Rainwater is a free, perpetually-renewing water supply.
Installing a rainwater tank can be relatively simple and inexpensive, and the benefits are ongoing. Rainwater is a good source of water for garden use and other household uses such as washing clothes, flushing toilets and - if it is properly treated or purified - for drinking.
Is a rainwater system for me?
If your property is not connected to a mains water supply, rainwater may be a viable water supply option.
Even if you’re connected to the mains water supply, you may want to consider using rainwater for your garden. By doing this, you will be able to reduce your water charges and demand on mains water supply.
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.
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 with a tap or connection to a soak hose.
In urban areas, if want to install a rainwater system for more than garden use, please contact the Council on 0800 727 059 and discuss your specific requirements.
Rainwater can contain:
- Bacteria such as giardia, salmonella and E.coli
- 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
- Leaves, soil and other debris
For health reasons, you need to ensure your rainwater system is property set up and maintained.
Roof materials and pipes
Some roofing materials are not suitable for rainwater collection - check with the manufacturer. A typical rainwater system is set up to minimise contamination.
A leaf filter helps keep your rainwater tank free from contamination. 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. The Council may 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. An annual check is recommended for drinking water.
Regular maintenance is vital 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
- Checking the roof and guttering for debris
- Keeping the roof clear of overhanging vegetation
- 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
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.
Buying your rainwater system
A basic rainwater collection tank can be easy to install and relatively inexpensive. Costs vary depending on the tank material, and installation and other requirements such as a building permit (if required). Other costs may include the pipes, filters or treatment, any plumbing requirements, building consent fees, and annual inspection fees.
The size of tank you will need depends on average rainfall, what you intend to use the water for, and whether you have access to mains water supply. 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 town and are short of space, you could install a 'slimline' tank that attaches to a wall on the side of your house. Water is heavy, so even slimline tanks need to be well-supported.
Contact your local rainwater tank supplier (under Water Storage and Tank Manufacturers in the Yellow Pages) or the Council for advice on the appropriate tank to purchase.
The most common tanks are made of plastic (polyethylene), concrete, fibreglass and galvanised 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.
Galvanised steel is relatively inexpensive, but its lifespan is limited by corrosion. Water stored in galvanised tanks may have higher than normal concentrations of zinc, which may be a problem if used for human consumption.
Concrete and fibre tanks are strong and long-lasting. Plastic is tough, durable and relatively lightweight; choose a tank that’s algae-resistant. Plastic or fibreglass tanks are good options in earthquake zones.
Consult a structural engineer and the Council if you are considering placing the tank in the ground.
There’s a risk of overflow from rainwater tanks that are poorly installed or are not 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.
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.
You can fit a tap to the container, or a ‘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.