Rainfall Intensity

Heavy rainWhen most people think about rainfall they think about the amount of rainfall . . . . . . . . . but it’s just as important to understand how hard it can rain, the  ‘Rainfall Intensity’.

The ‘Rainfall Intensity’ is important when thinking about things like; Roof Gutters, Down Pipes, Stormwater Pipes , Tank Overflows, and even how high your house is above ground level.

Photo courtesy of www.mirror.co.uk

Recurrence Interval

When talking about heavy rainfall you will frequently hear talk on the news about things like a ‘1 in a 100 year storm’. What that means is that a statistical calculation indicates that a storm of that strength is only expected to occur once in any 100 year period, the ‘Recurrence Interval’.

There are two rainfall intensities that the building codes normally consider. They are based on the peak 5 minutes during a storm for vaious ‘Recurrence Intervals’.

Intervals can be from: 1 in 20 years to: 1 in 100 years (See this link for more information: Roof Choices)

Intensity Calculation

Statistical intensities have been calculated for all locations in Australia and are available at the Bureau of Meteorology (BOM) Website. at this link: Intensity Frequency Duration (IFD)

To Use the IFD

  1. Enter “your town name ”  “coordinates”  into Google and write down the coordinates
  2. Click on the Create An IFD button
  3. Enter the coordinates into the Decimal Coordinate box
  4. Enter the town name in the Location Name box
  5. Tick that you accept the Conditions of Use
  6. Press the Submit button
  7. Click on the Table tab
  8. The following is the top line of the table for Werribee in Victoria.


1 year


2 years


5 years


10 years


20 years


50 years


100 years

5 mins








The units are mm of rain in 5 mins (Multiply by 12 to get an hourly flow rate)

Low Rainfall Intensity

Parts of the Plumbing and Building Codes refer to areas of “Low Rainfall Intensity”.

A location  of “Low Rainfall Intensity” means the 5 minute rainfall intensity for an average recurrence interval of 20 years is not more than 125 mm/hour.


For more on rainwater plumbing see Drainage

Rainwater includes more on rainwater collection