to collect rainwater from concrete patios, driveways and other impervious surfaces.

Rainwater tanks may be constructed from materials such as plastic (polyethylene), concrete, and galvanized steel, as well as fibreglass which is rust and chemical-resistant. Tanks are usually installed above ground, and are usually opaque to prevent the exposure of stored water to sunlight, to avoid algal blooms.
Tanks are also covered and have screen inlets to exclude insects, debris, animals and bird droppings. Tanks often come with a plastic inner lining to both increase the life of the tank and protect the water quality.
Apart from rooftops, tanks may also be set up to collect rainwater from concrete patios, driveways and other impervious surfaces.
Sizes typically range in capacity from around 400 to 100,000 litres (100 to 25,000 US gallons). Larger tanks are commonly used where there is no access to a centralised water supply. Also affecting tank size is predicted rainfall and rainfall variability; the higher prices for larger tanks; intended use of rainwater and typical consumption for these uses; the area of roof draining into the tank; security of supply desired.