Rainwater harvesting system design

Collecting, filtering, and reusing rainwater is an expensive concept and lengthy process, but can benefit a facility with a reasonable return on investment.

By David E. DeBord November 28, 2011

In metropolitan areas, especially those with combined sanitary and storm sewers, it is often a requirement that a new building must include a system for storm water detention. This is a system that collects the rainwater and stores it so that it may be released at a rate slow enough that the flow can be safely assimilated by the existing sewer system. Unfortunately, and ironically, many of these jurisdictions that require rainwater capture and detention do not allow for it to actually be used.

Combined sewer overflows demographics

Combined sewer systems are remnants of the country’s early infrastructure and so are typically found in older communities. Combined sewer systems serve roughly 772 communities containing about 40 million people. Most communities with combined sewer systems are located in the Northeast, the Great Lakes regions, and the Pacific Northwest.

Rainwater harvesting is most often referred to as an “emerging technology”; however, rainwater cisterns are not a new concept. In the Middle East in 2000 B.C., typical middle-class dwellings stored rainwater in cisterns for use as a domestic supply as well as private bathing facilities for the wealthy. The reason that the construction of this type of large public underground cistern may have been abandoned as a rainwater harvesting system strategy could be due to the fact that the construction of underground cisterns is considerably more expensive than the construction of dams and some other alternatives. Our failure to appreciate the value of rainwater, coupled with the perception of construction costs, has rendered rainwater harvesting somewhat of a lost art.

System design professionals

Alternative water source systems (AWSS) should always be designed and sized by design professionals possessing either a Certified in Plumbing Design (CPD) certification from the American Society of Plumbing Engineers (ASPE) or a Professional Engineer (PE) license, and certification from the American Rainwater Catchment Society of America (ARCSA) as an ARCSA Accredited Professional (ARCSA-AP).

1. Collection

The first part of the system that the rain comes into contact with is the roof itself. Although the plumbing engineer does not usually design the roof, he or she is often consulted or at least has an opportunity to contribute some input. The roof is a critical component of the rainwater harvesting system and will define some of the other system components such as filtration. Recommended roofing materials include standing seam metal, ethylene propylene diene monomer (EDPM), and slate or tile.

The following roofing materials are not recommended for rainwater harvesting:

  • Asphalt: One of the most common roofing materials used in the U.S. is asphalt. Unfortunately, this is not a good surface from which to harvest rain that is intended for potable use. Crumbling asphalt roofing material debris can be separated during filtration, but this type of shingle can also leach petroleum products into the water. These contaminants can also be removed from the captured rainwater, but it is preferable to keep them from entering our system in the first place.
  • Wooden shingles: Wooden shingles are porous and can be a fertile medium for the growth of mold and fungi that will be introduced into our rainwater harvesting system. They are also treated with chemicals that are intended to protect the wood, but are neither intended nor acceptable for human consumption.
  • Unprotected metal: There have been cases of unacceptable levels of heavy metal contaminants in vegetables that were irrigated from rainwater harvested from metal roofing materials. Tests have indicated that these bare metal roofs can contribute to heavy metal levels that exceed the allowable limits. If this water is used to irrigate non-edible vegetation, then it might be acceptable. However, as there could still be a risk of contamination, unprotected metal should not be included in a rainwater harvesting system. Note that some modern metal roofing materials may be exempt from these comments, especially if they include protective coatings.

The following roofing materials are recommended for rainwater harvesting:

  • Standing seam metal: Standing seam metal roofs can be an excellent material for rainwater harvesting. A standing seam roof is constructed of many interlocking panels that run vertically from the roof’s ridge to the eave. The interlocking seam where two panels join together is raised above the roof’s flat surface, allowing water to run off without seeping between panels.
  • EPDM: Ethylene propylene diene monomer (EPDM) is a rubber-like compound that has been used in roofs in the U.S. since the 1960s and is one of the most common types of low-slope roofing materials. This is because it is relatively inexpensive, simple to install, and fairly clean to work with when compared to conventional built-up roofs.
  • Slate or tile: Slate is a good surface to harvest rain from as long as it is kept clean. One possible advantage of slate and tile is that they do not deteriorate. These materials do not include asbestos tiles, which have been mainly used as siding in the past but could potentially be found on some roof surfaces.

2. Storage

Storage tanks come in many shapes, sizes, and materials. They can be located below grade, above grade, near the roof, or in many other locations.

The appropriate tank size depends on:

  • Roof collection area
  • Anticipated local rainfall, and
  • Our intended use of the collected water.

There could be some benefits to sizing the tanks so that they overflow regularly to provide skimming of the water surface, but large enough to provide a reliable water supply. The storage tanks are typically the largest expense in a rainwater harvesting system. This makes it critical for calculating the return on investment (ROI) or payback time.

3. Filtration and disinfection

There are many facets of this process to consider. We will generally need several types of filtration and disinfection components to produce a suitable quality of water. The quality of water may be subject to our intended use of it, but generally we will be required by the authorities to deliver water of potable standards, even though they usually will not allow it to be used as potable water.

First flush, roof washers, or rain diverters flush off the first water of a storm before it enters the storage tank. This water most likely would be contaminated by particulates, bird droppings, and other materials on the roof surface. We can improve the quality of collected water and increase the efficiency of our filtration/disinfection systems by diverting this first flush of water to the storm sewer rather introducing it into our collection system. This could be accomplished with a manufactured device or a diverter pipe of sufficient liquid holding capacity, or with some type of constructed basin with overflows. The manufactured devices usually are touted as “self-cleaning” but still require access for occasional inspection and maintenance.

General rules of thumb:

  • First flush devices should remove about 10 gallons of water per 1,000 sq ft of roof/catchment area.
  • Remember to size for each downspout of the catchment/roof area.
  • If harvesting rainwater in an area with lots of dust or other pollution sources, a larger first flush device is recommended.
  • First flush devices must be easy to open and maintain. If not, the device will most likely be disabled.


4. Delivery

One of officials’ biggest concerns is the potential for cross-connections with potable water supply. Private and municipal potable water systems must be protected from this contamination, and such protection may be provided by physical air gaps, break tanks, or reduced-pressure-type vacuum breakers (RPZ), as required/approved by the local authority having jurisdiction (AHJ).

Pipe identification also requires some consideration. There is still a lack of industry-wide agreement on how to label the piping and what color it should be. Piping manufacturers are producing pre-labeled piping in purple plastic, but it is still not an agreed-upon standard for all jurisdictions. From many AHJs’ perspectives, there are only two types of water: potable and nonpotable. Implementing the use of purple pipe for all nonpotable water is intended to prevent cross-connection between the two. In addition, gas industry engineers are concerned that a yellow alternate water pipe could be confused with a natural gas pipe, which is also yellow.

That being said, it seems that the general consensus is that piping that carries water from any AWSS shall be purple in color and labeled every 5 ft as to its contents. The label should note that the nonpotable water is not for consumption, unless the system is approved by local authorities to be used for potable purposes.

The type of pumping system that we include in our system will depend on other design components. One of the most popular systems provides submersible pumps within the stage tank system. These pumps should always include floating inlets with integral strainers. These inlets float below the surface of the water so that they are presumably capturing the cleanest of the stored water. The inlets lie below potential contaminants that are floating on the surface, and above any sediment that might have accumulated on the bottom of the tank.

Sizing rainwater systems

Historical rainfall data is needed in order to be able to base the system design. The rainfall rate, or intensity, is a term that relates the quantity of rainfall to a unit of time.

The rainfall rate used for designing roof drainage systems is related to the average frequency of occurrence and the time that it takes runoff to reach the collection device from the most remote portion of the contributing roof area. The average frequency of occurrence, also referred to as the return period, is an indication of the average number of years between storms that will produce rainfall rates equaling or exceeding a given amount. The amount of time that it takes the runoff to reach the collection device from the most remote portion of the tributary area is known as the time of concentration.

The duration of a storm equals the time of concentration and is the period of time during which the heaviest rainfall occurs and, in theory, when the greatest amount of runoff occurs. The rainfall rate used for the design of roof drainage systems is based on a 10-year return period and 5-minute duration. The 10-year, 5-minute rainfall rate is recommended as a minimum for use in designing roof drainage systems. Historically, it has been relatively accurate.

An example of a simplified method of cistern sizing is a 15,000-sq-ft roof in Minnesota, during the month of July, with a 4.34-in monthly rainfall. Since each square-foot of roof surface can generate approximately 0.62 gal of usable water per inch of rainfall, this particular roof could generate 40,362 gal of water.

In this case, a 40,000-gal cistern may not be practical, but cisterns generally should be as big as the project can afford in terms of both expense and space. They are the biggest and generally the most expensive part of the system.

Detailed cistern sizing method:

  1. Obtain monthly rainfall averages for a given location.
  2. Determine the catchment footprint area.
  3. Determine the runoff coefficient from tables published by ASPE and ARCSA.
  4. Determine the safety factor based on knowledge of the site and discussion.
  5. Obtain the monthly catchment potential by multiplying the column entries on each row.
  6. Calculate the annual catchment amount by multiplying the total annual rainfall rates in the last row of columns B, C, D, and E.
  7. Double-check the results by comparing the annual catchment to the sum of each month found.
  8. Review the monthly supply amount in column F.
    1. Identify highs and lows
    2. Observe patterns in data
    3. Identify the highest 4 months
    4. Identify the lowest 4 months 
    5. Identify the months where capture potential is zero.

For outdoor irrigation use, rainwater requirements will vary based on:

  • The climate of the location
  • The types of plants
  • Amount of exposure to direct summer sun
  • Soil conditions
  • Presence or lack of mulch
  • Size of the area irrigated.

All of these factors will determine how much irrigation water is needed. Large landscapes with large water demands may not be readily accommodated by rainwater catchment systems.

Determine site water requirements with the following equation:  


Q = Flow in GPM

A = Area in square feet

WR = Water Requirement in inches per week

0.6234 = volume of water in 1 sq ft 1-in. deep

D = Number of days per week available for irrigation

H = Hours per day

DU = Distribution Uniformity or System Efficiency

*Consider that the WR = Water Requirement should be roughly equivalent Q + 25%.

Rainwater harvesting system cost

The cost of a rainwater harvesting system depends on the size of the system. A simple system for a commercial building could be as little as $10,000, while a complete system for a commercial building could be $100,000 or higher.

Developing a budget for a rainwater harvesting system may be as simple as adding up the prices for each of the components and deciding how that coincides with the budget expectations. The largest expense is the storage tank, and the cost of the tank is based upon the size and the material. Costs can range from as low as about $0.50 per gallon for large fiberglass tanks to up to $4.00 per gallon for welded steel tanks. As tank sizes increase, unit costs per gallon of storage decrease. There is also an installation cost, which can also vary greatly. An underground tank will cost much more for excavation and anchoring. An improperly anchored underground tank can "float" out of the ground and cause damage even if it only moves a little. System providers as well as Means Cost Data should be consulted when preparing cost estimates.

Operating costs

Operating costs should be considered as you prepare your budget. As with any water treatment system, the cleaner the water needs to be, the greater the efforts required to maintain the system. Fortunately, with filter cartridges, this just means regular replacement of the cartridges, and with the disinfection system, following the manufacturers’ recommendations for regular maintenance. But proper operation and maintenance of the system does add to total costs.

Filter cartridges should be replaced per the manufacturer’s specifications, based upon the rate of water use. Some of the operating costs and time expenditures necessary for system maintenance are regularly cleaning gutters and roof washers, checking the system for leaks by monitoring water levels, and paying close attention to water use rates to determine if an invisible leak has sprung. Although the “do-it-yourselfers” can handle all of these tasks with little added financial burden, the time for regular maintenance and operation must be set aside to operate a successful system.

Comparing to other sources of water

In some areas, the cost of drilling a well can be as high as $20,000 or more, with no guarantee of hitting a reliable source of water. The deeper the well, the more expensive the effort will be. Also, well water can have very high total dissolved solid (TDS) levels in some aquifers, resulting in “hard” water. Rainwater is naturally soft and has become a preferred option in some parts of the country with costs lower than or equal to those of drilling a well, and reliability high enough to justify reliance on weather patterns, rather than on an aquifer’s water quality and quantity.

ROI and financial budget calculations

In terms of financial considerations, rainwater budget calculations can be used for a basis of water savings, but cost data, such as the costs of water and sewage, is needed to evaluate the system further. Sewer charges are normally based on the water meter readings.

Plumbing codes and treatment

In the U.S., we routinely design buildings that use clean drinking water for flushing toilets and for landscape irrigation. Only about 1% of municipally distributed potable water is used for purposes that actually require potable water: drinking, cooking, and bathing. Green building practices are becoming more desirable and prevalent, and more emphasis is being placed on water conservation in building design. The U.S. Green Building Council’s LEED 2009: Technical advancements to the LEED rating system is one example of a demonstration of growing appreciation for this vital resource, as water credits have increased significantly from previous versions of LEED Certification requirements. Some of the more common water-conserving strategies that are being implemented include specification of low-flow toilet fixtures including dual-flush toilets, sensor-operated lavatories, and waterless urinals. Other measures include the use of gray water, reclaimed water, and rainwater harvesting (rainwater catchment). Stored rainwater collected from a large catchment surface is a cost-effective source of water for landscape irrigation and toilet flushing. The use of rainwater as a domestic-water source not only decreases the amount of clean drinking water used by a building, but it also decreases the amount of storm water runoff from the site, which in turn lessens its effect on erosion and decreases the load on storm sewers and waterways.

Authorities worldwide, including the Uniform Plumbing Code (UPC), the International Plumbing Code (IPC), ASHRAE, and others, are writing codes to address alternative water systems. While the national standards are currently behind in the realm of the design and installation of rainwater harvesting systems, some state and local authorities have issued codes, guidelines, and ordinances to address these systems. The IPC and UPC are both working to fully address the subject of rainwater harvesting and provide guidance to the design community on a national and international level.

Some states that already have in place, or under consideration, some form of regulations, guidelines, tax incentives, or assistance include:

  • Alaska
  • Arizona
  • California
  • Colorado
  • Florida
  • Georgia
  • Hawaii
  • Illinois
  • New Mexico
  • North Carolina
  • Ohio
  • Oregon
  • Texas
  • Utah
  • Vermont
  • Virginia
  • Washington
  • U.S. Virgin Islands


  • Australia
  • India
  • United Kingdom

Alternate materials and methods

Almost all codes include sections that deal with alternate materials, methods, systems, and equipment. An alternative material or method of construction shall be approved where the code official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method, or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.

This provides the code official with the flexibility to approve the installation of a rainwater harvesting system even if that system is not specifically addressed by the pertinent code. The design engineer must submit an application known as an Alternate Materials and Methods Request (AMMR), available from the building department. Approval is subject to review on a case-by-case basis. In some jurisdictions, separate approvals may be needed for state and local authorities. Alternatively, some jurisdictions will actually require rainwater capture and detention, and some even require rainwater to be reused on-site.

Usually, a licensed professional engineer is responsible for the proper operation of the system. Some states and codes acknowledge the CPD (formerly CIPE) designation that is certified by ASPE. The most important consideration is that if an alternative system is contemplated, it must be submitted to and approved by the AHJs. To expedite approval, the following checklist may be helpful.


Additional strategies

Enlist help. Seek out knowledgeable experts and resource people, including sympathetic code officials, to support your position.

Show respect for their position. Consider the building department a resource rather than an adversary. Maintain a cooperative, open-minded, and positive attitude, acknowledging that the building officials have the authority to approve alternatives that meet the intent of the code.

Develop relationships. Nurture trust, both in your design approach and in your willingness to meet the intent of the code. Having a good relationship with the building department is essential.

Meet and share information with building officials. Arrange an initial meeting to formally discuss the project and proposed alternatives.

Network. Cultivate relationships with experienced building officials who have approved and worked with the materials or methods in question, or who are open-minded and receptive to alternatives.

Ultimately, the AHJ has the last word. The local and state authorities have an obligation to ensure that the public water supply is safe and secure, in both quality and quantity.

Going forward

To learn more about these systems, consult ASPE and ARCSA publications. Both organizations also provide technical seminars and training opportunities on these subjects, and are involved with the writing of standards to define these practices and systems.

David E. DeBord CPD, LEED AP BD+C, ARCSA AP, has over 30 years in the consulting business. David is employed as a Plumbing Engineer at Environmental Systems Design in Chicago. He is a Senior Associate and serves as the TA (Technical Authority) in the International-Special Projects Group. David is the Legislative Vice President of ASPE (American Society of Plumbing Engineers) at the Society level and an Adjunct Assistant Professor at Illinois Institute of Technology. He is member and a Past President of the Chicago Chapter of ASPE. He is also a member of the American Rainwater Catchment Society of America (ARCSA), American Solar Energy Society (ASES), the Geothermal Heat Pump Consortium (GHPC), USGBC, and serves on code committees of the ICC and IAPMO. He has been published in several publications and writes data book chapters for ASPE. He also presents webinars and lectures for ASPE and other organizations.

References and additional information

Refer to publications from ASPE (American Society of Plumbing Engineers) and ARCSA (American Rainwater Catchment Systems Association), including the jointly published Rainwater Catchment Design and Installation Standards publication, as well as the IGCC (International Green Construction code), the Green Supplement from IAPMO, and various publications of the Texas Water Development Board. Some of the information above references these materials. The websites of the EPA and National Oceanic and Atmospheric Administration (NOAA) also have a large amount of relative information.