Harvesting Rain: System Design for Strategic Rainwater Capture

Conserving water through rainwater harvesting saves natural resources, providing water for use in buildings and for site irrigation
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Sponsored by Watts Water Technologies, Inc.
Celeste Allen Novak, AIA, LEED AP

System Components

There are six main design components in a rainwater harvesting system.

  • Collection or catchment surface
  • Gutters and downspouts
  • Downspout filtration
  • Storage - above and below ground
  • Pumps and controls
  • Treatment and disinfection

Collection Surfaces

Rainwater for the Garden

In 2009, water levels in Georgia were at record lows and residents were encouraged to conserve water. This homeowner in Watkinsville, Georgia installed a 1500-gallon above-ground polyethylene tank to water their garden, flush toilets and wash clothes. The system included an internal pump, first flush diverter, floating suction, related controls and the tank, which was hidden under the homeowner's backyard deck.

Photos courtesy of BRAE

Rainwater storage is placed under the deck and the hose bib is carefully labeled as nonpotable water to be used for irrigation only.

Metal, clay, asphalt, wood, tar, slate, vinyl or rubber-the type and texture of a roof surface will affect the quality of the water runoff. A smooth surface will allow for more water collection and limit the possibility of evaporation or overflow from gutters. Powder-coated metal roofs have a smooth surface and resist corrosion, outperforming most other choices for a rainwater system. Tile, slate and clay roofs are porous and can lose as much as ten percent of potential rainwater due to absorption, insufficient flow or evaporation. Sealants can increase the flow on a porous surface and some coatings have a catalyst that neutralizes bacteria on a roofing surface. Green roofs do not make good surfaces for rainwater capture as they absorb much of the rainwater to provide plant moisture as well as contain particulates from the growing matter. When collecting water from green roofs the water is best used for outdoor irrigation purposes.

Many residential roofs have asphalt shingle roofs. As the roof ages asphalt grit is washed into downspouts. It is important to keep as much of this grit as possible out of the storage vessel. Most commercial roofing has a vinyl or rubberized surface with thermal or chemical welded seams. Design professionals should always take into account the end uses of the water before determining how or whether or not the composition of the roof will affect the quality of water collected. The end use for the water will determine the feasibility of using a particular roof system.

Gutters and Downspouts

Roof gutters and downspouts direct the flow of rainwater to the catchment area. Steep roofs, long distances between downspouts and inadequate maintenance can cause water overflow and loss of rainwater capture. Securely attached to downspouts, continuous or seamless half-round gutters are the most efficient conveyors of rainwater. Common downspout and gutter materials include PVC, vinyl, seamless aluminum and powder-coated steel. The critical component of this portion of the rainwater system includes the drop outlet. A gutter drop outlet routes water from the sloped, horizontal gutter to the vertical downspout. Roof hardware, brackets and straps, keep the gutters and downspout fastened tightly to the building. Complex roof forms may have roof valleys that direct more water to a gutter area often causing water overflow. If necessary, overflow roof dams will increase rainwater capture. Gutters and downspouts should not hold any water in the summer or the winter months and the roof installed as per code to reduce ice dams.

Filtration and First Flush

Filters, simple screens and first flush diverters are part of a complete rainwater system. Rooftop rainwater is filtered at the roof level and in the downspout to collect debris and sediment before water enters the storage tank. Leaf screens are attached along gutters above the downspout. They keep large debris from clogging the water channel. They are usually made of plastic or wire mesh and need regular seasonal maintenance.

A downspout filter shaped like a funnel with a stainless steel screen filters roof runoff. This filter is installed just above the highest level in a storage tank or along the pathway of the downspout at a level that is most likely to be maintained by the owner. Strainer baskets with fine to course mesh filters can be placed into the inlet of a tank to provide additional filtering. The type of screening and filter mechanisms depends on the climate, leaf or other litter loading as well as how the rainwater will be used. If the building is in a climate where there are infrequent heavy rains, accumulated roof dirt will require greater filtering. Typically, less filtering is necessary for rainwater used for irrigation than when the end source is for indoor applications.

Located downstream from filters, a first flush diverter can direct the first flow of rainwater that may have collected roof debris, built up chemicals, sediments and bacteria to a landscape area and away from the storage tank. The chamber in the PCV standpipe fills to prevent the initial water flow from entering the tank. Sealed by a floating ball, additional rainwater is diverted to the storage tank. Rainwater from the first flush is drained away from the storage tank and the system is reset for the next rainfall. A first flush diverter improves water quality, reduces maintenance and extends the life of pumps and fixtures that use rainwater. It is important to note that if a first flush diverter is not regularly cleaned and maintained they may be of little value and may even negatively impact the system.

There is no one size fits all solution to the filtering of rainwater before entering the cistern. Ultimately the cleaner the water coming into the tank the better the water quality will be on the way out. Rainwater systems consultants will evaluate rainwater quality including acidity caused by pollutants when choosing a filtering system.


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Originally published in June 2013