What to consider when retrofitting a tank

From 1 October 2005 the Toowoomba Planning Scheme required all new buildings, whether residential, commercial or industrial, to be fitted with rainwater tanks. State legislation (Qld Development Code 4.2) made this requirement mandatory in 2006. However, when retrofitting to an older existing home there are several things you need to consider (refer to the related document below for a summary of these points).

What size tank should I buy?

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%.

How much water will the tank collect?

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 the following:

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.

Are there any restrictions on the use of the tank water?

Depending on the way the tank has been connected to the household's plumbing system, tank water maybe 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 (refer to related document below).

Are there any restrictions on where the tank should be sited?

If you would like information on how to install a rainwater tank or any other regulations applying to tank installation please refer to Planning & building - Building - Rainwater tanks. Check the related Plumbing information sheet INFO 26 Requirements for Installation of Rainwater Tanks.