Appropriate Roof Surface Materials for Rain Harvesting Systems
Your roofing material plays a key role in ensuring you harvest quality rainwater - not dangerous rainwater
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.
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.
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.