Many houses and buildings in India have flat roofs. With some simple adjustments, a channeling system can be built to guide rainwater from the roof to a water tank for storage in dry times. The water is used as drinking water and for irrigation. In the upper part of the tank there is a filter of sand and pebbles.

See here an example of the roof top water tank at a school.

Roof top watertank

And this is how a roof top water tank is built