Harvesting rain from a roof
Choose the size of a roof and the rain in a month, and work out how many litres of rainwater a family could collect in a tank, and for how many days it would give a family of five their water.
Core concept
1 mm of rain on 1 m² of roof is 1 litre of water, so a roof collects area × rain × 0.8 litres (some splashes or soaks away): even a small roof can store a lot of water in the monsoon
Topic: Our planet and sky
When rain falls on a roof, it can be led through gutters and a pipe into a tank instead of running away. This is rainwater harvesting, and many homes in Nepal, India and Bangladesh use it. Working out how much you can collect is simple: 1 mm of rain falling on 1 square metre (m²) is exactly 1 litre of water. So a roof of 50 m² in a month with 100 mm of rain receives 5000 litres. Some water splashes off or wets the roof, so we collect about 80% of it. A family of five uses about 250 litres a day for drinking, cooking, washing and cleaning.
What you will learn
- Work out the rainwater a roof collects: area × rain × 0.8.
- Compare how much water different roofs and months give.
- Explain why harvesting rain helps save water.
Before you start
- Know how to find the area of a rectangle.
Materials
- A rain gauge or a straight-sided jar and a ruler
Equipment
- None
Procedure
- Record the water from a 50 m² roof with 100 mm of rain.
- Choose the biggest roof and a monsoon month of 400 mm, and record.
- Choose a 20 m² roof in a dry month of 50 mm, and record.
How this simulation is simplified
- 1 mm of rain on 1 m² is 1 litre; about 80% of it reaches the tank.
- A family of five uses about 250 litres a day.