Tanks specifically marketed for rainwater are usually cheaper than others, which meet technical standards for food products, storing oils or other uses.
Rainwater tanks have to be strong – a litre of water weighs one kilogram, so a relatively small 1,000 litre rainwater tank will have to support a tonne.
Prices vary, but a good, French made plastic tank, treated against ultra violet light, holding 2,075 litres is advertised for €845. The ultra violet light treatment means it can be sited on the ground, where it will need a solid base able to support the two tonnes of water it will hold when full.
Most people though will want to bury it – large plastic water tanks are not pretty.
Electric pumps for tanks are now relatively cheap, €300 should buy a good one, or a good quality petrol pump will cost around €800.
Of course, if your property is on a slope, and you can arrange your guttering system so that the water flows out at the top of the slope, putting a tank high up will mean that no pump is needed as gravity will do the job.
One of the cheaper ways of building a rainwater tank is to use cardboard boxes and earth instead of wooden shuttering.
Scout around until you can find large cardboard boxes of the sort that used to be used for delivering things like biscuits to grocery stores.
When you have enough, dig a hole for the tank making sure that it is 10 cm wider and longer than the boxes.
Put a good layer of hardcore into the bottom of the hole them pour in enough concrete for a solid 10cm base, working it to be as flat as possible.
Leave the base to set for two days then put the boxes into the hole, making sure there is the 10cm gap to the sides, and fill the boxes half way up with soil.
You will then be able to pour concrete into the gap, stopping to put more soil into the box when needed.
Remember to place inlet and outlet pipes before the final pour, being careful to make sure they slope very slightly the right way, and leave the concrete to set for a week.
You will then be able to dig out the soil and rip away the cardboard, leaving you with a concrete tank, which only needs hard cement rendering to be completely waterproof.
If you are feeling flush, you can even use waterproof renderings, containing resins, from the builders shop.
For a top, treated, close fitting oak planks are ideal – it is important that they fit well to stop the tank becoming a breeding ground for mosquitoes.
A small box can be used in a similar manner for a filter bed, with volcanic rocks, similar to those used for septic tanks, being ideal.
The outlet should lead to a soak-away, again, using a mechanical digger to dig it will save hours of labour. If possible put the soak-away where you will have a vegetable or flower bed – the extra moisture does wonders for plants.
If you have access to 44 gallon (220 litres) oil drums, they can also serve as rainwater tanks, lining them up in a row, with the overflow from the first serving as the inlet for the second and so on. Again, it does not look very elegant, so some form of screening will probably be needed.
More on this topic: How to use rainwater in France
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