Since there are no currently active contests, we have switched Climate CoLab to read-only mode.
Learn more at
Skip navigation
Share via:


Rainwater harvesting refers to the collecting of rainwater, mostly on a roof, from where it flows through gutters into a collection tank.



Rainwater harvesting is a way of capturing rain as it falls and retaining it in the soil or in tanks below ground so it can be later used as a source of clean water. By constructing ridges of soil along the contours of fields rainfall is held back from running off the hard-baked soils too quickly, so that crops have enough water to grow. Even when rainfall levels are low, families can harvest enough food. Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site, rather than allowing it to run off. Rainwater can be collected from rivers or roofs, and in many places the water collected is redirected to a deep pit (well, shaft, or borehole), a reservoir with percolation, or collected from dew or fog with nets or other tools. Its uses include water for gardens, livestock, irrigation, domestic use with proper treatment, and indoor heating for houses etc. The harvested water can also be used as drinking water, longer-term storage and for other purposes such as groundwater recharge.

In a normal scenario the rainwater is collected from roof buildings and then stored inside of a special tank. Rainwater harvesting systems are designed after assessing site conditions that include rainfall pattern, incident rainfall, subsurface strata and their storage characteristics. Rainwater harvesting is popular all across the world, although in countries that are very dry, such as Australia, it is even more popular.

Category of the action

Mitigation/Adaptation, Changing public attitudes about climate change

What actions do you propose?

Build water tank for every building for roof top rainwater harvesting

Roof top rainwater harvesting is a system of catching rainwater where it falls. In rooftop harvesting, the roof becomes the catchments, and the rainwater is collected from the roof of the house/building. It can either be stored in a tank or diverted to artificial recharge system. This method is less expensive and very effective and if implemented properly helps in augmenting the ground water level of the area. Rain Water Harvesting actively draws upon traditional management system and knowledge. It uses simple technology, which can be maintained at the household level. No new organizational structures are needed for operations and maintenance.
One way to collect water is rooftop rainwater harvesting, where any suitable roof surface — tiles, metal sheets, plastics, but not grass or palm leaf — can be used to intercept the flow of rainwater in combination with gutters and downpipes (made from wood, bamboo, galvanized iron, or PVC) to provide a household with high-quality drinking water. A rooftop rainwater harvesting system might be a 500 cubic meter underground storage tank, serving a whole community, or it might be just a bucket, standing underneath a roof without a gutter. Settlement of community on ridges and crests make accessibility to water, a problem for the people. Rainwater harvesting is a viable technological option that has been practiced for centuries in these communities. Rain Water Harvesting has been very useful in rural settlements where the pressure head is not sufficient to supply water from the conventional source such as springs and streams.

Rainwater can be harvested and used by those in the city who have running city water just as well as it can be used by those in more rural towns where adequate water supply may be unavailable. It is simple to replace many things that you are using regular city water with rainwater and enjoy numerous benefits when you do. Not only it helps you to conserve water but can also help you to save energy as the need to maintain centralized water system can be bypassed.

Components of the roof top rainwater harvesting

The system mainly constitutes of following sub components:

  • Catchments
  • Transportation
  • First flush
  • Filter


Fig : Components of Rainwater harvesting


The surface that receives rainfall directly is the catchment of rainwater harvesting system. It may be terrace, courtyard, or paved or unpaved open ground. The terrace may be flat RCC/stone roof or sloping roof. Therefore the catchment is the area, which actually contributes rainwater to the harvesting system.


Rainwater from rooftop should be carried through down take water pipes or drains to storage/harvesting system. Water pipes should be UV resistant (ISI HDPE/PVC pipes) of required capacity. Water from sloping roofs could be caught through gutters and down take pipe. At terraces, mouth of the each drain should have wire mesh to restrict floating material.

First Flush

First flush is a device used to flush off the water received in first shower. The first shower of rains needs to be flushed-off to avoid contaminating storable/rechargeable water by the probable contaminants of the atmosphere and the catchment roof. It will also help in cleaning of silt and other material deposited on roof during dry seasons Provisions of first rain separator should be made at outlet of each drainpipe.


There is always some skepticism regarding Roof Top Rainwater Harvesting since doubts are raised that rainwater may contaminate groundwater. There is remote possibility of this fear coming true if proper filter mechanism is not adopted. Secondly all care must be taken to see that underground sewer drains are not punctured and no leakage is taking place in close vicinity. Filters are used fro treatment of water to effectively remove turbidity, colour and microorganisms. After first flushing of rainfall, water should pass through filters. A gravel, sand and ‘netlon’ mesh filter is designed and placed on top of the storage tank. This filter is very important in keeping the rainwater in the storage tank clean. It removes silt, dust, leaves and other organic matter from entering the storage tank. The filter media should be cleaned daily after every rainfall event. Clogged filters prevent rainwater from easily entering the storage tank and the filter may overflow. The sand or gravel media should be taken out and washed before it is replaced in the filter.

Method of roof top rainwater harvesting

Storage of Direct Use

In this method rain water collected from the roof of the building is diverted to a storage tank. The storage tank has to be designed according to the water requirements, rainfall and catchment availability. Each drainpipe should have mesh filter at mouth and first flush device followed by filtration system before connecting to the storage tank. It is advisable that each tank should have excess water over flow system.

Fig : A storage tank on a platform painted white

Excess water could be diverted to recharge system. Water from storage tank can be used for secondary purposes such as washing and gardening etc. This is the most cost effective way of rainwater harvesting. The main advantage of collecting and using the rainwater during rainy season is not only to save water from conventional sources, but also to save energy incurred on transportation and distribution of water at the doorstep. This also conserves groundwater, if it is being extracted to meet the demand when rains are on.

Construction of ferrocement tank

Ferrocement water tanks are made of steel mesh and wire, covered on the inside and outside with a thin layer of cement-and-sand mortar. The walls may be as thin as 2.5 cm. The tanks can be used for individual households or for whole communities, and they provide a relatively inexpensive and easy-to-maintain storage method. To avoid bending forces in the material, most ferrocement tanks have curved walls, in the form of a cylinder, a globe or an egg. Compared to concrete reservoirs, ferrocement tanks are relatively light and flexible.
To protect the water from contamination, the tank is covered with a lid or a roof that can be made of various materials, but is usually ferrocement. In this case, an aeration pipe with a screen is needed to allow fresh air to circulate in the tank, while keeping out rodents and insects. A manhole in the roof gives access to the tank for cleaning and repairs. Water flows into the reservoir through an inlet pipe, which is normally above the water level.

Suitable conditions

  • Avoid making sub-surface ferrocement or anthill/lime/cement tanks in unstable soil.
  • Do not site tanks near big trees whose roots might crack the walls.
  • Do not site tanks where heavy vehicles will pass close to tank wall.
  • Do not site sub-surface tanks in areas of high water tables to reduce risk of flotation.


Construction, operations and maintenance

General advice on cement: A common cause of cracks in structures and linings (e.g. in tanks, dams, waterways, wells) is errors in mixing and applying the cement. First of all, it is important that only pure ingredients are used: clean water, clean sand, clean rocks. The materials have to be mixed very thoroughly. Secondly, the amount of water during mixing needs to minimal: the concrete or cement needs to be just workable, on the dry side even, and not fluid. Thirdly, it is essential that during curing the cement or concrete is kept moist at all times, for at least a week. Structures should be covered with plastic, large leaves or other materials during the curing period, and kept wet regularly.

Specific advice: Range of volume: From 1 m3 to over 80 m3. Range of depth: Usually, between 1.5–3.0 m.

Build the wall of the tank

  1. Cut the chicken wire/hexagonal wire mesh using the wire cutters. The tank size will dictate the necessary sizes of the mesh.
  2. Cut the electro-welded mesh using the bolt cutters. The tank size will dictate the necessary sizes of the mesh.
  3. Lay the chicken wire and electro-welded mesh flat to create layers. The layering should be as follows: 2 layers of chicken wire, 1 layer of electro-welded mesh, 2 layers of chicken wire.
  4. Using wire ties and the pliers, fasten the flat layers of mesh together.
  5. Know that once the mesh layers are securely fastened, roll one end of the mesh to the other and attach the two ends together using the wire ties. This will create the circular body structure of the tank walls.
  6. Stand the mesh frame upright on the tank foundation and attach, using the wire ties, the frame to rebar protruding from the tank foundation.
  7. Tie the reinforcement wire near the top of the mesh frame and connect the wire to the ground. Be sure that the wire is in tension. This will prevent the walls from buckling during the construction and concrete curing stages. Four reinforcement wires around the perimeter of the frame are sufficient.
  8. Place scrap pipes as form placeholders for the inflow, outflow, overflow, and drainage pipe locations.
  9. Slap the concrete mortar onto the outside of the mesh frame. The mortar should be applied so that the holes of the mesh are completely filled.
  10. Allow the mortar to cure.
  11. Repeat step 9 with the inside of the mesh frame. Use a ladder when climbing in and out of the tank structure.
  12. Allow the mortar to cure. Keep the walls wet during the curing process. Once all the mortar has been applied, the walls will be about 2 inches thick.

Build the roof of the tank

  1. Cut the wood boards so that one edge is curved outward. This can be done while the tank walls are curing. The tank size will dictate the necessary sizes of the forms.
  2. Once the walls are fully cured, position the wood beams inside the tank. One log will be the center pole sticking up vertically from the center of the foundation. Two logs will be cross bars near to the top of the walls to support the wooden roof forms. Additional vertical logs will also be used to support the wooden roof forms; these logs should be placed along the inside circumference of the walls.
  3. Nail the cut wood board forms to the support poles. The wood will create a domed roof form.
  4. Nail the plywood sheets flat against the wood board forms.
  5. Leave one square space open in the forms. This will provide the tank lid opening.
  6. Lay concentric circle of rebar on top of the plywood sheets.
  7. Tie supporting wires to the rebar circles using wire ties. The wires should run from the dome center down to the base of the roof.
  8. Place and tie chicken wire mesh to the supporting wires.
  9. Slap concrete mortar mixture onto the roof forms. Be sure that enough concrete gets below the rebar circles.
  10. Allow the concrete to cure. Keep the concrete wet during this process.
  11. Once the concrete has cured, enter the tank through the open square space.
  12. Carefully remove the wood beams, forms and plywood.
  13. Paint the inside of the tank with the waterproof sealant.
  14. Install the square metal lid into the open space of to roof.

Optional: Paint the walls and roof of the ferrocement tank with any desired color.

Governmental Policy

  • Government prepared a flexible, socially and economically acceptable policy for rainwater harvesting.
  • Government highly prioritizes the roof top rainwater harvesting method for poor and marginalized community.
  • Government compulsory standardize to build water tank for every building for roof top rainwater harvesting.
  • Government gave more subsides for construction of rainwater harvesting tank for every building.
  • A global level with micro-credit for rainwater harvesting
  • Promotion of rainwater harvesting tanks through a combination of subsidy and credit from micro-finance institutions (MFIs).

Who will take these actions?

Government, private sector, and individual

Where will these actions be taken?


What are other key benefits?

The key benefits are:

  • A reduced amount of water consumption in the home, thus all of your utility bills will greatly shrink in size.
  • Collection of rainwater and harvesting it for everyday use has an ample number of exciting benefits.
  • Easily accessible
  • Easily adaptable to different types of communities
  • Flushing of toilets
  • Rainwater harvesting systems are cost effective, provide high quality water, reduce dependence on wells. The surplus rainwater can also be used to recharge ground water aquifer through artificial recharge techniques
  • Reduced dependency on water storage dams
  • Set-up costs are low, with construction easy, materials low-cost and maintenance cheap
  • Sustainability is higher
  • Use of excess rain water to recharge ground water table
  • Use of rain water for gardening and washing vehicles

What are the proposal’s costs?


Rainwater harvesting tank vary in size and costs. In Nepal, Ferrocement tank construction cost: US$35 - $75 per m3 storage depending on size (e.g. 11m3 for US$700, 46m3 for US$1,500).

Time line

Time line

Within 1 year baseline survey

1 to2 years installation of roof top rainwater harvesting system

Above 2 years cost benefit analysis.

Related proposals