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How to install a water tank

Valued Contributor

Difficulty: Intermediate

Once you have chosen your water tank, it’s time to get it installed. Getting the installation right will ensure you have many years of reliable storage and supply from your tank.


The process of installation will vary based on your tank size, shape and connection type, but the basic steps are consistent across all tanks. Building the base is a straightforward D.I.Y. job and we have included the steps for creating a timber sided box frame base.


Remember that you will need to a plumber and possibility also an electrician if you need a pump.


Step 1

Once you have determined the location for your tank, you now need to set out for its base. You could lay a concrete slab, but it would need to be reinforced and 75 to 100mm thick. Instead, we choose to build a timber box frame filled with compacted road base.


The general rule is that your base should be 200mm wider on all sides than your tank. The only exception to this would be where a tank is going flush against a wall, or if it is a small tank (less than 10,000l) where a 100 to 150mm spacing is acceptable. We used 200mm extra on each side.


For square, rectangular and oval tanks take the dimensions, length and width, add a suitable extra spacing to each side and end and then mark this on the ground. So, for example, a tank that is 2000mm x 1000mm you would mark for a base that is 2400mm x 1400mm if using a 200mm additional spacing. A circular tank will need a square base that is 200mm wider on all sides than the tank diameter so a 2000mm diameter tank would need a 2400mm square.


If your tank is being installed close to another structure, such as a wall or path edge, use the adjoining structure to keep your set-out straight by checking each end of your proposed base is an equal distance from the edge of this existing structure. Then use your builders square and straight edge to mark the sides and finally the furthest side.


To set-out for a circular tank you can either set-out a square or (as we did) start by marking the tank circumference plus the extra width. We used this technique as we did not have an existing edge to easily mark from.


To mark a circle, hammer in a stake where you want the centre of the tank. Put a small nail or screw in the top of the stake. Hook your stringline over this and then roll the string out to the tank radius (1/2 the diameter) plus 200mm. Holding your set-out spray at this length on the stringline hold the line taut and walk slowly around the peg while spraying to mark a circle. Now mark the sides and then use your square and straight edge to mark the square around this.


1.1 Mark a circle.jpg1.2 Mark the sides.jpg

Step 2

You now need to check the accuracy of your set-out. It is worth remembering that your base does not have to be a 100% accurate. As long as you have allowed some extra space there is a little room for error.


To check your set-out, hammer a stake into each corner, put a nail in the top and from this work your way around to measure the length of all sides to make sure they are as expected. If not, adjust as needed by moving the stakes.


To check your set-out is square you check the diagonals. Run your tape from the top of the stake at the rear left to the top of the stake at the front right. Write the figure down. Now repeat in reverse – top of rear right to top of front left – and compare the figures. If square they should be the same. A minor difference of say 10 or 20mm is okay at this stage as you can make adjustments as you start to install. If the difference is large use your square to check all corners and move the corner stakes as needed before checking the diagonals again.


2 Check set out is square.jpg

Step 3

Time to determine how much you need to excavate for your box frame. This will be reasonably simple on a flat site and only a bit more complex on a slightly sloping site such as ours. We are using 200mm sleepers and our base only needs to be around 100mm tall so on a flat site it’s simple, you’ll need to dig a 100mm trench all around. On a sloping site determine which point will be the highest and start measuring from here.

We decided to allow 100mm above ground at the highest point, which happened to be a corner, and set a stake accordingly. Depending on the length of your level and straight edge you may need to set more stakes to allow you to take measurements.


Now transfer your levels. The aim is to project the level from your first stake to all of the other stakes. Again, 100% accuracy isn’t needed at this stage.


Finally, make a simple plan. We recorded our height differences on a simple plan to make our excavating more accurate. To translate that record, a height of 160 means we need to excavate only 40mm. 200mm means our sleeper will be sitting on ground level. Anything above 200mm tells us we will need an extra sleeper through this area.


3.1 Set your first stake height.jpg3.2 Transfer your levels.jpg3.3 Make a simple plan.jpg

Step 4

Start excavating by following your measurements. It’s best to start in a corner where you will have a post. Work your way around making your trenches one spade width wide. This gives you room to move your sleepers to adjust square as you make your final measurements. Throw the excavated material into what will be the internal area of your base. This way any excess can be used as fill. Your excavations only need to be approximate at this stage as you’ll fine-tune them once you start checking levels.


4 Excavate as needed.jpg

Step 5

Position a corner post using a sleeper temporarily positioned to get the post plumb and in-line. Add some water to the bottom of the hole, then add around half a bag of the concrete, tamp this down, add a little more water, then more concrete mix until the post hole is adequately filled. Check the post is plumb and level as you are doing this. A magnetic level is very handy as it frees-up your hands.


5 Position and concrete post.jpg


Step 6

To check excavation levels, position your long level on top of this first post and use this to check the depth. You can now trim your excavated areas in more accurately. Work towards the next corner (or centre post) and when you reach the next corner (or post) you can repeat Step 5 to add the next post using your sleeper and level as a guide to set the post at the correct height. You can now fix your first sleeper in. Check it is level and sitting flush with the top of the posts and then pre-drill through metal and temporarily secure with wafer screws.


6 Check excavation levels.jpg


Step 7

Position your next sleeper, trimming in excavations if needed, and this sleeper can be secured at the correct height with one wafer screw. In our case this corner was a low point so the sleeper was clear of the ground. Before concreting in the next corner post use your builders square to check the corner. It should be square and this is where any set-out issues with the diagonal measurements not matching can be rectified.


Once checked and adjusted use the sleeper to align the next corner post and concrete the post in, then fixing the sleeper off. Repeat this process until you arrive back at your first post.


To check your level is remaining consistent on your side sections, run your level diagonally across from one side to the other before fixing off the next post.


7.1 Add sleepers and adjust excavations.jpg7.2 Check levels diagonally.jpg

Step 8

The box frame is now complete. We just had gaps beneath some sleepers. This was resolved by positioning sleeper off-cuts inside the frame, tamping them down and then screwing them to the fitted sleepers using the large bugle-headed batten screws.


8 Finished frame.jpg

Step 9

Time to spread the excavated material and add the drainage mat. Use the excavated fill to back fill the open areas on both outside and in of box frame. Evenly spread the remaining fill though centre of box. Cover the area with drainage fabric lapping it up the sides, if needs be you can staple this in place.


Now commence filling with road-base. It is important that this filling be done in even stages as each layer must be adequately compressed. Add around 30 or 40mm before running over the area with the vibrating plate compactor until solid underfoot. Add another layer of the same thickness and repeat until filled. You can very lightly water the road-base to aid compaction but only sprinkle on a very small amount of water.


9.1 Spread excavated material.jpg9.2 Add and compact roadbase.jpg

Step 10

Use a soil spreader or the back of the rake head to bring to a finished level shaving off any excess road-base. You can use a long piece of timber or your straight edge as a screed bar. Keep working the surface until the bar can rest on the side boards with no bumps in the road-base.


10 Level excess roadbase.jpg

Step 11

Pre-drill as needed and add at least two hex head screws to each sleeper end on each post face. Remove the temporary wafer screws and replace them with a hex head. These hex heads will keep your sleepers more secure over time. Your tank base is now complete.


11 Add and replace screws.jpg

Step 12

Time to add your tank. Check with your supplier as to how many people will be required on site to move your tank in. Our tank was a 22,700L poly tank (in Native or Wallaby Grey) and weighed 400kg so it took three people to move and position.


Once onto the base, take care to get the tank centred and ensure that the inlet, outlet and overflow were all correctly positioned.


12.2 Check position and alignment.jpg

Step 13

As they can be easily damaged in transit, many tanks will come without the outlet valve fitted. The delivery driver may fit this for you once in position but if you have to do it yourself it can seem like a bit of a logic puzzle as everything needs to be done from the outside of the tank. Here are the easy steps for adding your outlet.


First, you might need to drill a hole for your outlet. Use a hole-saw of a suitable size and blade type fitted to your power drill. You should position the outlet low enough to maximise access to water but not so low that it will be difficult to attach fittings. For poly tanks ensure that you drill into the lower, thicker section of the tank wall.


13 Drill outlet hole.jpg

Step 14

Create a feed rope. Your length of feed rope needs to be around three times the height of your tank. So, a 1.2m tall tank you need a length of around 3.5m. Tie a loop in one end and then tie your weight onto the rope. With steel tanks avoid using a weight with any sharp edges so you do not damage the internal lining. While firmly holding the long end drop this looped end into the tank as close to the outlet hole as possible.


Now pull the feed rope through. Put your hooked wire into the tank through the outlet hole and fish around to hook the rope. Pull a metre of so through to the outside.


14.1 Create feed rope.jpg14.2 Pull feed rope through.jpg

Step 15

Time to add the tank fitting. Slide the internal tank fitting onto your rope ensuring that the rubber seal is pointing upwards. Tie your weight behind this fitting to stop it sliding on rope. You can tie a slide-stop in front too if you need to avoid the fitting impacting the tank surface. Lower this into the tank and start pulling through from the outlet hole and the fitting will emerge through the hole.


Slide outside fitting nut onto end of rope all the way to the protruding fitting and finger-tighten. Pull rope back out through the top of the tank. Now use spanner or pliers to firmly tighten your fitting.


Add a couple of wraps of pipe tape to the thread of the fitting ensuring it is wound on the direction of the thread. Now firmly screw on the tank valve. You tank is now ready for plumbing connection.


It’s at this point that each installation becomes different and you’ll need a plumber to ensure compliance with various regulations. Your plumber will need to perform the final connections for the pipes and this should include making the downpipe changes (such as creating diversions and adding fittings like rain head filters and first flush devices). They will need to be adding new lines and taps, making any connections to your house supply, connecting to the pump and adding the tank overflow line, ensuring this is discharging in an appropriate way. You might also need an electrician at this stage if you are adding a pump.


15.1 Add tank fitting.jpg15.2 Secure fitting.jpg15.3 Add the tank valve.jpg

Step 16

In our case the tank was installed away from the house and was fed with “charged” (constantly water-filled) pipes. This meant that trenches had to be dug for the incoming stormwater pipes and for the outgoing water supply line. Two downpipes were connected and these were joined with a Y-junction underground.


In the case of a charged line system the incoming pipe will generally run from ground up the side of the tank.


16.1 Water-filled charged lines.jpg16.2 Charged line tank input.jpg

Step 17

The tank screen needs to be installed on the tank mouth as soon as possible after installation.


The outlet section is generally angled to reduce splashing. It is wise to just friction fit the final outlet stage (don’t use pipe glues) so that the outlet can be swung aside easily to remove or clean the screen.


17.1 Install tank screen.jpg17.2 Angle outlet.jpg

Step 18

Your new water tank is now installed. Just remember that all taps supplying rainwater must be clearly marked as such. Now you can look forward to that magic moment when the first rainwater starts filling your new tank.


18.1 Add signage.jpg18.2 Let it rain.jpg



Your choice of water tank.


To create a timber-sided, box-frame base you’ll need:


  • Hardwood set-out or survey stakes

  • Treated Pine or hardwood sleepers (we used 200 x 75 x 3000mm treated Pine)

  • Concrete-in corner posts (and joiner posts if needed) to suit sleeper size

  • Bags of high mpa rapid set concrete mix (Allow at least 2 bags per post)

  • Set-out spray paint

  • 35mm galvanised wafer head screws

  • Hex head timber or timber-metal screws at least 50mm long

  • Drainage fabric

  • Road base or crusher dust

  • Bugle-head batten screws (galvanised or climacoat) at least 100mm long and driver bit to suit (optional).


For adding the tank outlet valve, you’ll need:


  • Tank outlet and valve fittings (these should come with your tank).



To create a timber-sided, box-frame base you’ll need:


  • Long and short levels

  • Straight edge

  • Stringline (for larger or circular set-outs only)

  • Tape measure

  • Large framing or builder’s square

  • Shovel

  • Power saw

  • Battery drill with suitable driver bits

  • Drill bit to suit to pre-drill metal posts for wafer head and hex head screws

  • Bit to suit hex head screws

  • Landscape rake and/or soil spreader

  • Vibrating plate compactor (hire).


For adding the tank outlet valve, you’ll need:


  • Light rope

  • Weight/slide-stop for rope (a large nut or bolt is suitable)

  • Hooked wire around 500mm long

  • Suitable sized spanner or locking pliers

  • Pipe tape

  • Power drill and hole saw (optional).


1.1 Mark a circle.jpg

1.2 Mark the sides.jpg

2 Check set out is square.jpg

3.1 Set your first stake height.jpg

3.2 Transfer your levels.jpg

3.3 Make a simple plan.jpg

4 Excavate as needed.jpg

5 Position and concrete post.jpg

6 Check excavation levels.jpg

7.1 Add sleepers and adjust excavations.jpg

7.2 Check levels diagonally.jpg

8 Finished frame.jpg

9.1 Spread excavated material.jpg

9.2 Add and compact roadbase.jpg

10 Level excess roadbase.jpg

11 Add and replace screws.jpg

12.1 Bring in your tank.jpg

12.2 Check position and alignment.jpg

13 Drill outlet hole.jpg

14.1 Create feed rope.jpg

14.2 Pull feed rope through.jpg

15.1 Add tank fitting.jpg

15.2 Secure fitting.jpg

15.3 Add the tank valve.jpg

16.1 Water-filled charged lines.jpg

16.2 Charged line tank input.jpg

17.1 Install tank screen.jpg

17.2 Angle outlet.jpg

18.1 Add signage.jpg

18.2 Let it rain.jpg

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