Design of box gutters and valley gutters in housing

Did you know that the minimum slope of a box gutter is between 1:40 - 1:200 and that a valley gutter cannot be installed on a roof with a pitch of less than 12.5 degrees?

Australian Standards and the Housing Provisions of the NCC contain separate pathways for compliance which differ. Knowing their limitations will avoid poorly designed or under-designed box and valley gutters that have the potential to cause major damage in a building.

What standards apply to roof gutters, valleys and box gutters?

The Deemed to Satisfy (DTS) pathways for designing and constructing roof drainage systems in Part H2D6 of Volume Two are:

• Part 7.4 of the ABCB Housing Provisions (HP); or 
• AS/NZS3500.3 Plumbing and Drainage – Stormwater drainage.

It is important to understand the key differences between AS/NZS 3500.3 and Part 7.4 of the Housing Provisions, given the different rainfall intensities and various construction methods across Australia. Part 7.4 of the Housing Provisions has limited solutions that may not suit all circumstances.

For example, there is no option within the Housing Provisions for gutter falls less than 1:500. This option only exists with AS/NZS 3500. Similarly, the Housing Provisions do not provide a solution for box gutters. Knowing these limitations at the design stage will minimise the risk of rework or damage in a rain event.

Key elements that determine gutter design

All gutters are designed according to the rainfall intensity in mm/h. This is determined based on an ‘Annual Exceedance Probability’ (AEP). AEP is the rainfall total accumulated over a given duration that is likely to be exceeded in any one year.

This differs in gutter application. For example, an eave gutter that is external to the building presents lower risk and may not be required in some locations. They also may not be required in applications such as a Class 10 building which is separate from a Class 1.

Eaves gutters are only required to remove water arising from rainfall events with a 5 minute duration and an AEP of 5% (provided they are not blocked).

A 1% AEP applies in other cases and dedicated or continuous overflow measures would be required. The NCC contains several options  including rainheads or weirs that can be used. They are not required on a verandah or an eave that is greater than 450mm in width, which has no lining or is sloping away from the building.

The catchment area of the roof's area is one of the other key determinants for gutter and downpipe design. This is based on the distance from the ridge to the gutter.  

Design considerations for a box gutter

An important area of building design is the correct sizing and grading of box gutters. As box gutters typically have a low fall and sit atop the building, they need to be designed against overflow. An AEP of 1% is applies to box gutters and Appendix D in AS/NZS 3500.3 provides the rainfall intensities for various locations throughout Australia.

The Housing Provisions no longer provide a solution for box gutters. Please refer to the relevant Australian Standard AS/NZS 3500.3.

There are three design options for box gutters using AS/NZS 3500.3 for: 

• Box gutters, rainheads and downpipes 
• Box gutters, sump/side overflow devices and downpipes 
• Box gutters, sump/high-capacity overflow devices and downpipes.

For each one of these box gutter designs there are relevant figures and flow charts for the design procedure.

There are limitations placed on the design method in the Standard. For example:

• The slope of the box gutter needs to be in the range of 1:40 to 1:200
• Minimum ‘freeboard’ (extra height in the valley gutter to minimise the chance of overflow) of 30mm  
• For rain heads the design flow should not exceed 16 litres per second, and 
• The size range of vertical downpipes and the limitation of solution for sumps with overflow devices shall both be within limitations of the Standard.

An informative Appendix provides worked examples for all three design methods.

The installation requirements for box gutters include:

• Gradients shall not be less than 1:200 for sole (base of box gutter) widths equal to or less than 600mm 
• Lap joints shall comply with the requirements of the Standard 
• The box gutter shall have a continuous support system for sole widths greater than 450mm, and discontinuous support using brackets for sole widths equal to or less than 450mm 
• Outlets shall be provided through rainheads or sumps
• Where necessary, expansion joints shall be provided. 

Designing valley gutters

Housing Provisions and AS/NZS 3500.3 contain a table (e.g. 7.4.4c and Table 3.6.2), to determine gutter width and depth based on rainfall intensity of the location.

There are minor differences between the NCC 7.4.4 and AS/NZS 3500.3. Depending on your state or territory, plans may need to specify which compliance pathway is being followed.

Valley gutters can only be used for roof pitches of 12.5 degrees or more. This is clearly specified in the Housing Provisions, AS/NZS 3500.3 and HB 39 Metal roof and wall cladding installation.

The Housing Provisions (Clause 7.4.4) also require valley gutters: 

• have dimensions in accordance with Table 7.4.4c for the relevant rainfall intensity; and  
• have a side angle of not less than 12.5 degrees
• have minimum ‘freeboard’ (extra height in the valley gutter to minimise the chance of overflow) of not less than 15 mm.

AS/NZS 3500.3 Clause 3.6 requires the:

• nominal valley gutter side angle shall be 16.5 degrees; and
• catchment area shall not exceed 20m¹

What about valleys in roofs less than 12.5 degrees?

The Housing Provisions explanatory information notes “Where roofs have pitches less than 12.5 degrees valley gutters may be designed as box gutters in accordance with AS/NZS 3500.3 or as a Performance Solution by a professional engineer or other appropriately qualified person”.

For roofs with roof trusses manufactured with roof pitches between 5 - 10 degrees valleys designed as box gutters this may impact on truss design and layout. Adopting a Performance Solutions will need to be considered at the design stage, as a different method of construction may be required to accommodate a deeper gutter in a valley.

Victorian variation

Victoria has a variation to NCC Volume Two clause H2D6, only permitting AS/NZS 3500.3 as the sole method for the design gutters and downpipes. This includes using valley gutter dimensions and sizes in Table 3.6.2 of the standard.