One of the most important components of your rainwater storage tank installation is the base. I tell this to every customer that has a Site Assessment with me, and people are often surprised by the comment.

Why is the base of the water tank so important?

Well lets say you are installing a 3000 litre slimline water tank against your building in the side passage. The first thing you need to understand is that the tank will weigh over 3 tonne when it is completely full. That’s a lot of weight to hold, so the water tank base must be able to support the tanks weight.

The last thing you want once you have installed your tank, is to then be waiting for the first rainfall, and then once the tank fills the base gives way and the tank begins to lean on the house. Have 3 tonne leaning against your building is not a desirable situation to have.

Over the past 6 years I have visited several sites where this has actually happened. I remember a call out by an elderly lady who had called in a panic explaining that her tank was leaning against her house. On arrival to her home I found a 5000 litre round tank leaning against her weatherboard wall. Several of the panels had been broken and it was only a matter of time before the sheer weight of the tank would have collapsed the wall completely.

Fortunately the tank was only have half full making it only around 2.5 tonne of weight.

The base was only constructed using some river sand which had been thrown slap shot under the tank. There had been no timber frame to hold the sand in place, and over time with rain etc the sand had been slowly washed away and eventually the tank base was undermined and started to lean.

So to avoid this situation the best thing to do is to ensure your tank base is constructed with the right materials and suitable for the location and the tank you are installing. If you are unsure as to what is required you can give me a call on 1300 885 547 and I will gladly assist.

There is generally two types of base’s that we install, a frame using a timber base filled with sand and/or crusher dust, and a concrete reinforced slab.

1.  The Timber Frame Base

This base is generally useful when the site is very flat and in on a grassed/soil area. You cannot use this base when there is already a concrete slab.

The first thing to do is to ensure the area has been leveled out and is on firm ground. If the area consists of fill, then you need to make sure it has been very firmly compacted.

The next step is to make a rectangular or square frame depending on the type of rainwater tank, ie – slimline tank or round tank. You want the timbers to be treated and have a width of around 150mm. This will allow you to fill the frame to a height of around 120mm with the sand, blue metal or crusher dust.

Always ensure to make the timber frame at least 100mm longer at either end of the tank and at least 50mm wider than the tank. This allows rooms for the weight of the tank when full to be dispersed over the base. You do not want the edges of the tank to be nearly touching the timer frame.

We prefer to use crusher dust or quite course sand, although you may also use very fine blue metal. The blue metal can be used when the site does have water running past our through it, as the larger fill of blue metal will be more resistant to been washed away and the base been undermined. Always check with the manufacturer of the rainwater tank been installed as to the type of base that should be used and which will not breach the warranty.

Once the frame is made, the fill is then poured in and distributed evenly within the timber frame. Once again ensure that it is compacted well before moving the tank on to the base.

Once the fill is well distributed in the frame and is even and compacted, the tank can be put into place. As this is been done sometimes the fill will move around as well. The best thing to do is to get the tank into the centre of your base and have one person at either end moving it too and thro until it finds its level on the base.

2.  The Concrete Base

The concrete base is generally the best type of base to install for your rainwater tank system. The base is must be used when the tank is to be placed on an area with existing concrete. If the existing concrete path or base that is on site is not totally flat and void of large holes/cracks then a new base must be placed on top.

For the concrete base up using your timbers and again remember to allow an additional 100mm either end of the slab and at least 60mm in width. You do not want to have the tank sitting right on the edge of the slab, as over time the slab will begin to crumble away on the edges. You need the extra length and width on the base to disperse the weight of the tank. Remember a 3000 Litre tank weighs over 3 tonne, and this is why the slab construction and size is so important.

Once the slab has been formed up, then put some F62 steel mesh through the mix to reinforce the slab. The concrete must be at least 25mpa and I suggest making the slab a minimum of 100-120mm thick.

The slab should have a troweled finish and should be screed flat and be level with no high or low spots.The base of the tank needs to be supported across the entire surface.

2000 litre thintank


  1. Hi,

    i am building a granny flat & need a 2500 litter wall tank it will be on a concrete base but i only have 450mm width & plenty of height 6m & length is 8m. can you make them custom built & if so how much.

    • Hi Talal,

      Thank you for your enquiry.

      Please refer to your inbox where I have sent you a reply.

      Kind Regards,

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