When looking for a place for your tank, try to find somewhere out of the way, where it does not completely block a passage and there is ready access to a downpipe from a large roof catchment. Do not put it across access hatches under the house or where it blocks off a window. Tanks filled to capacity are heavy, with each litre of water weighing 1 kilo. Therefore a 2500-litre tank weighs in excess of 2.5 tonnes when full. Small tanks can be mounted on paving slabs or compacted road base, but larger tanks are best mounted on a 100mm-thick poured concrete slab. Check with your local council as to any regulations regarding water tanks in your area.
Gather your supplies:
• Formwork sides (2)125 x 25 x 2850mm pine
• Formwork ends (2)125 x 25 x 650mm pine
You’ll also need:
For a 2800 x 650 x 100mm thick slab: 2700 x 550 x 2020mm tank; timber pegs; set- out paint; 2-3 bags washed sand; 7mm steel reinforcing mesh; 150 x 1.25mm tie wire; 50-65mm bar chairs; premixed bagged concrete (20-21 bags); cement mixer (hired); rainwater head; first flush diverter kit; 90mm PVC pipe and fittings; polypipe and connectors; poly ball valve; garden tap
Select a suitable area that’s accessible and complies with local council guidelines; here, tank can be no closer than 450mm to boundary. Remove green matter that could rot under slab.
Order tank. Steel tanks are normally specially made, so you can customise yours to suit your requirements or limitations; here, 2700 x 550mm in area and 2020mm high, giving a capacity of just under 2870 litres. For this tank, make formwork 2800 x 650mm internal size. This allows a 50mm skirt of concrete around outside of tank all the way around. Screw sides of formwork to ends.
Use formwork to mark size of excavation needed on ground using set-out paint. Do not rest slab directly on house or garage footings
Excavate to an average depth of 50mm. Check that excavated surface is level.
Put formwork back in place and drive pegs around perimeter where possible. Drive to below top edge of formwork or cut off level with top of formwork using a saw.
Screw pegs to formwork from outside, checking that formwork remains level by rising and lowering on pegs as needed. If formwork is solid and level, you do not need to worry about your spirit level once you start pouring concrete, as you can be guided by top of formwork. This makes finishing concrete much easier. In places where wet concrete could get away because of a dip in the ground, provide infill pieces under formwork.
Spread a thin layer of washed sand on surface. As this is a water tank you do not need a vapour barrier, but sand aids in drainage under slab. It will be retained by surrounding ground once formwork is removed.
Cut reinforcing mesh as needed using a pair of boltcutters or an angle grinder. Its final size should be 40-50mm short of formwork all around so that it is never exposed to air, which would allow it to corrode. If it needs to be joined, overlap mesh by at least 200mm and use 1.25mm tie wire and pliers to twist wire to join bars.
Place mesh in formwork and raise to middle of height using 50-65mm bar chairs. Support formwork so it does not sag.
Using premixed bags of concrete, mix enough concrete for slab; here, 20-21 bags. Using hired cement mixer, add 6 litres of water to bowl, then add 3 bags of mix. (On average you will need 2 litres of water to 1 bag of mix.) Mix until uniform in appearance, then pour into a wheelbarrow to pour into formwork. As it is a narrow space, spread concrete to other end using a short-handled shovel. Add more concrete until it is about 10mm above formwork level.
Tip! Buy a little extra concrete to allow for wastage, spillage and any irregularities in ground. You do not want to have to leave the job to buy another bag or 2 near end of pour.
Use a short length of 90 x 35mm or similar to tamp concrete into corners and edges of formwork to remove air, then use it across top of formwork to screed concrete from 1 end using a sawing and chopping motion. Work in 1 direction, leaving a smooth but very wet surface as you go. When you are happy with level, let concrete just set and lose its surface or bleed water (usually a few hours).
When surface water has evaporated, trowel surface with a wooden float for a slightly rough, sandy finish. Use an edging tool to round edges for a neat appearance that otherwise may be brittle. Tool in a dummy joint halfway along slab to help control cracking. Remove formwork.
Put tank on concrete base and start making connections. You will have to cut into downpipe that will supply tank, ideally just under gutter. Install rainwater head with leaf guard. This has a fine mesh to ensure most particles are expelled to outside water-collection system.
From head, run 90mm pipe at a slight downward angle to T joiner supplied with first flush diverter kit, located adjacent to tank. Then continue pipe on to tank. Install a 1800mm-high x 90mm pipe at T join as a first flush chamber. At bottom of chamber, install ball float mechanism. When it first rains, dirty wash off roof will go into chamber until ball floats to top and blocks the T, at which time clean water then flows on to tank. Water is released slowly from this chamber over time so it can do its job next time it rains.
Continuing on to tank, install an elbow so water flows into grate at top of tank and add filter, leaving enough room so filter can be removed and cleaned. Fit an overflow pipe to outlet at top of tank and reconnect to your stormwater system.
At bottom, use poly fittings and inline poly ball valve to take water to where it is to be used and shut it off if necessary. At end, fit
a wall plate and add garden tap.