Design of box gutters for domestic installations (NAT)

One of the more important areas of building design relates to the correct size and grading of box gutters. A poorly or under-designed box gutter has the potential to cause major consequential damage in a building so it is extremely important that correct size and fall ratio is achieved.

There are two methods that can be used to design a box gutter by either adopting the Building Code of Australia Volume 2 Housing Provisions (BCA) or using the relevant Australian Standards for plumbing stormwater work AS/NZS 3500 part 3, or domestic installations for housing AS/NZS 3500 part 5. Both contain information on domestic stormwater installation.

The first and foremost consideration is the rainfall intensity for any chosen area. The BCA and the Australian Standards provide information on Australian Rainfall Intensities (ARI) and as box gutters need to be designed to a high level of security against overflow generally the 100 year ARI is adopted. This is based on the five minute rainfall intensity in millimetres per hour every one hundred years which is a worst case scenario.

The steps to designing box gutters differ slightly depending on whether the BCA or Australian Standard method is adopted. The BCA provides a somewhat simpler method in that once the rainfall intensity is determined and the roof catchment area per downpipe is calculated, the tables in the BCA will provide a size based on minimum cross sectional area of the required gutter.

The BCA provides a limit where if the catchment area and rainfall intensity exceeds a certain level the Australian Standards must be adopted for design; the BCA also requires a minimum fall of 1:100 for box gutters.

Using the Australian Standard requires more calculation and under the domestic installations Standard provides solutions for ‘small box gutters’ where the size and flow rate of the stormwater is limited. For larger gutters a more complex design criteria must be adopted under AS/NZS 3500 part 3.

The domestic installations Standard provides a worked example where the rainfall intensity is determined along with the roof catchment area, which considers the slope of roof; a formula provided in the Standard then allows the flow rate into the rainhead to be calculated.

Using the flow rate the required box gutter width and depth is given under a table in the Standard with various width and depth option for different flow rates.

A maximum flow rate of eight litres per second is allowed which equates to a maximum box gutter size of 300mm wide x 130mm deep but there are various size options provided for various flow rates.

The maximum gradient under the Standard is 1:200 which differs from the BCA provisions and there are other considerations such as the width of the rainhead that must equal the box gutter width and the rainhead must be fully sealed to the box gutter with overflow provision.

Box gutters also need to be straight and have uniform slope.

If it is envisaged that the box gutter will be trafficable, in other words expect foot traffic, then the support system must be designed appropriately and if the length of box gutter exceed 12m then expansion joints must be provided.

Valley gutters

Valley gutters may need to be designed as box gutters if below a certain roof pitch and the size of roof catchment exceeds certain limits.

The Standards limit valley gutters to a roof slope of not less than 12.5 degrees with nominal valley gutter side angles of 16.5 degrees and a roof catchment area for the gutter not exceeding 20m².

It should never be assumed that a valley gutter will be suitable for lower pitched roofs and this is something that will need to be addressed at design stage as a different method of construction may be required to accommodate a deeper gutter in a valley.

It is essential that whether designing box gutters under the BCA or Australian Standards the correct size and detailing is achieved to ensure trouble free performance.

For further information HIA members can contact HIA’s Building Services staff on 1300 650 620 or hia_technical@hia.com.au.   

If you would like to become a HIA member, contact 1300 650 620 or enquiry@hia.com.au

The above is intended to provide general information in summary form. The contents do not constitute specific advice and should not be relied upon as such. Formal specific advice should be sought by members with respect to particular matters before taking action.