A Municipal Guide to Promote Rooftop Rainwater Harvesting ~ Rainwater Harvesting Systems


In New Jersey, there is interest in harvesting rain water from roofs of buildings and greenhouses at businesses and farms.  The most basic form of rainwater harvesting is simply diverting roof runoff away from stormwater infrastructure and onto permeable areas like lawns and landscaped areas.
There are many benefits to harvesting rainwater such as reducing runoff during rain events, conserving potable water, having a source of relatively high quality water for supplemental irrigation, and avoiding state permits for additional water allocations.  Reducing runoff and conserving potable water contribute to a wider goal of sustainable development, particularly in densely populated areas.

A rainwater harvesting system and control system installed at a school at the Samuel J. Mickle Elementary School in East Greenwich, NJ

What are the rain harvesting system components?

A basic rainwater harvesting system consists of:

  • The catchment surface;  for roof collection systems, this is the roof surface

  • Gutters and downspouts

  • The storage tank

  • A water delivery system;  this may be a simple spigot and a hose on a rain barrel or a pump system that supplies an irrigation system

  • Leaf screens may be useful to keep debris out of the collected water;  such screens will need maintenance to prevent clogging as they collect leaves and other debris

  • First-flush diverters may be useful to remove the first flush of water after runoff begins

Other design considerations and components:

  • Consideration should be given to the disposition of water that overflows the collection tank during large rain events.  Consultation with a qualified engineer may be advisable.

  • If harvested rainwater will be used for critical irrigation, it may be necessary to use municipal water or well water as a back-up source of water.  This is sometimes called a dual supply system.

  • The potential for backflow can be prevented by leaving a physical air gap between the potable water supply outlet and the potentially contaminated water or by installing anti-siphon devices such as double check valves or vacuum breakers.

  • Depending on location, local conditions, and materials used, tanks and their plumbing may not need any special preparations for winter weather.  Valves may need testing the following spring as they are susceptible to damage by freezing water.  If modifications are needed, it may include a method to divert water from being collected during winter months and a valve to drain the water from the tank before the onset of freezing temperature.

  • It is important to know the local regulations.  In all cases, building codes, health department stipulations and water supply company requirements should be followed to prevent cross connection.  Obtaining permits, using certified plumbers, or allowing inspections of completed installations may be required.

What types of tanks are available?

  • The most common vessel is a closed tank.  Less common vessels include stone cisterns or open basins. 

  • Plastic tanks are available commercially in a variety of sizes and shapes.

  • Plastic tanks for above-ground use should be made from plastic that is resistant to ultraviolet light and opaque in color.  

  • For underground use, tanks manufactured for this application should be used.

  • Larger commercially-available tanks made from fiberglass, galvanized steel, or a variety of other materials are appropriate when a plastic liner is used. 

  • If collection vessels are recycled from other uses, those used to store toxic materials must be avoided.

  • Matrix systems which consist of modular milk-crate-like plastic forms are also available.  These systems provide flexibility in size and shape of the reservoir area and can be used where a load-bearing vessel is desired.  Pre-packaged and modular systems are available commercially, and some manufacturers use certified contractors for installation.

What type of considerations should be made when selecting a tank?

  • The capacity of a rainwater harvesting system can range from a single 55-gallon barrel to a series of cisterns capable of storing thousands of gallons. 

  • Larger tanks will provide greater capacity for storage.  If only a small portion of collected water will be used, a smaller tank may achieve the same efficiency in the long run.

  • It is important to determine the roof area that is available and practical to be used as a collection surface.  Tanks can be sized to meet a specific need.

What is involved with draining the tank?

  • Collection tanks are often fitted with some method to facilitate draining and to remove any debris that may collect in the tank. 

  • A drain or a manhole opening can be used for this purpose.

  • For maximum collection efficiency, a spare tank can be connected to collect water while the main tank is taken offline for cleaning.

  • The ability to drain or pump out stagnant water is also helpful.  Tanks may need to be vented to allow filling with water, but all openings should be screened to prevent access by mosquitoes and/or small animals.  Tanks should be located both as close to the collection surface and as close to where water will be used as is practical.

How to quantify the water quantity and demand?

There are simple formulas that can be used to calculate the potential volume of rainwater harvested from a storm.  Examples are available in Rooftop Rainwater Harvesting For Plant Irrigation I: Design Concepts and Water Quantity, Rutgers Fact Sheet FS1162  (http://njaes.rutgers.edu/pubs/publication.asp?pid=FS1162).

For more information about rainwater harvesting for plant irrigation, municipalities also are referred to Rooftop Rainwater Harvesting For Plant Irrigation II: Water Quality and Horticultural Considerations, Rutgers Fact Sheet FS1165 (http://njaes.rutgers.edu/pubs/publication.asp?pid=FS1165).

Safety Considerations

  • It should be remembered that a gallon of water weighs more than eight pounds, so a 55-gallon drum of water will weigh over 400 pounds, and a 500-gallon cistern would weigh over 4,000 pounds.

  • Any water collection vessel should be installed on a level and solid surface to prevent the possibility of anyone being crushed or trapped by an unstable tank.

  • A base of soil, sand or pea gravel may be a suitable platform for a collection vessel, but a concrete pad may offer a more stable surface that is less susceptible to erosion. 

  • Consulting a professional engineer to be sure of the stability of the soil or constructed base is advisable.

Legal Considerations

State laws and local ordinances should be consulted when designing any rainwater harvesting system.

Cost and Benefits

  • For sites serviced with municipal or well water, per gallon savings of using harvested water are unlikely to offset the costs of installation and maintenance of a rainwater collection system in the short-term.  Instead, the most valuable benefits may be environmental gains of water conservation and stormwater management and for businesses, the demonstration of good environmental stewardship. 

  • Rainwater harvesting may be a good solution for sites without easy access to municipal or well-water.


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