Why Your Rainwater Tank Should Supply More Than the Garden

Many homeowners think of a rainwater tank as something that supplies a single outdoor tap for watering the lawn or garden. While this is useful, it does not take full advantage of the water collected from the roof.

A properly designed rainwater system can also supply toilet cisterns and the cold-water connection for a washing machine. When these connections are planned during construction, rainwater can replace treated mains water for routine household activities that do not require drinking-quality water.

In urban areas, NSW Health supports the use of rainwater for non-drinking purposes, including toilet flushing, washing clothes, garden irrigation, car washing and several other household uses. Where a reliable public water supply is available, mains water generally remains the recommended source for drinking and cooking.

The problem with using rainwater only in the garden

A garden-only rainwater tank may not be used consistently throughout the year.

During dry weather, the stored water can be valuable for lawns, vegetable gardens and landscaping. However, during wet periods, the garden may need little or no additional watering. This is also when the tank is most likely to be full.

If no one is using the stored water, the tank may remain full and simply overflow during the next rainfall. Although the system is still managing some stormwater, much of the available rainwater is not being used.

Connecting the tank to frequently used internal fixtures creates regular demand. Toilets are flushed every day, and most households use their washing machine several times each week. This means rainwater can continue to be used even when the garden is already wet.

NSW water-saving guidance specifically recommends connecting tanks to toilets or washing machines so that the stored water can be used throughout the year, including during periods of high rainfall when outdoor watering is unnecessary.

Using rainwater to flush toilets

Toilet flushing is one of the most practical uses for collected rainwater.

There is little reason to use drinking-quality mains water to fill a toilet cistern when suitable rainwater is available. Once the system has been installed, the household does not need to do anything differently. The toilet operates normally, but the cistern is supplied from the rainwater tank whenever sufficient water is available.

For a family home with several occupants and multiple bathrooms, toilet flushing can create regular demand on the tank. This helps reduce mains-water consumption while also creating additional storage capacity before the next rainfall.

The system should normally include an automatic switching device or another approved backup arrangement. When there is water in the tank, the pump supplies the connected toilets. When the tank becomes empty, the system switches to mains water, allowing the toilets to continue operating without interruption.

Using rainwater in the washing machine

Rainwater can also be connected to the cold-water supply for the washing machine.

The connection may be provided through a dedicated rainwater tap in the laundry or through an integrated rainwater supply system designed by the plumber. The pump and pipework must provide suitable flow and pressure for the appliance.

Using tank water in the washing machine can be particularly effective because laundry is a regular household activity. Unlike garden watering, it continues during rainy weeks and throughout winter.

This regular use helps turn a rainwater tank from an occasional garden accessory into a functioning part of the home’s water system.

The tank, roof catchment, first-flush device, screens, pump and any filters must be properly maintained. Clean gutters and effective inlet screens reduce the amount of leaves, sediment, insects and roof debris entering the tank. A first-flush device diverts the initial roof run-off after a dry period, when dust and bird droppings are most likely to be washed from the roof.

How an automatic rainwater system works

A typical system collects water from selected sections of the roof through the gutters and downpipes. The water passes through screens and, preferably, a first-flush device before entering the tank.

A pump then supplies the nominated fixtures, which may include:

• toilet cisterns;
• the washing machine’s cold-water connection;
• outdoor garden taps; and
• an irrigation system.

An automatic controller gives priority to rainwater. When the water level becomes too low, the system changes to mains water or activates an approved mains-water backup arrangement.

The drinking-water system and rainwater system must remain properly separated. Backflow protection, identified pipework and compliant switching or top-up equipment are essential to prevent rainwater from entering the mains drinking-water supply. Sydney Water advises homeowners to check site-specific backflow requirements when connecting a tank to toilets or a washing machine.

Why the connection should be planned during construction

It is possible to add internal rainwater connections to an existing home, but retrofitting can be significantly more complicated.

The plumber may need to install new pipework through completed walls or ceilings, excavate sections of the yard, add exposed external pipes or find a practical route between the tank, toilets and laundry. Electrical work may also be needed for the pump and switching equipment.

During construction, the builder and plumber can plan the entire system before the walls are closed and the landscaping is completed. This allows space to be provided for:

• the tank and its supporting base;
• suitable roof catchment and downpipe connections;
• separate rainwater pipes to the toilets and laundry;
• a pump and weather-protected electrical supply;
• an automatic mains-water switching device;
• tank overflow connected to the stormwater system;
• maintenance access; and
• compliant backflow protection and identification of rainwater services.

The tank location should be considered early. A large tank becomes extremely heavy when full: 1,000 litres of water weighs approximately one tonne. It therefore requires an appropriate stable base and must not be placed over water or wastewater mains. Planning the location early can also reduce pipe lengths, improve access and avoid losing a useful part of the backyard.

Tank size is only part of the decision

Homeowners often focus on the number of litres the tank can hold. Capacity is important, but it is not the only factor determining how useful the system will be.

A large tank connected to only a small roof area may take a long time to fill. A tank connected to a large roof catchment but rarely used may frequently overflow. A smaller, well-designed tank connected to regularly used fixtures may sometimes provide better practical value.

The design should consider:

• local rainfall;
• available roof catchment;
• the number of occupants;
• the number of toilets;
• expected laundry use;
• garden and irrigation requirements;
• available space;
• pump performance; and
• any BASIX commitments or council requirements.

In NSW, BASIX considers the capacity of the tank, the roof area connected to it and the household uses to which the collected water will be allocated. These details should match the approved documentation and the completed installation.

Greater savings from the same tank

A rainwater tank represents an investment in the tank itself, its base, downpipes, plumbing, pump and installation. Connecting it only to one garden tap may leave much of that investment underused.

Sydney Water states that a properly installed tank plumbed into a home could save up to 40 per cent of the property’s drinking-water supply. Actual savings will depend on rainfall, roof catchment, tank capacity, household consumption and the fixtures connected to the system.

The strongest results generally come from combining outdoor and indoor uses. Garden irrigation may consume substantial water during dry periods, while toilets and laundry create a steadier demand throughout the year.

Regular use can also leave more empty capacity in the tank before another storm, helping the system capture more roof run-off rather than immediately discharging it through the overflow.

Installation and maintenance still matter

A rainwater tank is a low-maintenance system, but it is not a no-maintenance system.

Gutters, downpipes and inlet screens should be kept clear. Mosquito-proof mesh must remain sealed and undamaged. The first-flush device should be checked and cleaned, and the tank should occasionally be inspected for sediment accumulation.

The pump and automatic switching system should also be tested periodically. A simple practical check is to flush a connected toilet or operate the washing machine and confirm that the rainwater pump starts correctly.

Internal connections should be completed by a licensed plumber. Depending on the installation and local requirements, plumbing inspections, backflow protection and a Certificate of Compliance may also be required. Local council and water-authority requirements should be confirmed before construction.

A small decision that can improve the home for years

A rainwater tank should not be treated as an isolated object placed beside the house at the end of the project. It can be part of a practical whole-of-home water strategy.

Connecting it to the toilets and washing machine can reduce reliance on treated mains water, make better use of rainfall throughout the year and increase the value obtained from the tank, pump and plumbing system.

The most important time to make this decision is before construction begins. Once the pipe routes, tank location, electrical supply and nominated fixtures are included in the plans, the system can be installed neatly and efficiently as part of the build.

How ARK Houses can help

At ARK Houses, we consider how a rainwater tank will actually be used, not merely where it will be placed. When planning a new house, duplex or dual-key development, we can incorporate suitable roof catchment, tank positioning, pump infrastructure and connections to toilets, laundry and outdoor taps so the completed system provides practical water savings long after construction is finished.

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