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Rainwater collection

Make use of every precious drop

There’s something special about using rainwater around the home. Rainwater is free and there are no restrictions on when or where you can use it. By making the most of every precious drop you can save money on water bills at the same time.

It’s possible to start small with one rainwater tank and then add another later. You can start with a tank simply for the garden, then stage it so that you make rainwater connections to the house as you can afford them.

Rainwater can potentially be used for all purposes you use water for around the home. It can be collected off surfaces outside the home as stormwater, and directed to the garden where it can soak into the soil. The limits on what you use it for will depend on how much you have available and on the quality of the rainwater.

Explore the topics below and use Renew’s free Tankulator tool to plan a rainwater system for your home.

 

Special gutters, flap valves and leaf diverters can help to keep rainwater clean. Image: David Johns.
Where can I use rainwater in the home?

Water Not Down the Drain author Stuart McQuire says you can use rainwater for anything you use mains water for within the home. He reduced his mains water use to just two buckets a day at one point by using rainwater in the bathroom, laundry, toilet, kitchen and garden.

Stuart recommends working out how much rainwater you have available and then matching it to a particular use.

WORK OUT WHERE YOU WANT THE TANK. ITS LOCATION WILL DETERMINE MANY TANK SPECIFICATIONS

The first decision you have to make is where the tank will be located. Where you place the tank will determine its size and shape, and possibly even its colour if it needs to blend into the surrounding vegetation or dwelling walls. A large yard offers a number of options. You could place it next to the house or shed, or even under the house.

You also need to consider how the water will get from the roof into the tank, as well as overflow piping. However, there are a number of different systems for plumbing a tank to a home’s gutters that allow a tank to be situated some distance from the home, so this should probably not be an overriding consideration.

Use Tankulator and other methods to work out how much rainwater you can collect

To work out how much rainwater is available you need to know how much rain falls in your area, as well as the size of your roof. Use Tankulator, an online rain harvesting calculator developed by Renew to help work out this equation and plan the right-sized tank. You can also work out the figures yourself using the following method.

The roof of your house, garage or shed is your catchment for rainwater. The horizontal dimensions of buildings will give you the roof area, by multiplying the length by the width. Pace around the outside of the buildings or use a tape measure to get the dimensions. It may not be possible to harvest water from the whole roof, but this will give you the figure for the maximum roof area available as catchment for rainwater.

You can use the rainfall records for your area to calculate how much rain falls on your roof. Some rain will be lost due to evaporation or spillage, so assuming that 85 per cent is harvested, with 15 per cent lost, use the figure of 0.85 to convert the rainfall into the amount that you could potentially harvest in tanks.

Volume of rainwater available for tanks = roof area x rainfall x 0.85

Use Tankulator here.

What size does a rainwater tank need to be?

The access and space for tanks will be limiting factors, but beyond that, how big should tanks be? The ideal size will depend on what you want to use the water for, the number of people in the household, and also your budget, roof size and rainfall. Households without mains water supply would typically have rainwater tanks in the range of 50,000 to 100,000 litres. For a suburban or urban situation you are unlikely to have space for tanks that big. Bigger tanks are usually needed for outdoor use on the garden, but smaller tanks can be used when rainwater is used indoors. For example, tanks just for the toilet or laundry can be much smaller than tanks for the garden. This is because the garden may need a whole lot of water at once, rather than small amounts regularly.

The Tankulator was built by Renew staff as a tool for householders who are either seeking to install a rainwater system in their homes or improve on their current rainwater system. We have come across many tanks which are left full of water in backyards, going unused, whereas an efficient tank is one which cycles water regularly and provides an important, independent source of water for householders and businesses.

SIZING A TANK TO COVER TOTAL HOUSEHOLD SUPPLY

If your rainwater tanks are to be your only water supply then you will probably need tanks in the range of 50,000 to 100,000 litres. Use Renew’s Tankulator to help with the sizing equations, and also ask local tank suppliers to advise what size is appropriate, based on their experience in the local area.

Stuart McQuire in his book Water Not Down the Drain recommends using a calculation that checks the expected rainfall in the area against your expected water use. To use this method you need to know the average monthly rainfall figures for your area, along with your typical water use. Find out more in Water Not Down the Drain.

Find out more about tank materials and their pros and cons

The six most common rainwater tank materials are concrete, fibreglass, plastic (usually polyethylene, often just called ‘poly’, or PVC, used in flexible bladder tanks), Aquaplate Colorbond (thin sheet steel with a colour coating on the outside and a waterproof coating on the inside), galvanised steel and stainless steel. Each of these materials has advantages and disadvantages, which we explain further, including the pros and cons of each in Renew magazine’s Rainwater Tank Buyers Guide.

Considerations include durability, reparability, material sustainability and water quality. For example, from a durability viewpoint, a plastic tank might be the best solution for bore or dam water, as plastic tanks are relatively immune to damage from salty water. However, if your tank only needs to hold rainwater, then any tank material should be suitable.

From a material sustainability point of view, stainless steel tanks generally don’t corrode to the level that coated steel tanks do, so when it comes time to scrap them, most scrap metal dealers will actually pay you for them, as stainless steel is quite valuable. There’s a lot to find out, more info in the latest Buyers Guide here.

UNDERGROUND, ABOVE GROUND, UNDER HOUSE – WHAT TANK CHOICES AND STYLES ARE OUT THERE?

Once you have decided on the tank’s material, you then have to work out the size (capacity) and shape, and whether you want it underground or above ground.

Above-ground tanks are usually of the traditional round variety, or of the at-sided oval or rectangular ‘slimline’ style for narrow spaces. There are even tanks that are so narrow they can be used as walls, and others that consist of small modular tanks that can be assembled into larger tanks of almost any shape.

If you highly value aesthetics, then poly tanks may well be your first choice as they have clean, smooth lines and come in a huge range of colours and styles.

For high-density living, underground tanks can often be the solution, and these are available as poly tanks or concrete, which is often cast in-situ. Concrete tanks are usually reinforced internally and as such can support heavy loads, so they can be placed under driveways. Some underground poly tanks are also well reinforced and can withstand considerable loads.

If your house is mounted on stumps, or you have a post-mounted deck, you could consider a bladder tank. These consist of a metal frame around a flexible plastic or rubber bladder. As the bladder fills it expands to its maximum size.

Getting rainwater into the home

To use rainwater in the house, plumbing connections will need to be made to the pipe network either at an incoming point or at different points of the house. You will need to get a licensed plumber to make these connections and authorised backflow prevention devices will need to be used where connections are made with mains water pipes. These prevent water flowing from the rainwater supply into the mains water supply.

Water Not Down the Drain includes more information on difficulties accessing pipework in newer houses.

With a large enough rainwater supply you may decide to go all the way and connect rainwater to everything in the house including drinking water. This makes plumbing relatively easy because you won’t need to refit plumbing to all the different fixtures in the house. A plumbing connection can be made at the point the mains water comes into the house, before it branches to all the different fixtures.

Keeping your rainwater supply clean

The greatest risk of contamination comes not from your tank, but from your roof and gutters. There are a number of tank accessories that can help avoid any contamination problems, as well as other accessories that can make owning a tank that much more useful.

Between downpours, the average roof collects contaminants from sources such as bird and other animal droppings, pollution from vehicles, stoves and heaters, and roof coatings and sealants. All of these wash down into your tank with the first flow of rainwater—unless, of course, you have fitted a first-flush device.

These divert the initial flow of water from the roof to the stormwater drain, thus preventing most contaminants from entering your tank. Many water tanks are supplied with these devices as part of the package, but if not, then make sure you fit one, even if you are not intending to drink the water, as it will reduce the build-up of sludge in the tank.

Find out more about devices for keeping rainwater clean in the Renew magazine article Keeping it Clean.

PLANNING A RAINWATER SYSTEM IN A BUSHFIRE ZONE

If you live in a rural area, or even the urban fringe, you should take into account the effects of bushfire on your water tank.

Contamination of tanks from floating ash can be a big issue, and a first-flush device can reduce this problem considerably. Residents in bushfire-risk areas will also need to attach fire-brigade fittings to their tanks and ensure that fire tankers can access the rainwater tank.

Of course, fire doesn’t just produce a lot of ash, it also has a habit of burning things. Poly tanks can melt down to the waterline if a bushfire gets too close, but in severe fires even metal and concrete tanks can become unusable. If you live in a fire-prone area then underground tanks might be your best bet. The 2007 Bushfire Cooperative Research Centre/CSIRO report Research into the Performance of Water Tanks in Bushfires details the effect of fire on different tanks.

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