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Designing a domestic level Roof Rain Water Harvesting Systems
This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.
2
Table of Content
Sr.No.
1.
2.
3.
4.
Description
Introduction
a. Advantages
1.1. System Components:
2. Roof
3. PVC gutters
4. Downpipes
5. First Rain separator
6. Filter
7. Storage tank
8. Overflow pipe
9. Tap
Common problems and portable solution
Challenges
Calculation for Rain Water Harvesting:
1. Determination of Roof area:
2. Determination of pipe diameter
Page No.
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This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.
3
1. Introduction :
Rain water Harvesting:
It is the process of collecting and storing of rain water for future and productive use.
Uses include water for garden, water for livestock, water for irrigation, water for
domestic use with proper treatment, and indoor heating for houses etc.
a.




Advantages:
Recycling of free sourced rain water.
System can be constructed from readily available and low cost materials.
Easy and simple construction method.
Low maintains cost.
2. Objective:
 Documenting steps involved in designing of a domestic level Roof Rain Water
Harvesting Systems
3.
A)
B)
C)
D)
E)
F)
G)
H)
System Components:
Roof
PVC gutters
Downpipes
First Rain separator
Filter
Storage tank
Overflow pipe
Tap
A) Roof :
The existing roof is made use of to collect rainwater. Rainwater is pure as it falls
from sky, so it is necessary that roof be kept clean for it to remain pure when is
collected.
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AND PRAJ Foundation Technology development project.
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Slope:
Slope of the roof affects how quickly water will runoff during a rain event. A steep
roof will shed runoff quickly and more easily clean the roof of contamination. A
less-steep, flatter roof will cause the water to move more slowly.
The size of the catchment area of roof:
Roof will determine how much rainwater that you can harvest. It is quite
easy to collect rainwater falling on roof; rooftop rainwater harvesting is the
process of collecting rainwater falling on rooftops in a tank.
Note: Cleaning should be done by adult never by children.
RCC rooftop (slab) (Clean terrace)
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Tin Roof Water Flowing Over Roof (steep slope)
B) PVC Gutters:
 Gutters of PVC collect the rainwater from the roof and transfer it is to filter.
 On slopping roof PVC gutters can pick up leaves, dust, small twigs and other
organic matter.
 Gutters need to be cleaned at least once in week.
 During the rainy season PVC gutters should be inspected and cleaned daily if
possible.
 Gutters should fix to roof or to walls with clamps.
 The gutters /pipes should be in sloppy direction to the storage tank and not
away from it.
 PVC pipe cut in middle. PVC Gutter pipe should always have an end cap.
This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.
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A clean PVC gutter to catch every drop of rain
 End cap to allow first little amount of rainwater to be collected
separately. This has most of dust and dirt in it.
End cap for the gutter
C) Downpipes:
PVC down pipes brings the water remain gutters /pipes vertically down should
clamped firmly to wall and should be never be loosely fixed.
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AND PRAJ Foundation Technology development project.
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Downpipe
D) First Rain Separator:
 First rain separator also is used when roof is being cleaned.
 It has a valve or an end cap to allow the first little amount of rainwater
to be collected separately. This has most of the dust and dirt in it.
 It is important to ensure that the first rain separator is always kept in the
closed position and never left open.
 After every rain it should be opened carefully and the waste water allow
to flow out. The pipe should then be cleaned and the valve or the end
cap closed.
 Sometimes the first rain separator can get packed due to the dirt or dust
in it. In such a situation the valve or the end cap should be carefully
replaced.
This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.
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First Rain Separator
E) FILTER:
There are different types of filters are used for Rain Water Harvesting
1) Sand Gravel Filter
2) Charcoal Filters
3) PVC –Pipe Filters
1) Sand Gravel Filter :
 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 can removes silt, dust, leaves and other organic matter from entering the
storage tank. The ratio will be 1:1:2 for sand:gravel: bricks.
 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 gravel media should be taken out and washed before
it is replaced in the filter.
 These are commonly used filters, constructed by bricks masonry and filleted by
pebbles, gravel, and sand as in figure. Each layer should be separated by wire
mesh.
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AND PRAJ Foundation Technology development project.
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2) Charcoal Filters :
Charcoal Filters can be made in in –situ or in pebbles gravel, sand, Charcoal as
shown in the figure should fill the drum or chamber .Each layer should be
separator by wire mesh. Thin layer of charcoal is used to absorb odors.
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AND PRAJ Foundation Technology development project.
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3) PVC –Pipe Filters:
 This filter can be made by PVC pipe of 1 to 1.20 m length.
 Diameter of pipe depends on the area of roof. 6 inch diameter pipe is
enough for 1500 Ft2 roof and 8 inch diameter pipe should be used for
roof more than 1500 Sq2.
 Pipe divided into three compartments by wire mesh.
 Each component should be filled with gravel and sand alternatively as
shown in figure.
 A layer of charcoal could also be inserted between two layers.
 Both ends of filter should reducer of required size to connect inlet and
outlet.These filters could be placed horizontally and vertically in the
system.
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AND PRAJ Foundation Technology development project.
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F) Tank: Rain water tank is the device which is use collecting and maintained the
harvest rain water.
There are several materials available for a storage container
1. Corrugated Steel and Enclosed Metal
2. Concrete
3. Plastic
1. Corrugated Steel and Enclosed Metal :
Advantages:
 It is leak proof.
 It has a unique design prevents insects from entering & contaminating
the water.
 It does not rust or corrode hence water is hygienically clean.
 No growth of bacteria in the stored water.
 They are light in weight & easy to install in the long run saves money.
 Available in different capacities ranging from 250 - 4000 liters.
 It has 10 years warranty.
Corrugated Steel and Enclosed Metal
Disadvantages:
 Temperature of water gets increased.
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AND PRAJ Foundation Technology development project.
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2. Concrete Tanks:
Concrete Water Tank
Advantages:
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Readily available.
Have long life and relatively low maintenance, increase its economic benefits.
It is not as likely to rot, corrode, or decay as other building materials.
Concrete is a non-combustible material which makes it fire-safe and able to
withstand high temperatures.
It is resistant to wind, water, rodents, and insects. Hence, concrete is often used
for storm shelters.
Disadvantages:
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low ductility
low strength-to-weight ratio
Concrete is susceptible to cracks
3. Polyethylene and Polypropylene (Plastic) tanks:
Advantages:
 They are corrosion proof. They will not rust as metal tanks can and not
affected by ordinary chemicals.
 Low in cost
This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.
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 They are light in weight compared to concrete and are easier and cheaper to
install, and easy to move.
 They are very water tight and lids and connections fit very tightly so odor is
less of a problem.
 Easier to repair than concrete or metal tanks if they get damaged.
 It has a manhole extensions
Disadvantages:
 Polyethylene tanks, when placed above ground, can be subject to algae growth
as well as the possibility of a short life i.e. 20 years due to normal UV exposure
in sunlight.
 As they are light but is possibility to get punctured or otherwise damaged
during transport.
Plastic Water Tank
Sometime tanks are installed over the ground and some are under the ground
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AND PRAJ Foundation Technology development project.
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a. Underground Water tank :
Advantages of underground water tanks:
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Space required is limited, tanks can be placed under driveways or lawns
Water stored underground is safe from interference and spoilage from
important where civil unrest/ civil destroyer.
Underground tanks are protected from fires and other natural disasters.
In an agricultural or wildlife situation, underground tanks are protected from
animals that may damage above ground tanks.
Manhole is present.
Disadvantages of underground water tanks:
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More expensive.
Pumps need to extract water from underground tanks so more electricity will
be required.
Sometimes cleaning may be difficult.
b. Over ground water tank :
Advantages of over ground water tanks:
 Least expensive.
 Manhole is present, so cleaning is much easier.
 No pump / motor will be required.
Disadvantages of over ground water tanks:
 More spaced will be required.
 More interference and spoilage from where civil unrest/ civil destroyer, tank
should be protected from animals that may damage above ground tanks.
F) Overflow Pipe:
 The storage tank will have an overflow pipe from the top of the tank.
 In case of heavy rain, the overflow pipe will allow the excess rain water to be
safely disposed of without causing any flooding.
 It will have a mesh at the bottom to prevent rats, squirrel sand cockroach from
coming in.
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AND PRAJ Foundation Technology development project.
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 The mesh should be checked weekly and if torn or open should be repaired or
replaced immediately.
Overflow Pipe
G) TAP:
 A tap is provided in every tank to draw the rainwater out. Sometimes a tank
canhave more than one tap.
 A broken tap will result in the entire system going to waste as all the collected
rainwater will flow out.
 If there is any leak in the tap, that too should be taken care of by replacing the
washer or by getting a good plumber to repair it immediately.
This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.
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Tap
This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.
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Disadvantages: (Care need to be taken for)
Common problems in Roof Rain water
harvesting systems
Supplies can be contaminated by bird
/animal droppings.
Many times blockage can occur due to
leaves and trashes.
PVC pipes can be suffered from growth of
algae.
Initial investment cost is high.
Portable Solutions
Frequent cleaning should be done.
Surface and guttering structures needs to
be cleaned / flushed before use.
Pipesneeds to be clean when growth of
algae is seen.
It can be reduced by using commonly
available materials.
Challenges:
 Droughts / long periods of time with little or no rain can cause serious
problems with your supply of water.
Calculation for Rain Water Harvesting:
This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.
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Calculations necessary in rain water harvesting are:
1. Determination of Roof area
2. Total water harvested as per roof area
3. Pipe size for water removal
Roof areas are of two types
a. Flat roof
b. Sloping roof
For Flat roof area can be calculated as= 𝐿𝑒𝑛𝑔𝑡ℎ ∗ 𝐵𝑟𝑒𝑎𝑑𝑡ℎ
And if shape of terrace is flat then,
Then roof area calculated as:
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AND PRAJ Foundation Technology development project.
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A1 = X * Y
A2 = X* Y
Then,
total area = A1+ A2
(A1)Area of one part of the roof = a ∗ y
(𝐴2)𝑎𝑟𝑒𝑎 𝑜𝑓 𝑜𝑡ℎ𝑒𝑟 𝑝𝑎𝑟𝑡 𝑜𝑓 𝑟𝑜𝑜𝑓 = 𝑎 ∗ 𝑦
𝑇𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎 = 2 ∗ 𝑎 ∗ 𝑦
Example:
If area of slab roof is2479 Sq. meters with annual rainfall of 292.1395 mm
(millimeters)
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Roof yield = 2479 X 292.1395 = 724,213 liters/year
Water yield = 714,213 liters/year
SIZING VERTICAL DOWNSPOUT PIPES (Metric measurements)
Based on Maximum Allowable Catchment Surface Area (in square meters)
at various rainfall rates
Maximum Potential Rainfall Rate
Diameter of
25 mm/hr
50 mm/hr 75 mm/hr
100
125
downspout pipe
mm/hr mm/hr
2
2
2
75mm
600 m
300 m
200 m
150 m2 120 m2
100mm
1,286 m2
643 m2
429 m2
321 m2 257 m2
125mm
2,334 m2
1,117 m2
778 m2
583 m2 467 m2
Example :
For 23m2 area what should be size of pipe
Ans :
Diameter =2*√𝑎𝑟𝑒𝑎/3.14
=2 ∗ √23/3.14
=5.41 cm
NoteThis document is prepared as educational content / manual with the purpose of providing basic information
on said topic and technical information related to it. Information published in this document is collected from
Open source public domain and through VIGYAN ASHRAM own in this field. Users are requested to consult
experts from this field for any further details.
This manual is prepared under financial & technical support of “Department of Science & Technology (DST)
AND PRAJ Foundation Technology development project.