The ADCSWS assessed the viability of providing roof top rainwater in the coastal belt. Within one square meter of area and one millimetere (mm) of rain, the water that can be harvested is one litre. This means that the roofs have tremendous potential – with 10 mm of rainfall, a 50sq m roof can collect 500 litres of water. Since 1997, the organisation has constructed 52 rooftop rainwater harvesting structures. The rainwater is being used by the people successfully at a cost of Rs 10,000 per household. The water samples have been tested and found safe for human consumption.
source: http://www.rainwaterharvesting.org/Rural/Pallithode.htm