Recharge pits: send roof rainwater back underground

In a dry year, groundwater gets less rain to refill it. A recharge pit helps put some back by letting filtered roof water soak into the ground instead of running into drains.

How it is built (fact). The Bureau of Indian Standards' guideline for rooftop rainwater harvesting, IS 15797:2008, describes recharge pits about 1 to 2 metres wide and 2 to 3 metres deep. They are filled in layers: boulders of 5 to 20 cm at the bottom, gravel of 5 to 10 mm in the middle and coarse sand of 1.5 to 2 mm on top. The top sand layer catches fine silt.

Which size? The standard says pits suit roofs of about 100 square metres. For larger roofs of 200 to 300 square metres, a recharge trench, a longer and narrower version, works better. Hyderabad's water board, HMWSSB, suggests a pit of about 6 cubic metres for a 100 square metre roof.

Keep dirt out. Fit a mesh on the roof outlet and use a desilting chamber before the pit, so leaves and mud do not clog it.

Maintenance. Clean or replace the top layer of sand periodically, ideally before the monsoon.

Get it checked. Ask a qualified plumber or your water board where a pit can safely go on your plot, for your soil and building.

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