New Builds in France - Utilising Rainwater

These days we all want to minimise environmental damage and with careful planning, every new builder can help save the planet – often at minimal cost.

Water is a precious resource and rainwater harvesting is one of the oldest methods of collecting water for domestic use. In France the amount of water used on average per day is 175 litres per head; meaning that a household of four will have an average annual water bill of around €382 to €534 (£259 to £362).

With some regions already paying as much as €4.57 (£3.22) per m³ and prices predicted to rise by a minimum of 10 per cent over the next ten years, saving and storing rainwater is set to become ever more important. And for the people who have down-shifted to France and are growing their own vegetables and raising livestock, even on a small scale, there are huge economies to be made.

Rainwater also has several other benefits. Its natural softness means that you will be using less detergent and appliances will not suffer a build up of limescale. There will also be no need to use anti limescale products. The garden will require less fertiliser due to its natural properties. People with sensitive skin will find that rainwater is far milder and thus less likely to irritate.

When planning a system, it is important to take into consideration how the water will be used as this will impact on the storage method. It also makes sense to estimate how much water is likely to be collected, as rainfall will vary in its quantity and frequency from region to region. For example, it rains less often but more heavily in the South West than the North. This affects the choice of storage method of and it is important to choose the right size reservoir to enable the system to function efficiently. Residents of those regions likely to experience sudden, heavy downpours will need a system able to cope with these.

So how can the potential harvest be calculated? Well, the equation is quite simple – when you know how.

The amount of recoverable water will depend on the surface area of the roof and the coefficient of the pitch. This is 0.8 for a pitch greater than 15 per cent and 0.7 for a pitch that is less than 15 per cent. The surface of the roof multiplied by the coefficient multiplied by the average rainfall for your region = the volume of water.

For example, with an annual rainfall of 858l/m², a roof area of 150m² and a pitch of 30 per cent, the amount of water collected over a year will be a staggering 102,960 (150x0.8 x858) litres of water. This would cover the needs of a family of four (excluding drinking water) for nearly twelve months. Clearly it is essential to save, conserve and use all the water that is possible. This is even easier for people building a new house as they will be able to design their system from scratch and should be able to maximise their rainfall collection system to get every last drop.

There are three basic methods of harvesting rainwater. The first is the simplest and best known – rain butts by any other name! These are now very reasonable in price and can be bought for as little as €17 (£11.54) complete with lid and tap. By adding a couple of extra fittings – a kit de collecteur and an ensemble de jumelage, two butts can be connected to enable one to be filled after the other, thus minimizing any waste. Above ground or surface tanks can be made from a variety of materials. Metal, wood, blocks and fibreglass can all be used as well as the much more common plastic.

Interior reservoirs are usually sited either in the cellar or an outbuilding. With the addition of a pump, this system allows for domestic usage, ie washing machines and toilets as well as exterior and garden use. This is a quick and easy system to install and it is possible to connect several reservoirs.

The third system is the most expensive to install and consists of a giant plastic reservoir or cuve which is then sunk into the ground outside. Materials and designs for these types of sub-surface tanks must be able to resist the soil and soil water pressures from outside when the tank is empty. The great advantage of this system is the sheer volume of water, which can be stored, up to 9,000 litres. Kits complete with pumps, filters etc are priced at around €2,500 (£1,697) for a 6,500-litre reservoir.

The reservoirs must be installed at least a metre below ground and in an area where there is no vehicular traffic. They should also be placed at a sufficient distance from any trees whose roots may damage the structure below ground. In terms of a new build, it is sensible to think about the location of the reservoir very early on. It will be cheaper to install the tank at the same time as the foundations are being dug. The system can be carefully planned to fit in with all the households needs and the plot can then be left ready for landscaping. The household’s water requirements will also have a bearing on the type of pump required. Unsurprisingly, a large garden surface area and numerous water points will require a more powerful pump!

The water can then be used both inside and outside the house. In the garden it can be used for watering plants and lawns, washing the car, dustbins, driveways and of course filling the pool. Inside, the water can be used to flush loos, run washing machines and after analysis and filtration, can even be classified as eau potable! These home treatment systems are still relatively rare but it is likely that the next few years will see them becoming much more widely used. It is also possible to install a device known as a tableau de jauge, which gives a permanent reading (as a percentage) of the amount of water available in the reservoir.

It is important to remember that any storage system will only be effective if it is being supplied with water! Gutters and guttering are of huge importance to the eco-builder and should be planned to maximise rainfall harvest. In properties with multiple roof areas, it is important that each surface has adequate guttering. Blocked and broken gutters are of no use so it is vital to check for damage and obstructions regularly.

Water Consumption

Flushing an old loo 7-10l

Flushing a new loo 4-6l

Taking a bath 80l

Ordinary shower 8l per min

Power shower 20l per min

Washing machine 50-100l per cycle

Dishwashers 15-25l per cycle

Garden hose 10l per minute

Did you know?

Under French law, every owner has the right to use the rainwater that falls on his land (art.641 of the civil code). In complex cases, semi-detatched housing, shared grounds etc, it is sensible to take the advice of a notaire.

Installing a rainwater harvesting system before 31 December 2011, will give a 40 per cent tax rebate on the cost of installation and equipment up to a maximum of €5000 (£2,891).

Remember to ask at your mairie if your commune has any tax credits/grants available for the installation of a rainwater collection system.

Install an overflow device to divert water from your first reservoir when full.

Think carefully about the amount of water you could collect and the amount you use and create a system that is best adapted to your individual needs.