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Average rainfall, ideal tank size
and roof surface area

Knowing your average rainfall is vital if you want an appropriately-sized rainwater tank for your needs. There are many factors you need to consider when choosing your tank size. These factors include how much water you typically use around your property, the size of your roof and whether you will be relying solely on rainwater for your water needs. But before you worry about any of that, you need to know your average rainfall, as based on historic data.

Knowing your roof size is vital if you want to calculate how much rainwater you can harvest. Your roof surface area is the size of your roof, expressed in square metres (or feet). For single storey homes, your roof surface area will often be greater than your floor area (unless you have a flat roof).  As your roof surface is the catchment area for harvesting rainwater, it’s important to know how large or small it is so you can calculate how much rainwater you can expect to harvest.

Historical Rainfall

Access historical rainfall data

National or local weather services are an excellent source of historical rainfall data for your area. In Australia, the Bureau of Meteorology offers this information online and free of charge.

When considering historical rainfall data, try and look at data for the last 100 years or the longest period available. When looking at this information, consider monthly and yearly data as well as overall averages. This includes considering the longest periods without rainfall.

Looking at the data in this way is important for two reasons.

First, some areas get a great deal of their yearly rainfall at a particular time of year – such as locations with a “wet season”. In these places, a tank or tanks with a larger storage capacity would be necessary to harvest more rainwater during these times and store it for use during times without rainfall.

Secondly, averages can conceal periods of drought or even just year-to-year variations. For example, one town had an average annual rainfall of 1,562mm over 20 years, but this ranged from 1,089mm to 2,892mm in the individual years during this time. In such areas, you may want to consider using a tank or tanks that can handle the larger volume, not the average.

Calculating your roof surface area

Building plans offer the easiest way to calculate your roof surface area – after all, the numbers should all be on there. This calculation option will also give you a very accurate result. Google Maps or other overhead mapping software can also be used to calculate your roof surface area. The accuracy of your results will depend on which tool you use, however, it should give you a good rough calculation.

Alternatively, you can simply head outside with a tape measure and roughly measure the size of your roof at ground level. This will provide a rough calculation that is still useful enough for your purposes.

Aerial Roof Surface
Roof Water Dripping

Calculating your catchment capacity

Roughly speaking, 1 millimetre of rain over 1 square metre of roof equals 1 litre of water.

This capacity can be calculated using the following formula:

Annual rainfall (in millimetres) x Roof surface area (in square metres) = Roof catchment capacity.

This is an important figure to understand so you can design a Rain Harvesting system and choose a tank size that will meet your water volume requirements.

Great Rain Harvesting starts with a great system.
Build yours by following the 4 Pillars of Rain Harvesting System Design.

Find out more

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