Territory variations, deletions or additions that may be relevant to this topic, you should contact the relevant State or Territory Building/Plumbing Authority for more information.

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