When retrofitting to an older existing home there are several things you need to consider such as what size tank and how much water will be collected.
When planning to install a rainwater tank, you should determine:
The intended use of rainwater
Volume of water needed for these uses
Roof area supplying the tank
Using Toowoomba rainfall records, a 10,000 litre (2,200 gallon) rainwater tank internally connected to the toilet and washing machine could be expected to generally supply 20-30% of the average daily water needs for a typical household. Using a 22,000 litre tank (5,000 gallon) would increase this figure to 30-40%.
Amount of water to be collected
It is important to consider the roof catchment area that can service the tank. Many installations use only about half the roof’s total area. Increasing the tank’s catchment is an effective means of improving the tank’s reliability of supply.
An easy way to work out how much water you may collect in a rainfall event is to remember that 1 litre of water = 1 square metre of roof area x 1 millimetre. This means that if you received 10 millimetres of rainfall then every 1 square metre of roof area draining to the tank will collect 10 litres.
Restrictions on tank water usage
Depending on the way the tank has been connected to the household's plumbing system, tank water may be regarded as town water. If this is the case, under medium to extreme water restriction levels, tank water would be subject to limited or no outdoor water use
If you would like information on how to install a rainwater tank or any other regulations applying to tank installation please refer to INFO 26 Requirements for Installation of Rainwater Tanks.