Gutter and Downpipes

A report recently released by one of Australia’s prominent property services providers published a list of the most common building defects impacting Australian properties.

Featuring high on that list were reported issues with: water leaks (42%), exterior water penetration (40%), guttering problems (25%), defective roof coverings (23%), and plumbing faults (22%).

Compliance with AS/NZS 3500.3:2021 specifies that eaves gutters, including downpipes, shall be designed and installed so that water will not flow back into the building. This article looks at the process of determining gutter size and the number and size of required downpipes for a Class 1 building with hipped or gabled roofs with a constant slope.

Roof drainage systems shall be designed for the average exceedance probability (AEP) for the site to prevent what is termed overtopping (water rising above the highest level of the gutter).

Rainfall intensity is expressed in millimetres per hour (mm/h) for a rain event of five-minute duration for any location in Australia and for sizing external eaves gutters an AEP of 5% is generally accepted. 5% is equal to 20 year average recurrence interval (ARI). Appendix D in AS/NZS 3500.3.2021 supplies references for selected Australian cities. The rainfall intensity for other locations may be determined from data gained from the Bureau of Meteorology.

Step 1: To calculate the size of an eaves gutter and the required number of down pipes, we must first calculate the size of the rainwater catchment area. For this example, we will use the area of the roof. This can be determined from building plans. The true area in m² can be calculated by multiplying the roof area expressed in m² by the relative multiplier factor that relates to the roof slope expressed in degrees (refer to AS/NZS 3500.3:2021, Table 3.4.3.2).

Example:
Roof area 250m² Roof Slope 22⁰
Area = 250 X 1.2 (multiplier factor from Table 3.4.3.2)
= 305m²

Step 2: Select the rainfall intensity for the location from Appendix D, Table D.1. For this example, we will choose Townsville as the location where we will use the AEP 5% (20 years ARI intensity) of 235mm/h.

Step 3: The ABCB Housing Provisions requires that eaves gutters must be installed with a fall of not less than 1:500 and the maximum length of gutter served by each downpipe is not greater than 12m. A gradient ≥ 1:500 is considered effective so as not to cause any permanent ponding. AS/ NZS3500.1:2021 also requires that any deviations from nominal gradients shall be smooth and not cause any obstructions to flow.

Step 4: To select a type and size of eaves gutter to be used, a reference must be made to the manufacturer’s literature. For this example, we reference Lysaght Rainwater Solutions Technical data. The table shown (left) provides information about a range of eaves gutter types. We have selected the Trimline gutter fitted with overflow slots. From the literature supplied the effective cross section of this gutter is 6244mm² and you can select between 90mm round or 100mm x 50mm as minimum downpipe sizes. The effective cross section of a gutter relates to the carrying capacity of the selected gutter.

Step 5: As has become common practice, we nominate 90 diameter downpipes for this project. From the graph shown, we can determine the number of downpipes required using the effective cross section area for the chosen gutter shown horizontally at the bottom of the graph. From that point, we draw a line vertically until it intersects with the design rainfall for the area of 235mm (point selected by interpolation). From that point, move horizontally to the left to determine the recommended catchment area in m² for each vertical downpipe.

From the chart, a gutter with a cross section area of 6244mm² and a design rainfall intensity of 235mm with a roof catchment area of 305m² will require a minimum of 12* x 90 diameter downpipes each with a catchment area of 26m². *((305 ÷26) = 11.73 rounded to 12).

The design flow in litres per second for each downpipe can be calculated using the following method:

The table and chart used in the article are adapted from the Rainwater Solutions QLD publication issued by Lysaght, dated September 2023. Lysaght recommends referring to the complete version of the publication when designing eaves guttering systems. The more recent version of the publication is available at lysaght.com.