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Rainwater Catchment Systems: The Basics Catching Rain for Re-Use


Rainwater Catchment Systems: The Basics

Catching Rain for Re-Use

By Stacey Meinzen

California’s extreme drought has us all worried. The possibility of an El Niño leaves us hopeful. However, El Niño is likely to come more in downpours than in a gentle drawn-out season of rain. Clearly mulching and swaling are key to helping recharge the aquifer beneath our feet. However, rooftop catchment can play a key role in water security for our region and our households, as well as connecting us to the seasons.

Rainwater can be used for landscapes, emergency in-home use, storm water control, wildlife and livestock watering, and fire protection. Before installing a system, you must first decide what the ultimate use for that water will be – namely, does it need to be potable?

Major components of a rainwater catchment system:

The major components of a rainwater catchment system are the collection surface (usually the roof), gutters, downspouts, pre-filtration systems or first-flush devices, storage tanks and distribution systems — which can include sanitization. (Source:

The amount of rainwater you can harvest will be determined by the size of your roof. The “footprint” of the roof can be calculated by finding the area of the building and adding the area of the roof’s overhang. To calculate how much rainwater you can harvest, use this equation: Harvested water (gallons) = catchment area (sq. ft) x rainfall depth (inches) x 0.62 (Source:

The Texas Commission on Environmental Quality has a report, “Harvesting, Storing, and Treating Rainwater for Domestic Indoor Use” that helps homeowners who are interested in harvesting rainwater. Many of the recommendations in this article come from this report.

Roof Material & Slope: Ideally, a smooth, nonporous roof material is best because it improves the efficiency of your system by absorbing less water and reducing the chance that microbes and debris will collect in the pores and seams of the roof.  For POTABLE WATER - Avoid using composite, asphalt, or asbestos shingles for rooftop catchment. It is also extremely important that you not use any roofing material that contains fungicides, algaecides, or any other biocide compound. As these products age and decay, they release a variety of organic and inorganic chemicals that can pose a health threat if consumed. 

There are some technologies - such as activated carbon filters followed by reverse osmosis that can remove these contaminants, but you should not risk exposing yourself to these health threats.

If possible, your roof should have a steep slope. During the periods between rainfall events, dust and debris can accumulate on the roof. It is much easier to rinse the debris from a well-pitched roof than it is from a roof that is level or that only has a slight incline. 

If you are going to coat your roof with a sealant or paint and you want rainwater that is potable, use a paint that has been certified under NSF Protocol P151 or ANSI/NSF Standard 61. NSF Protocol P151 was developed specifically for testing rainwater harvesting and collection systems while ANSI/NSF Standard 61 is a more general standard that applies to all materials that come into contact with potable water. Products certified under either of these requirements are approved for potable water applications and are unlikely to contaminate your water if properly applied. 

Gutters, downspouts, and piping: are used to collect the water harvested from your roof and transport it to the tanks that will store it before treatment. A seamless aluminum gutter or a plastic pipe may be best for your gutter. Again, if you need your water to be potable, you must be sure that the materials you use are not toxic. 

Aluminum oxidizes very slowly, thus, seamless aluminum should be relatively safe to use and allows you to avoid joint seams, which can harbor bacteria and algea. PVC pipe that meets the requirements of ANSI/NSF Standard 14 or Standard 61 is readily available. However, PVC gutters will probably not last as long as seamless aluminum gutters, because plastics are more vulnerable to sunlight damage and decay than aluminum. You will need to consider several other factors as you design your gutters, downspouts, and piping.

• Your gutter should gradually slope toward the downspout. If you do not properly slope the gutter, it will not drain completely and algae and bacteria will grow in moist areas. For best efficiency, the bottom of the gutter should drop 1 inch for every 8 feet of gutter length. 

• If your roof tends to capture leaves, twigs, or other large debris, you should install a self-cleaning leaf guard, gutter screen, or other similar material on the top of your gutter. If you don’t have much trouble with leaves and twigs, you will probably find that a basket screen or funnel screen located at each gullet downspout is enough. 

• If you cannot design the piping system so that it drains completely between rainfall events, you must incorporate additional equipment to ensure that stagnant water is flushed from the piping before it enters the storage tank.

First-Flush Diverters: While roof-based collectors are much less susceptible to chemical and biological contamination than land-based collectors, even well-designed roofs will collect some contaminants during the periods between rainfall events. You will need to use a first-flush diverter to keep most of the dust, dirt, chemical contaminants, or animal and bird droppings out of the untreated-water storage tank. Most of these contaminants will be rinsed off the roof during the first few minutes of a major rainfall event. In order to prevent them from reaching your storage tank, you need to install a first flush diverter on each downspout that discharges to the tank. 

There are two types of first-flush diverters. A “fixed volume” diverter diverts a fixed volume of water before the tank begins to fill and a “fixed rate” diverter begins filling the tank only after the rainfall rate reaches a certain level. For a “fixed volume” diverter, it is best to size it so that it will flush at least 1 to 2 gallons of water for every 100 square feet of roof surface. For example, if your roof is 30 feet wide and 50 feet long:

• 30 feet × 50 feet = 1,500 square feet (total roof area) 

• 1,500 square feet ÷ 100 square feet = 15 units per 100 square feet 

• 15 × 1 to 2 gallons = 15 to 30 gallons

Piping:  Although other piping materials are available, plastics are probably your best choice, as they are less susceptible to corrosion. You may want to paint the outside of the piping that is exposed to sunlight in order to protect it from UV rays. Due to the corrosive tendencies of rainwater, you should NOT use thin-wall copper pipe or tubing.

Water Storage Systems:  An above ground tank is best because it’s more accessible and can be easily inspected, cleaned, and repaired. However, above ground tanks are exposed to sunlight and need to be protected from UV degradation, especially if you use plastic or other UV-sensitive materials. All the openings in your tank need to be sealed, screened, or covered, in order to prevent insects, lizards, birds, and rodents from getting into your tank. You must also make sure that the open end of all vents and overflows face downward and are covered with a corrosion-resistant window-screen material. In order to prevent flies and mosquitoes from using your tank as a breeding ground, there must be no gap larger than 1/16th of an inch at any opening into the tank. Since the gutter and downspout are open to the atmosphere, you should probably install an insect screen or a prefilter the inlet to the storage tank. 

Operation & Maintenance 

Rainwater collection and storage facilities require some routine maintenance, including: 

• Removing debris from the roof, leaf guard, gutter, gutter screen, and first-flush diverter. 

• Inspecting and repairing vent screens. 

• Siphoning sediment from the tank. 

• Testing the coliform bacteria levels in your untreated-water storage tank. 

• Disinfecting the untreated-water storage tank if total coliform levels reach 500 colony-forming units per 100 milliliters (500 CFU/100 mL) or if fecal coliform levels reach 100 CFU/100 mL.

Buying a Rainwater Catchment System

Rainwater catchment systems are available at many locations throughout Sonoma County. Ask your local hardware store about their offerings. These systems can be financed through Property Assessed Clean Energy (PACE) Financing, such as the Sonoma County Energy Independence Program. For more details, go to or call (707) 565-6470.