When you are buying a rainwater tank for your house, what do you need to know .

When you are buying a rainwater tank for your house, what do you need to know .


If you are searching around at the moment looking at the different options for rainwater storage for your house, then there are a couple of things that you need to consider.  The main things are how much water do I need, how much water can I catch and what can I afford to build for my budget.

How much water am I going to need?

The first thing that you should be considering when planning a large rainwater tank is how much water you are going to be using. In the WA climate, we catch most of our rainwater during the winter months and then we need to ensure that the tank of water is then going to last through summer so that we don’t run out before the rains start again.

Generally with rainwater caught off your roof, it is used for household water inside the house. Even though you may have 200 or 300,000L of water, this is nowhere near enough to irrigate your gardens and farm for the season so you will need to find other sources (a bore, dam or river) for outside uses.

According to recent studies by the Water Corporation, the average WA household uses 56,000L of water per person annually for indoor water use. This means that for every person that is living in your house, they consume approximately 155L each per day. So with a family of 4 that has visitors staying occasionally, you could need 225,000 -250,000L of water per year.

As your tank should fill up by the end of winter, you may not need a 250,000L tank but simply enough water in storage to get you through the 6-7 months of dry weather with a bit of buffer. If you use the example of the 4 person family, then you will need 125,000L plus a safety margin to get you through summer. This means that you will need at least a 150,000L rain water tank to get your family through the year without running out. As a rainwater tank has a 25-30 year life span, you also need to plan for if your family will grow too. If another baby comes along or more people move in, then your are going to need more water.

The table below gives you an estimate of how much water your family will need.

Table 1 Average water requirements for a WA house (litres)

Number of People in house 1 2 3 4 5 6
@ 155L each per dayAt home for 355 days per year 55800 111600 167400 223200 279000 334800
With a 10% safety margin 61380 122760 184140 245520 306900 368280


How much water can I catch?

The next part of the equation is that you need to work out how much water it is possible to catch from either you current or planned house and sheds. If the buildings are only at planning stage at the moment then this is a good check to ensure that you will be able to catch enough rainwater with the roof area you are building. If the buildings are already there, then you can easily calculate the amount of water you should be able to catch. If this matches up with the amount of water that you are going to use or you will have a surplus then everything will work. If you have a deficit in the amount of water that you are catching then you can:

  • Build more roof space.
  • Add guttering and downpipes to catch from more of the existing roof areas.
  • Supplement the rainwater from an alternative supply (ie a bore.

gutter catching Rainwater












To calculate the amount of rainwater that you can catch, you need to know 2 things:

  1. The combined roof area that you are going to catch rainwater off in square meters (m2).
  2. The average annual rainfall where your house is (in mm.)A millimetre of rain equals 1L/m2. So if 1mm of rain falls, you will be able to catch 1L for every 1m2 of roof catchment you have.

So as an example, if you have a 300m2 house in the Bunbury area which has an average rainfall of 700mm, then you should be able to catch 300 x 700 = 210,000L of water off your house. The form below lets you know how much water you can catch off your roof which determines the size of tank you can install.

Go To Our Rainwater Tank Size Calculator

Even though you can theoretically catch a certain amount of water off your roof every year, there are certain things that need to be considered still before you can work out what the most suitable tank is for your household.

The first is the layout of your property and the available space you have for a water tank. A 150,000L tank that in the South West would cater for a family of 5, has a very large footprint and there may not be enough space to fit this where it will catch all the water. Remember that you need to have a minimum of 300-500mm of fall from your lowest gutter to the tank inlet so that the water will flow in properly. So generally your water tank needs to be at or slightly below the level of your buildings so that it will fill up properly. It also has to be close enough so that the downpipes are easily plumbed in and the overflow from the tank will not flood an important part of your house.

Table 2 Dimensions of common tank sizes

Tank Volume (L) Wall Height Diameter Tank Pad
50,000L 2.32m 5.25m 7.25m
75,000L 2.0m 7.0m 9.0m
97,000L 2.0m 7.88m 9.88m
120,000L 2.0m 8.75m 9.75m
145,000L 2.0m 9.63m 10.63m
173,000L 2.0m 10.5m 11.5m


If the footprint of the tank you need is too big or won’t work for the layout of your property, then there are alternative wall heights and diameters to fit a larger volume in a smaller space. You could also look either at a poly sump tank and pumping the rainwater up to a tank in a more suitable spot or having two smaller tanks that will hold the desired volume of rainwater.

What does a rainwater tank cost?

There are 4 main costs that are involved in installing a large rainwater tank. They are:

  1. Site preparation and earthworks.
  2. Supply and installation of the tank.
  3. Installation of the downpipes and inlet pipework.
  4. Installation of a pressure pump and water filtration.

While the tank maybe only quoted at $8000-$12,000 installed, this will only cover the supply and installation of the tank itself. This only a part of the total costs of the job and you need to ensure that you have budgeted for everything you will need.

The site preparation and sand pad for a large water tank should be completed by an experienced earthworks operator. Generally the costs will be between $1000 and $2000 and when it is completed the pad will be even, level and made from a quality screened brickies or river sand. If a cheap fill sand is used, it may not hold the weight of the tank properly and over time you could risk collapse. Also if the pad is not up to standard, then the tank install crew will either make it good (at an hourly rate) or walk away until you have got it up to standard.

Tank pad layout design



Once the tank is installed, then there has to be water placed into the tank on completion for ballast. If you don’t have clean bore water or rainwater on site, then a tanker of water could cost $400 delivered. In addition so that the tank is not undermined and the warranty is maintained, the surroundings of the tank need to have blue metal spread around them which could cost between $200 to $400 depending on the tank size.

If you want to save a few dollars, then downpipes are something that you can do yourself and get it right. If you want to leave it up to a competent contractor, then depending on the amount of trenching and pipe needed, it could cost anywhere from $500 to $2000 to complete all the required pipes. Also make sure that there a good first flush diverter system installed and that the fall of the downpipes is sufficient to let enough water flow into your tank.

The last and possibly the most important part of your water supply is the pressure pump and the filtration system. You need a pressure pump installed to supply constant water pressure into your house. For a quality pump you are looking at $600-$1800 and you pay more for better features, better quality materials and better support and warranty. If you are installing a pump, then you also need to ensure that there is power connected at the tank too. You will need at least a double outdoor GPO installed where the pump is installed and this installed by a competent contractor will cost somewhere between $500 and $1500 depending on cable distance and complexity.

While rainwater might seem pure, your house will catch a lot of dust, pollutants, leaves and bird droppings. This will combine to create a lot of impurities and microbes that need to be sterilised and filtered out of the water so that it is safe for your family to drink. This is also one reason why a good first flush diverter is important as this stops a lot of this getting into the tank to start with. A filtration system can be as simple as a single cartridge filter but it is highly recommended to install a double “stepped filtration” cartridge followed by a UV steriliser light so the water is completely safe for your family to drink. Filtration for a whole house will cost somewhere between $200-$1500 plus the installation of about $500. You should not overlook it though as it is very important for your families health.

So in summary, the costs of a tank install can be:

  • Site preparation $1000-2000
  • Tank install $8000-$12000
  • Water and bluemetal $600-$800
  • Downpipes $500-$2000
  • Pump and power point $1100 – $3300
  • Filtration $200- $2000

TOTAL ranges from $11,500 to $22,100

Make sure you get good advice on what is the best options for your house so you can make a budget and get what you need without cutting corners.


What is the process for installing a rainwater tank?

When installing a larger rainwater tank the normal process is:

  1. Get local council approval (if required).
  2. Do your budget and ensure you have the funds to achieve what will work.
  3. Get site located properly and tank pad prepared.
  4. Install tank and fill with ballast water.
  5. Spread bluemetal around tank to prevent erosion and to maintain your warranty.
  6. Install downpipes and overflows.
  7. Install pump, filtration and power (of required).

You must make sure that as you are building something that you get an approval of the local council first. Your tank builder can provide plans and drawings for this as required. This prevents you getting fined or having to modify or remove the tank at great expense later.

Once you have an approval, ensure that you have done a thorough budget on the costs and discussed what you want to install with an experienced install contractor. You might have an idea in your head but there are practicalities that might make this not work.

Once you are ready to start, you need to locate the tank properly and then get your contractor to prepare the site for installing the tank. The contractor will clear and then construct a sand pad made from the correct grade of sand. After the pad is prepared, then the contractor can build the tank. Once completed, always remember to put some water in the tank to ensure that it won’t blow over or the liner won’t be damaged.

On completion, you must then ensure that either yourself or the contractor spreads bluemetal around the base of the tank to prevent erosion and damage to the liner and walls. This is also critical to ensuring that the warranty on the tank, which can be up to 25 years on quality brands, is endorsed.

After this, either yourself or the contractor can then connect the downpipes and other pipework so that if the rains come, the tank will start to fill. At this point, the power, pressure pump and filtration should be installed and then the tank can start to be used.










This article hopefully gives you a bit of an outline into what is involved in selecting a water tank and some of the critical things that if remembered make the process smooth and enjoyable. For any advice on your rainwater tank installation and to discuss installing a quality Steelfab liner tank please contact us via email or on (08) 9721 3577.