Collecting and using rainwater
- Pic: Craig Robertson Photography for Beacon Pathway Ltd
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 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.
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 with a tap or connection to a soak hose.
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
If you’re connecting a rainwater tank to your household plumbing, you’ll need a building consent. You may also need a consent to install the tank.
Some councils will only let you use rainwater for drinking and some other household uses if it has been treated. Some will require you to have your rainwater collection system inspected annually.
If you’re connecting a rainwater system to the plumbing of a house that also has a mains supply, you’re required by law to ensure that the mains water is isolated from the rainwater system. This is achieved by using a ‘dual check valve mechanism’, or a backflow prevention device. A qualified plumber must install this and a building consent is required. You may also be required to have annual checks on this device.
Safety
Rainwater can contain:
- campylobacter, giardia, cryptosporidium, 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.
To protect your health, you’ll need to ensure your rainwater system is property 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 labeled ‘suitable for potable water supply’) before rainwater is collected.
Concern has also been expressed about the suitability of PVC, because of the possibility of chemicals leaching. There are alternatives.
- Typical rainwater system set up to minimise contamination
Preventing 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. Some councils require annual testing of rainwater tanks used for drinking water.
Testing
You can have water tested by a specialist water-testing laboratory. Look under Laboratories in the Yellow Pages (www.yellowpages.co.nz). An annual check is recommended for drinking water.
Maintenance
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 - this takes about 20 minutes
- 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
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.
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.
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 '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 www.yellowpages.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 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. Rainwater stored in galvanised tanks may have higher than normal concentrations of zinc, which may be a problem if you drink your rainwater.
Concrete and fibre tanks are strong and long-lasting. Plastic is tough, durable and relatively lightweight; choose a tank that’s algae-resistant.
If you live in an earthquake zone, plastic or fibreglass tanks are good options.
In-ground tanks
Consult a structural engineer if you are considering placing the tank in the ground.
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.
Garden water
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.
More information
From Smarter Homes
- Easy ways to save water
- Reducing water flow
- Re-using greywater
- Managing stormwater
- Outdoor water use
- Onsite sewerage systems
- Landforms and waterways
- Planting and landscaping
From ConsumerBuild
You’ll need to contact your local council before you install any rainwater collection system. Consumerbuild’s 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 (PDF, 251KB), covers safety issues relating to tank water.
The Ministry of Health pamphlet Household Water Supplies (PDF, 1.54MB), 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 Water (PDF, 5.4MB) and Savings in your House (PDF, 5.5MB), from the association's website.
