Rooftop rainwater harvesting for houses and apartments

After a weak monsoon, every litre that soaks into the ground or is stored helps. Your roof is the easiest place to catch it.

How much a roof can collect (fact). Hyderabad's water board, HMWSSB, estimates that a 100 square metre roof can harvest about 55 cubic metres (55,000 litres) of rainwater a year. That assumes Hyderabad's average rainfall of 780 mm and that 70% of it is captured. HMWSSB says this is enough for a family of five for about 100 days, at 110 litres per person per day. A 500 square metre apartment roof could harvest about 275 cubic metres a year. Even one 20 mm shower on a 100 square metre roof gives 2,000 litres.

The parts of a system. The Jal Shakti Abhiyan guide lists the basics: a mesh at the roof outlet to stop leaves, a first-flush arrangement to send away the dirtiest first rain, a filter, and then a storage tank or a recharge pit that sends water into the ground.

For apartments. HMWSSB suggests a recharge pit of about 6 cubic metres for a 100 square metre roof and about 30 cubic metres for a 500 square metre roof. Raise it at your next RWA meeting, and ask a qualified plumber or the water board for a site check.

Timing. Build and clean the system in the dry months so it is ready when rain returns.

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