Rainwater harvesting basics: How to direct roof top rainwater into the sump


In cities, almost all the houses have sumps for storing either corporation water or water from bore well, tanker, etc. These sumps can be used for storing roof rain water during the rainy season. This saves the cost of a separate storage for rain water.  

A 2000 litre sump can collect 20mm of rainfall from a 100 sq. meter roof area.

Preparation - Materials required

  1. In case of already constructed house, the existing pipes can be used with an additional few for directing water into the sump.
  2. In new constructions, 4” to 5” PVC down pipes are required. Size and number depends upon the roof area and roof design.
  3. For sloping roof, cast-in-situ half round gutter laid across the slope all along the edge will collect rainwater. Alternatively, GI sheet turned in the form of gutter, running all around the edges of the roof can collect rain water. Clamps are required for securing the GI sheet to the roof.
  4. ‘Nahni trap’ or floor trap
  5. PVC Elbow junction for connecting pipes at right angles
  6. PVC horizontal pipes – 4” to 5” diameter – for leading water from down pipes into the sump.
  7. Over-flow pipe to allow excess water from the sump to flow out
  8. Filter for filtering the rain water before it reaches the sump

Site specification

  • Can be done in any kind of residence, office, school building etc.
  • Can be done on any building structure - kattcha or pucca
  • Construction of a sump is not possible in hard rock areas or water logged areas

Construction technique

Step 1
Connect the discharge point in the roof to the down water pipe. Place a ‘nahni trap’ at the discharge point to prevent fallen leaves from entering the pipe.

In case of sloping roof: Construct a cast-in-situ half round gutter across the slope, all along the edge, to collect rain water. Alternately, a half turned GI sheet, running horizontally all along the edge of the roof (across the slope) can be clamped to the roof. Place a ‘nahni trap’ at the inlet of the down water pipe to prevent fallen leaves from entering the pipe.

Step 2
Fix a first rain separator before the filter.

Step 3
At the end of the down water pipe connect the filter (for various kinds of  filters – see details given in the same section)

Step 4
Using horizontal pipes, lead the filtered water into the sump.

Step 5
Ensure that the sump has an overflow pipe to allow excess water to flow out.


  • For a new sump construction, the cost should be taken at Rs. 5 per litre.
  • In an existing house with a sump tank the cost could be between Rs. 1000 to Rs. 5000.


  • Sweep the roof before every rain. The roof should be devoid of all debris like fallen leaves and flowers, bird droppings, paper, plastic, etc. These might clog the ‘nahni trap’.
  • Always check water quality before drinking.
Post By: rajshekar