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How to design rain water collection and distribution system?

Hi all, my wife and I are trying to work out how to decide what type and quantity of rain water tanks to add to our DA in country NSW. We have to add some information to receive a BASIX certification.
We see most guidance suggesting 5000l for a big garden and now we are thinking how to create the optimum storage and distribution given the size of the roof we have and the need to pump out to the various areas of the garden (landscaping and garden design still in progress). I guess my thoughts are based on the way the water is fed off the roof, and how it can all go to one tank given the roof size or if a sunken tank is the best option. We have a decent lawn it could be sunk underneath.
The main house apex roof is approx 12.5m x 8.9m with additional rear of the home having 16.8m x 4.5m roof, then we are adding two outbuildings of approx. 6m x 6m (flat is planned).
The home is approx 200m sq footprint on approx 1400m sq block of land.

I’m having trouble uploading a copy of the block and would like to start with the general question. Any websites or recommendations to support me with

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Comments

EricL

Hello @brigadoon64

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about rainwater collection.

Based on your roof areas, the total potential catchment comes to approximately 259 square metres, factoring in the main house, rear extension, and two outbuildings. Using a typical rainfall estimate of 700 millimetres per year for NSW and a runoff efficiency of about 85 percent, your roof could potentially yield over 150,000 litres of rainwater annually. This suggests that while 5,000-litre tanks are generally recommended for garden use under BASIX, your property could justify 10,000 to 20,000 litres of storage, particularly if your landscaping and garden design will include significant lawn or planting zones.

Tank configuration will depend on space, aesthetics, and your plumbing layout. A single large above-ground tank offers simplicity but may be visually intrusive. Multiple smaller tanks positioned beneath various downpipes can be easier to conceal and are flexible in design. If you're open to excavating, a sunken tank is a good option for saving space and maintaining an uncluttered look, especially if it can sit beneath a lawn area, but this comes with higher installation costs and some technical complexity. Tank maintenance and repair also becomes much more difficult.

To distribute the harvested water effectively, especially across different areas of your garden, you will likely need a pump system and should allow for flexible underground conduit runs in your garden design. Include first-flush diverters, leaf screens, and an overflow strategy in your planning. If you aim to connect the rainwater to toilets or laundry, as often required under BASIX, a licensed plumber will be needed for safe integration and compliance with Australian Standards.

For the BASIX process itself, you'll need to use the NSW Government's online planning portal, where you’ll enter details about roof area, tank size, and intended water use. The system will generate a certificate you include in your Development Application (DA). Useful tools like the YourHome.gov.au website can help you test different tank scenarios.

Given the scope of your project, integrating a rainwater tank system across multiple roof catchments, possibly using a sunken tank, with the intention to supply water for garden use (and potentially indoor use for BASIX compliance) the best approach is to engage a licensed plumber with experience in rainwater harvesting systems, but potentially as part of a coordinated effort led by either a builder or a hydraulic designer, depending on complexity.

Let me call on our experienced members @Dave-1, @Nailbag and @diy_hausdesigns for their recommendations.

If you need further assistance, please let us know.

Eric

Dave-1

Good Evening @brigadoon64

Now that is some potential catchment area! And really impressed with @EricL 's knowledge and suggestions. All I see is fun, fun and more fun in designing something that will fullfill your DA application.

I really cant suggest too much as have not had to design something to suit an application process. I do like the idea of a distributed system over a large central tank. The amount of water you can generate would be easier to handle in multiple tanks, If there is an issue with a tank you dont loose all the water. Tanks dont need to be burried, they could be set back or form a protected area fro the yard (A secret garden type deal) A lot will come down to the lay of your land and the ability to easily pump the water where its needed including fire suppresion.

Break it down into smaller watered areas may be a solution with an overall ability to shift teh water as needed may the best end solution with the ability for sections to function alone.

Looking foirward to seeing your process.

Dave

Nailbag

Hi @brigadoon64

This to me is a far more complex project to get the most accurate advice. I would be getting in touch with tank supplier/installers who can offer specialised custom solution options for you.

Nailbag

brigadoon64

https://www.workshop.bunnings.com.au/discussion/comment/55242#Comment_55242

Hi Eric,

I’m very excited about this opportunity to make our property much more sustainable and thank you for the initial guidance. I have skimmed the BASIX website and have been asked by our architect that we have to complete this prior to submitting our DA. As a side bar, I’m also very nervous about over reaching and need to find a balance between optimal and possible. We have space and the opportunity to develop and looking for anyone in the community that is qualified and interested to contact me for further discussion.
Looking forward to building my understanding and where possible my skills within the shared goldmine of resources within the group.

thanks for the additional shares and already some responses. Mick

brigadoon64

https://www.workshop.bunnings.com.au/discussion/comment/55245#Comment_55245

Hi Dave,

Firstly thanks for taking the time. I’m already thinking that the three buildings may be the simplest way to create separation with several tanks. As you say, fun is key to taking the steps forward as the end result benefits are obvious if enough thought is put into the design stages.

any thoughts on a reliable company that you’ve dealt with or had good reports about please? Unsure if that question is ok on this forum?

cheers

Mick

brigadoon64

https://www.workshop.bunnings.com.au/discussion/comment/55247#Comment_55247

Thanks for the response Nailbag,

It looks like you are a great source of experience and ideas for other things that will be going into the project. As I move through the stages I’ll come back to the board.
cheers

Mick

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