A cistern is a container with a
waterproof lining that is
designed to store rainwater.
Can be underground or above
Can range in complexity from
a barrel at the bottom of a
downspout to more extensive
systems with covered storage
tanks, pumps, and water
Collected rainwater can technically be used
for indoor, including drinking water (if
properly filtered or purified) and outdoor use.
According to the City of Portland rainwater
harvesting code guide, collected rainwater
cannot be used for drinking unless it is
treated and meets both state and federal
standards for potable water.
Saves money on water bills
Clean water without chlorine and other
Less dependence on public water supply
May help reduce storm water fees
Water conservation – helps the environment
About 1 inch of rainfall on a 2,000 sq ft roof
can create 1,250 gallons of reusable
The total cost varies depending on the
size, need, and material used.
A complete system with filtration and water
capturing for 800 gallons of water at a single
residential home may cost $1000 for parts (not
Savings over time can be great in an area with
regular rainfall, such as Portland. For example, if
an average water bill is $100 per month for a
family of 4, the savings could be $5000 over a
period of 5 years ($6,000 minus $1,000 for the
initial cost of the cistern).
The water collection system (roof, gutter, and
The filter to keep out debris, such as bits of
The water storage vessel (cistern). The size
can vary in range depending on user needs.
The cistern should be made out of stone,
concrete, or another non-corroding, non-
contaminating material. Wood and metal are
The steps involved in designing a complex
water harvesting system include:
Calculation (not absolutely required)
Design (above or below ground, for example)
Draw the site and all site elements to scale.
During rainfall, observe the area to find out the
existing drainage flow patterns. Show the water flow
direction with arrows. Indicate high and low areas on
Look for catchment areas for water harvesting, such
as a roof or paved area.
For outdoor use, find planted areas or potential
planting areas that require irrigation.
Locate above or below ground storage near planted
Decide how water will be moved from the catchment
area to the holding area or storage container. Rely on
gravity to move water whenever possible.
This step is not required, but it is
recommended for more elaborate systems.
Calculate the monthly supply (rainfall harvest
potential) and the monthly demand (plant
water requirement) for a year.
Next, calculate your monthly storage
requirement if you are designing a more
Here is a website that is helpful with the
calcuation step: http://www.green-
Use the site analysis info and potential supply
and demand calculations to size and locate
Roofs or shade structures can be designed or
retrofitted to maximize the size of the
For efficient use of harvested water, group
plants with similar water requirements
Use gutters and downspouts to transport the
water from the roof to the storage area.
Select gutters that are 5 inches wide.
Select galvanized steel (26 gauge minimum) or
aluminum (.025 inch minimum) gutters.
Slope gutters 1/16" per 1' of gutter, to enhance flow.
Use an expansion joint at the connection, if a straight
run of gutter exceeds 40 feet.
Keep the front of the gutter one-half inch lower than
Provide gutter hangers every 3 feet.
Do not exceed 45 degree angle bends in horizontal
Select elbows in 45, 60, 75, or 90 degree sizes.
Space downspouts a minimum of 20 feet
apart, a maximum of 50 feet apart.
Provide 1 square inch of downspout area, for
every 100 square feet of roof area.
Select downspouts in different configurations
-- square, round, and corrugated round,
depending on your needs.
Use 4-inch diameter Schedule 40 PVC to
convey water to the storage container or
For more in-depth information on installing a rainwater
cistern, the following resources may be helpful:
Planning, design, installation, and more:
City of Portland Code Guide – Office of Planning and
More on calculations: http://www.lid-