J'arrose ? Guides

Guides to water wisely

Collecting rainwater: the tank, its rules and restrictions

Published on November 10, 2026

The water that falls on your roof usually goes into the gutters and then into the storm drains. A tank lets you keep it for the garden: it is free, neither chlorinated nor hard, and it remains available when restrictions limit tap water, with differences between départements.

How much water can you collect

One millimetre of rain is one litre of water per square metre of roof. A 100 m² roof that gets 700 mm of rain a year therefore receives 70 m³; you collect a little less, because of evaporation and overflow when the tank is full. Depending on the region, expect 45 to 80 m³ a year per 100 m² of roof.

The difficulty is that rain is scarce in summer, when the garden needs it most. A tank of a few hundred litres empties in a few summer waterings: the bigger it is, the better it bridges the gap between spring rains and droughts.

Installing a tank

  • Under a downpipe: a diverter fits on the pipe and sends the water to the tank; when the tank is full, the water goes back down the downpipe.
  • Roof water only: the regulations refer to water collected "downstream of surfaces inaccessible to people". Not water from a patio or a path.
  • Covered: an open tank is an ideal breeding place for tiger mosquitoes. Some communes or départements actually require rainwater tanks to be covered.
  • Out of the light: an opaque tank limits the growth of algae.
  • Maintenance: a quick check once a year is enough, with no professional needed. Never put antifreeze in the tank.

What the law says

  • Watering, washing the car, cleaning patios: allowed with a simple tank, not connected to the house, with no formalities.
  • Toilets, washing machine, washing indoor floors: possible, but with a separate network, taps labelled "non-drinking water" (eau non potable), and a declaration to the town hall.
  • Never for drinking, cooking, washing dishes or washing yourself.
  • Never connected to the drinking water network: rainwater could flow back into it and contaminate it. Polluting the public network is an offence punishable by up to 3 years in prison and a €45,000 fine.

Can you water with rainwater during a drought?

The national drought guide provides that restriction measures do not apply to collected rainwater. But each prefect writes their own decree, and not all of them take up this exemption as it stands. A few examples of decrees published on VigiEau:

  • Saône-et-Loire: watering with rainwater collected from roofs is not restricted at all, whatever the level.
  • Nord and Pas-de-Calais, at heightened alert: with rainwater, watering is still possible, but only before 9 am and after 7 pm.
  • Savoie, at heightened alert: watering lawns and flower beds is banned, except between 8 pm and 8 am with rainwater stored beforehand.

So before watering from your tank, read the decree for your area. On your commune's page, J'arrose ? shows the original text of the decree under the hours: that is usually where a rainwater exemption appears. Note: the verdict and the hours shown apply to tap water; they do not take a rainwater exemption into account.

Making the supply last

The water in the tank is limited: use it like tap water, at the foot of plants, early in the morning or in the evening, and generously rather than often. The guide When and how to water gives the details. Mulching and ollas filled with a watering can make it last much longer.

Sources (in French)