Installing a rainwater tank is relatively simple and inexpensive, and the benefits are ongoing.

Rainwater is a good source of water for using on your garden. It can also be used for washing your clothes, flushing the toilet, and - if it's properly treated or purified - for drinking and other household uses.

Is it for me?

If your property isn't connected to a mains water supply, rainwater may be the only viable water supply.

Even if you're connected to the mains water supply, you may want to consider using rainwater for your garden or for other household uses. By doing this, you may be able to reduce your demand on mains water supply and water charges.

How do you collect and use rainwater?

In principle, a rainwater collection system is simple: rainwater is collected from your roof and stored in a tank until you need it.

Exactly how you set the system up will depend on how much rainwater you need and what you want to use it for.

To collect rainwater for watering the garden, you might not need anything more complex than a 44-gallon drum or a 200 litre rain barrel with a tap or connection to a soak hose. Generally, systems for outdoor use only rely on gravity with no need for pumps.

You can also collect rainwater for:

• flushing your toilet and doing your laundry
• other household uses such as drinking, bathing and using in the kitchen (but the water will have to be treated or purified).

Legal requirements

Some councils will only let you use rainwater for drinking and some other household uses if it has been treated. It is best to check what your council’s requirements are before you start.  There is usually no problem with installing smaller tanks for garden watering.

If you’re connecting a rainwater system to the plumbing of a house that also has a mains supply, you will need a building consent.  You are also required by law to ensure that the mains water is isolated from the rainwater system. This is achieved by using a backflow prevention device. A qualified plumber must install this and you may be required to have annual checks on this device.

You may also need a consent to install the tank if it’s very large or is elevated.

Choosing the right rainwater system for you

How big?

The size of tank you’ll need depends on your rainfall, on what you intend to use the water for, and whether you have access to mains water supply.  You don’t need to have a huge tank to make a difference – even a rain barrel will reduce your outdoor water use and provide water in an emergency.

For garden watering, you can install either a rain barrel (generally about 240 litres) or a rainwater tank (500 litres +).   In areas such as Auckland with year round rainfall a smaller tank could be enough for the garden, but if you live in an area with dry summers, or you water your garden a lot, you will need a larger tank.

If you are planning to use rainwater for indoor supply, you will need a larger tank.  In areas with year round rain, a 5000 litre tank will provide a good proportion of your water use.  In areas with dry summers, a much larger tank (10,000 litres +) will be required.

If rainwater is your sole source of water, you will need at least a 30,000 litre rainwater tank.

Other factors that might influence the size of your tank include:

• how big your property is - a large garden will need more water
• how big your roof is (if you’re collecting the rainwater off your roof)
• how much security of supply you require.

If you live in the city and are short of space, you could install a slim-line tank that attaches to a wall on the side of your house. Water is heavy, so even slim-line tanks need to be well supported.

Contact your local rainwater tank supplier (under Water Storage and Tank Manufacturers in the Yellow Pages www.yellow.co.nz) or your local council for advice on the capacity of your rainwater tank.

Tank materials

The most common tank materials include plastic (polyethylene), concrete, fibreglass, timber and steel. The type of material you select depends on your budget, the size of tank, water use and whether the tank will be sited above or below ground.

Modern steel tanks have a long life polymer coating on the inside and a wide range of shapes and sizes are available.  There are a range of slim-line designs suitable for urban sections available.   Timber tanks have a plastic bladder liner, and generally come in larger sizes.  

Concrete and fibreglass tanks are strong and long-lasting. Plastic is tough, durable and relatively lightweight, and – like steel – there are a wide range of sizes and shapes suitable for urban environments.

In-ground tanks

Consult a structural engineer if you are considering placing the tank in the ground.

Other considerations

Gravity-fed systems (without need for a pump) will need the barrel or tank on a stand.  Because a litre of water weighs a kilo, a rainwater tank stand needs to be fairly robust, and should be concreted into the ground.  It will need to be over 30cm and less than one metre high – tanks on stands over one metre high generally require a building consent.

A rainwater system supplying the house will need a pump to operate.

When designing your rainwater system, don’t forget an overflow outlet (see section on Overflow) and provide access for cleaning.

Costs

A basic rainwater collection tank is easy to install and can be relatively inexpensive. Costs vary depending on the tank material, and installation and delivery requirements.

Other costs may include the pipes, filters or treatment, any plumbing requirements, building consent fees, and annual inspection fees.

Overflow

There’s a risk of overflow from rainwater tanks that are poorly installed or aren’t big enough to cope with runoff from the roof. Overflow needs to be contained on your property or diverted to the stormwater system. Otherwise, overflow from your tank could damage your property and neighbouring properties, especially in built-up areas. If damage occurs, you could be liable.

Setting up for garden watering

To collect rainwater for your garden, any large, watertight container will do. Set it up so the downpipe from your roof feeds into it, and cover it with a strong insect screen to stop mosquitoes invading and animals from getting trapped. In periods of high rainfall you may need to reconnect your downpipes to the stormwater system to prevent overflowing.

You can fit a tap to the container, or fit a soak hose or ‘dripper’ irrigation system to it. If you use a tap, install it high up or put a lock on it so children can’t drink the water. A warning sign is also a good idea.

To prevent leaves, droppings and other organic matter from contaminating your rainwater:

• Use a 'first flush diverter'. This is a simple, inexpensive device that fits to your tank inlet. It prevents the initial flow of contaminant-laden water from the roof entering the tank when it rains. Contaminants drain off to a suitably planted part of the garden or soakage area.
• Ensure the tank is tightly covered - this also prevents evaporation.
• Use a screen over the tank?s inlet pipe to keep out insects, birds and animals.
• Install covered rainwater-collecting gutters to prevent debris from entering your water tank.