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ACollecting rain from roofs is one of the oldest ways of securing water, and it remains common in dry regions. Today many households use the same idea on a small scale. According to the US Environmental Protection Agency (EPA), a rain barrel captures water from a roof and stores it for later use on lawns, gardens or indoor plants. Larger tanks called cisterns can hold much more water and may be installed either above or below the ground.

BA typical system has several parts. The roof acts as the catchment surface, and gutters and downspouts carry the water towards storage. Leaf screens stop larger debris from entering. Because the first rain to run off a roof can carry dust, leaves and bird droppings that have built up, many systems include a first-flush diverter, which sends this initial water away so that it does not enter the tank. A manual published by the Texas Water Development Board describes these components in detail.

CThe benefits extend beyond the household. When rain falls on roofs and paved surfaces, it cannot soak into the ground, so it runs off quickly and may carry pollutants into streams. Storing part of that water reduces runoff from the property, especially during heavy storms. The stored water can then replace some treated mains water that would otherwise be used for watering plants, which may lower household bills in places where water is charged by volume.

DRainwater is not automatically clean, however. The EPA warns that water flowing over a roof can pick up bacteria from birds and other animals as well as chemicals from roofing materials, so extra care is needed when watering fruit and vegetables. The agency also advises against drinking collected rainwater unless it has been properly tested. Where a household does plan to drink it, the Texas manual discusses treatment and says that local health and building authorities should be consulted.