Rainwater harvesting at home: how it works

The idea

Your roof already collects rain. Harvesting means leading that water somewhere useful instead of letting it run into the drain. You can store it in a tank, or send it into the ground through a recharge pit to top up the groundwater that borewells draw on.

The Ministry of Jal Shakti’s Catch the Rain campaign promotes rooftop harvesting on public, commercial and residential buildings.

The parts of a simple system

1. A clean roof, the catchment. Keep it free of leaves, droppings and anything stored on it.
2. Pipes that gather the water from the roof drains.
3. A first flush: a way to throw away the first rain, which washes the dirt off the roof.
4. A filter to take out sand and leaves.
5. A tank to store the water, or a recharge pit to send it underground.

What could your roof collect?

A rough rule: roof area in square metres, times yearly rainfall in metres, times 0.8 for what’s lost along the way, gives cubic metres of water. One cubic metre is 1,000 litres.

For example, a 100 square metre roof where 700 mm of rain falls in a year could collect about 100 × 0.7 × 0.8 = 56 cubic metres, or 56,000 litres. Use your own city’s rainfall for a real figure.

Before you build

Stored rainwater isn’t safe to drink until it’s been treated, so plan to use it for flushing, washing and gardens first. A recharge pit must be built well away from sewage and septic tanks. Rules differ by city, and some buildings must have harvesting by law, so ask your local body or water board, and use a trained plumber for anything underground.