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Some gutters and downpipes requirements have a new home in the ABCB Housing Provisions.

2022 has been a wet year for many parts of the country, showing us how important working gutters and downpipes are for a roof’s drainage system. Equally as important is knowing what requirements are new, or have changed in NCC 2022.

What’s changed?

As you’re probably aware, Volume Two is restructured and looks a little different.

Before we dive into the technical changes, you need to know exactly where to find the requirements.

‘Gutters and downpipes’ is now in Part 7.4 of the new ABCB Housing Provisions Standard (2019: Part 3.5.3). It introduces new and expanded requirements for gutters and downpipes.

Although the requirements are mainly in the Housing Provisions, you will still need to refer to H2D6 in Volume Two, as it has important information to help you meet the performance requirements.

You can read more about how to use Volume Two and the Housing Provisions in this article.

Let’s look at some of the more technical changes.

Definitions (Schedule 1)

A key change to the NCC definitions includes replacing the existing defined term ‘average reoccurrence interval’ with ‘annual exceedance probability’ (AEP). AEP is the probability that expected yearly rainfall will be exceeded in any one year. So 1% AEP means there is a 1% chance that rainfall could exceed the expected rainfall in a given region.

This change aligns with AS/NZS 3500.3, which also uses AEP.

Rainfall intensities

The rainfall intensities for different locations are updated to suit the new AEP metric. These are in the various tables in clause 7.4.3 (2019:

There is an example in the explanatory information of the Housing Provisions (after Table 7.4.3d) to help you understand how AEP is used to achieve correct gutter and downpipe sizing for overflow based off your site location.

We recommend familiarising yourself with the updated tables, as it may affect what gutter and downpipe sizes are used on site.

The ABCB gutters, downpipe and overflow calculator may also help.

Installation of gutters

In clause 7.4.4 (2019: ‘Installation of gutters’, gutters must be supported by brackets and securely fixed at corners, as well as stop ends.  

There are also changes to valley gutter requirements, which now require a minimum 15 mm freeboard limit. This essentially means there must be a minimum of 15 mm extra height in the valley gutter to minimise the chance of overflow (see explanatory Figure 7.4.4).

A new table (7.4.4c), is included to provide more details for the construction of valley gutters. The explanatory information for pitched roofs (less than 12.5 degrees), allows the use of AS/NZS 3500.3 for box gutters or alternatively a Performance Solution can be developed.

Box gutters

An important change is set out in clause H2D6(3). It excludes box gutters from Part 7.4 meaning the only DTS Solutions are in AS/NZS 3500.3.

All references to a 1:100 fall for box gutters have been removed. You can still install box gutters, but you will have to use AS/NZS 3500.3 as a DTS Solution, or use a Performance Solution.

Changes to tables and clearer diagrams

Some tables have been changed into clauses with clearer construction diagrams. This change improves readability of the requirements.

This summary gives you a good insight into some of the changes, however, you should check NCC 2022 for the specific details.

Resources to help you

For more guidance, you may find our gutters, downpipes and overflow calculator helpful.

This article was updated on 24/07/2023