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Do I need a retention or detention water tank?

10月 8, 2015

What is the difference between a retention and a detention tank?

The quick and simple answer is that there is no difference between both tanks, rather there is a difference between how they are setup.

You can purchase a 3000L tank (under or above ground) and set it up to be either- or. This will fully depend on the application.


Detention tanks work by temporarily storing the rainwater runoff during a rainfall event and then slowly releasing the water through a controlled small diameter orifice into the public storm water system. This storage and slow release of the rainwater reduces the peak storm water flows during a rainfall event and hence reduces the impacts on downstream infrastructure and/or streams.

Detention_tank_setupDetention tanks are usually council specified in areas where potential storm water flooding risks apply. With the current housing crises and increase, Auckland’s public storm water systems are considerably undersized.  So generally if you are adding roof space by extending or building new in areas the storm water systems have not been renewed, you will most likely require a detention tank.

Detention tanks are also handy when you are managing your storm water onsite. The most common application is in combination with soak holes. Giving the ground enough time to “soak” excess water will prevent any surface or ground related flooding. Often causing damage or nuisance to person and property.

The following is a typical underground detention tank installation



Detention tanks can of course also be used to retain water as retention tanks. The best way to achieve this, is to oversize your detention requirement. Retention would be any water storage below the outlet orifice and can be used for hose taps, toilets, laundry and under certain circumstances for all general plumbing.

A normal water tank is simply considered as a retention tank, water is “retained” for use.

The following is a typical obove ground detention/retention tank installation.


For more information, please download the following relative Auckland city council documentation. NSC07-detention-tanks-v4

How do I calculate how large my detention needs to be?

In Auckland a rule of thumb based on several conditions including “storm of the 100 years” would be to allow for 100 liters per 1 SQM of roof surface area.  Depending on detention requirment and specifications, the outlet orifice could be anywhere between 10mm-40mm.

If you are looking for New Zealand made above ground water storage water tanks, please look at the assortment of these water tanks from rotational plastics.

If you need specialist advice on rentention or detention solutions, dont hesitate to give us a ring on 0800 TEAM EURO and have a chat to our qualified drainlayers!

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