Rooftop Rainwater Harvesting
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Rooftop Rainwater Harvesting. Pre-implementation guide for schools. At the city level Reduces water demand Reduces energy demand – lesser energy required for water distribution Improves groundwater situation

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Rooftop rainwater harvesting

Rooftop Rainwater Harvesting


guide for schools

Rooftop rainwater harvesting

  • At the city level

    • Reduces water demand

    • Reduces energy demand – lesser energy required for water distribution

    • Improves groundwater situation

    • In coastal areas, it can counter saline water intrusion into freshwater aquifers

    • Prevents urban flooding


Rooftop rainwater harvesting

  • At the school level

  • Reduces water bills

  • Sets an example for students and parents

  • Improves groundwater table if rainwater is recharged

  • Improves groundwater quality by dilution

  • (rainwater has nearly neutral pH and zero hardness)


Rooftop rainwater harvesting

The concept is simple




Recharge if there’s excess

Rooftop rainwater harvesting


Catchment – rooftop

Transportation – Down take pipes

  • Filters – First flush device

    • Leaf and grit filter

Storage – Tank / well

How much water can we harvest
How much water can we harvest?

Catchment Area


Amount of rainfall


Volume of water received






Rain Water Collected From one Side of the Roof

Rain Water Collected From Both Sides of the Roof

Rooftop rainwater harvesting

The water that leaves the rooftop may be 65 – 90% of

the water that falls on it

Roof material absorbs some water

Evaporation losses

More water loss if roof is flat

Keep the roof clean
Keep the roof clean

  • Remove leaves, plant debris, bird and animal droppings, dead rodents or birds

  • No tar / bitumen on the roof

  • No chemical pesticides or fertilizers for rooftop garden

Rooftop rainwater harvesting


Gutter (open on top)

Downtake pipe (cylindrical)

PVC pipes preferred. Do not use lead pipes

Pipe size is calculated based on peak flow rates

(rooftop area and peak rainfall rate)

Take care
Take care….

  • Clamp gutters and down take pipes well

  • Keep gutters clean of leaves

  • See that leaves do not clog a down take pipe

    (Jaali or leaf filter may be needed at rooftop discharge point but this may also be a problem if leaf debris collects on it and water stagnates on the roof)

  • Gutters and down take pipes should be gently sloping towards the storage tank, so that there is no stagnation of water in the pipe


Down take pipes

Over flow

Make your own filter with sand, charcoal and gravel




Out flow


Storage tank


Buy a filter off the shelf

First flush


Recharge tank

Check and clean filters before and after each rainy season

First flush
First flush

  • The first rain carries with it a lot of filth from the rooftop and dissolved air pollutants

  • This rainwater should be diverted away from the storage tank using a first flush device.

  • Some filters have a first flush system integrated in them

  • Check and clean first flush devices regularly.

First flush standpipe

Storage size

Depends on

  • Rainwater availability

  • Water requirement

  • Budget (storage tank is most expensive part of (rwh system)

  • Available space and aesthetics

    Excess water can be diverted into a groundwater recharge system

  • Do a water audit to check water requirement.


  • Above ground

    • Ground level or Intermediate floor level

    • Just below roof

  • Below ground

Save on pumping cost

Storage tank features
Storage tank features

  • Durable and no leaks

  • Built on a strong and stable substrate that can support the tank filled with water

  • Opaque – presence of light can cause algal growth in tank

  • Secure and impervious cover

  • Vents to be covered with insect proof mesh

  • Located as close as possible to demand and supply points to reduce plumbing

Storage tank features1
Storage tank features

  • Overflow pipe should be provided close to the top, to reduce dead space.

  • Overflow pipe diameter should be equal to inlet pipe diameter

  • Overflow pipe should lead excess water away from the foundation of tank and other buildings.

  • Preferably the overflow should be led into a groundwater recharge system or a garden

  • There will be sedimentation in the tank. Taps and pipes to draw water from the tank should

  • not be very close to the bottom to prevent sediments from entering the plumbing system

  • and clogging it.

What can we use the rain water for
What can we use the rain water for?

If used for drinking, it is absolutely necessary to check water quality in a lab and treat as required

Other factors
Other factors

  • Some changes may be required in plumbing, if retrofit

  • Clean rwh system regularly

    • Rooftop everyday

    • Tank after a rainy season

    • Filter after a rainy season