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Design of box gutters for domestic installations

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

Designing a box gutter

There are two methods that can be used to design a box gutter: 

  1. adopting the Building Code of Australia Volume 2 Housing Provisions (BCA) or 
  2. using the relevant Australian Standards for plumbing storm water work AS/NZS 3500 part 3. Both contain information on domestic storm water 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 exceed 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 storm water 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 rain head 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 rain head that must equal the box gutter width, and that the rain head must be fully sealed to the box gutter with overflow provision.

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

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

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

To find out more, contact HIA’s Building Services team.

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