In Australia, the number of downpipes required for a house is governed by the National Construction Code (NCC). A single downpipe must not serve more than 12 metres of gutter length. This ensures effective fall and adequate capacity to discharge all water during heavy rain or a storm. This 12-metre rule is just a baseline, and determining the actual number of downpipes depends on factors such as the roof catchment area and the eave gutter design.
What Determines How Many Downpipes You Need
Roof Size and Catchment Area
Roof catchment area refers to the portion of the roof that drains to a specific gutter run. Larger catchment areas deliver more water to the same section of gutter. As the catchment area increases, either the number of downpipes or their distribution must increase to prevent overload.
Roof Shape and Layout
Complex roof designs concentrate water runoff into certain gutters. Valleys, multiple roof planes, and long gutter runs all increase demand on downpipes. A long gutter run with only one outlet is more likely to overflow than a run with multiple outlets.
Rainfall Intensity
Rainfall in Western Australia is often short and intense rather than long and steady. Even brief storms can generate high runoff. This means downpipe systems must be able to cope with peak rain intensity, not just average rainfall.
Estimate How Many Downpipes Your House Needs
The estimated number of downpipes your guttering system requires can be determined using the lineal meterage of your guttering. Start by measuring the length of each gutter run from the ground. Add up the total length of your gutter system and divide it by 12 to determine the least amount of downpipes your house requires.
For example, if the total length of your guttering system is 56 metres, you would require at least 5 downpipes (56 ÷ 12 = 4.66, round up to 5).
It’s helpful to sketch a rough outline of your roof to help visualise your measurements. You can also apply the 12 metre rule to your rough outline measurements and select the best location for each downpipe. Make sure that any gutter runs that are longer than 12 metres are being served by at least two downpipes.
Please note that this is just an approximate figure. To get a more accurate number of downpipes required, you will need to determine the following information:
- Rainfall Intensity for your location: Typically measured as the Average Recurrence Interval (ARI) for 5% AEP (1 in 20) for 5 mins duration (mm/hr).
- Roof Catchment Area: Calculated using the roof area (m²) and the roof’s average slope (degrees).
Before starting any work, it is important to check your calculations with your local building authority. If you have any further questions about downpipes or gutters, contact Dynamic Steelform. Our team will work with you to find the ideal guttering system for your project.