Collecting and using rainwater
Rainwater is a free, perpetually-renewing water supply.
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).
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
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.
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.
Consult a structural engineer if you are considering placing the tank in the ground.
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.
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.
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.
Rainwater can contain:
- leaves, soil and other debris
- bird, possum and other animal droppings
- heavy metals such as lead from your roof
- ash and chemical residues - for example, from agricultural spraying and vehicle emissions.
To protect your health, you’ll need to ensure your rainwater system is properly set up and maintained.
Roof materials and pipes
Some roofing materials aren’t suitable for rainwater collection. Check with the manufacturer.
If there’s lead, chromium or cadmium in the roof materials, soldering, flashings, paint or any other part of the roof, you shouldn’t collect rainwater from the roof.
Rainwater can also react with uncoated metal roofs, so any metal roof should be painted (using a roof paint that is labelled ‘suitable for potable water supply’) before rainwater is collected.
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.
Treating and purifying water for drinking
If you want to drink your rainwater or use it for any household use other than flushing the toilet and washing clothes, you'll need to treat it or purify it. Options include:
- adding chlorine
- using a filter or purifier
- boiling the water for one minute
- ultraviolet light treatment.
This will involve added costs. Check with your local authority, your local public health service (under Public Health Service in the White Pages or on the Ministry of Health's website) or your rainwater tank supplier for guidance and requirements on water treatment. Some councils require annual testing of rainwater tanks used for drinking water.
You can have water tested by a specialist water-testing laboratory. Look under Laboratories in the Yellow Pages (www.yellow.co.nz). An annual check is recommended for drinking water.
Regular maintenance is vital especially if you use rainwater for household use. Maintenance should include:
- desludging your tank yearly, using the sediment removal tap at the base of the tank, if there is one - this takes about 20 minutes
- checking the roof and guttering for debris
- keeping the roof clear of overhanging vegetation
- making sure your roof remains clean, especially from bird droppings
- regularly checking and maintaining screens and filters
- washing out the first flush diverter every six months or so, depending on your rainfall (this only takes 10 minutes)
- checking the condition of the tank's pipes, fittings and structural supports and inspecting the tank for cracks and leaks, particularly before it gets dry over summer.
- servicing any sterilisation equipment (as recommended by the supplier).
It’s also a good idea to drain and clean your tank every so often. How often depends on what gets into your tank, and on how often you remove sludge and sediment. Every five years is recommended.
From Smarter Homes
- Easy ways to save water
- Reducing water flow
- Re-using greywater
- Managing stormwater
- Outdoor water use
- Onsite sewage systems
- Landforms and waterways
- Planting and landscaping
You'll need to contact your local council before you install any rainwater collection system. Consumerbuild' Council finder web page has contact details for New Zealand local authorities.
From other sites
The Ministry of Health pamphlet Water Collection Tanks and Safe Household Water, covers safety issues relating to tank water.
The Ministry of Health pamphlet Household Water Supplies, covers the selection, operation and maintenance of tanks and other individual household water supplies.
You can download NZ Water and Wastes Association booklets The story of drinking water and Savings in your House from the Association's website.
Before you install any rainwater collection system, contact your local council. Visit the Local Government website to check local authority boundaries and link to council websites.
Kapiti Coast District Council’s website has guides on getting old concrete tanks back into action for garden watering and using rainwater for garden irrigation.
The Ecobob website has articles and case studies on rainwater collection and re-use, and links to products and suppliers.