Rainwater harvesting: tanks and water butts

Rainwater harvesting: how to choose the right collection system

A 100 m² roof collects an average of 80,000 litres of water per year in a temperate climate with 800 mm of annual rainfall. That’s enough to cover the watering needs of a 200 m² garden throughout the dry season, and supply the toilet or washing machine of a four-person household for several months. Harvesting this water means choosing a container matched to the actual collection volume, not just the space available in the garden.

The first criterion is the connected roof surface and the material’s collection coefficient: standard tiles return around 85% of the water that falls, while a green roof returns closer to 50% since part is absorbed by the substrate. Multiplying the building’s footprint by local annual rainfall gives the theoretical volume available; dividing that figure by 12 gives a realistic baseline for sizing the tank around the driest month.

Underground tank or above-ground water butt: two different uses

An underground polyethylene tank, typically between 3,000 and 10,000 litres, makes sense once you’re aiming for wider domestic use: toilet flushing, washing machine, automatic watering. It requires excavation and either a gravity-fed connection or a submersible pump, with installation costs starting around €2,500 excluding groundwork for a 5,000-litre model. The above-ground water butt, from 200 to 1,000 litres, can be installed in a few hours under an existing downpipe and is more than enough for watering a vegetable patch or flower beds.

Filtration and maintenance: avoiding stagnant water

A leaf filter placed upstream of the tank catches coarse debris before it clogs the bottom of the reservoir. For indoor sanitary use (toilet, washing machine), a 100-micron mesh filter at the tank outlet, combined with UV disinfection or a 10 cm gap between the overflow and the high water mark to prevent backflow, meets the recommendations of DTU 60.11 for non-potable water networks. Without this minimal upkeep, stored water starts developing algae once surface temperature exceeds 20°C, especially in translucent above-ground tanks exposed to sunlight.

• Above-ground tank 200-300 L: occasional watering for a balcony or small garden, budget €80-150
• Above-ground tank 500-1000 L: regular vegetable garden use, several draw-off points, budget €200-400
• Underground tank 3000-5000 L: automatic watering plus one indoor use (toilet), budget €2,500-4,000 installed
• Underground tank 7500-10000 L: house with over 500 m² of garden and multiple indoor uses, budget €4,500 and up

The electrical connection is often the most underestimated point: a surface pump draws between 600 and 1,200 W and must be protected by a dedicated residual current breaker if it feeds a circuit inside the home. For simple outdoor use with a garden hose, a gravity-fed draw-off tap at the base of the tank is enough and avoids any wiring altogether.

Regulations and declaring to your local council

In France, using rainwater indoors for permitted purposes (toilets, floor washing, washing machine under certain conditions) requires a declaration to the local town hall under the decree of 21 August 2008, with a separate network marked by a “non-potable water” symbol at every draw-off point. Rainwater remains prohibited for drinking, dishwashing, and personal hygiene without additional potabilisation treatment, which few homeowners set up given the cost of advanced filtration.

Choosing between different models usually comes down to weighing your roof’s actual collection volume against your expected frequency of use: a slightly undersized tank that fills quickly beats an oversized reservoir that sits stagnant for weeks between rains.