Rainwater Harvesting: Definition & Professional Guide
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Rainwater harvesting is a water collection method that captures precipitation from roof surfaces, channels it through gutters and downspouts into storage tanks or cisterns, and distributes the stored water for irrigation, toilet flushing, laundry, or potable use after appropriate treatment. Rainwater harvesting reduces municipal water consumption by 40 to 60 percent for homes that use collected water for irrigation and non-potable indoor fixtures, and many states have enacted statutes that explicitly protect property owners’ right to install these systems.
First Flush Diverter, Catchment Sizing & Storage Tank Pressurization
Rainwater harvesting begins at the roof catchment area, where every 1,000 square feet of roof surface captures approximately 600 gallons per inch of rainfall. In high-rainfall regions averaging 60 inches annually, a 2,000-square-foot roof can theoretically capture over 72,000 gallons per year. Gutters and downspouts channel water from the roof to the filtration and storage components.
A first flush diverter is the critical component that discards the initial volume of roof runoff, typically the first 1 to 2 gallons per 100 square feet of roof area. This initial flow carries the highest concentration of bird droppings, dust, pollen, and roofing material leachate. After the first flush diverts, cleaner water flows into the storage tank through a leaf screen and sediment filter.
Storage tanks range from 55-gallon rain barrels for basic garden irrigation to underground cisterns holding 5,000 to 20,000 gallons for whole-property supply. A submersible or external pump connected to a pressure tank delivers stored rainwater to fixtures at consistent pressure matching the home’s plumbing requirements.
Rain Barrel, Above-Ground Tank, Underground Cistern & Potable System Types
Rain barrel systems are entry-level collection setups using one or more 55-gallon barrels connected to a single downspout, providing gravity-fed irrigation water. Cost runs $100 to $300 installed.
Above-ground tank systems use polyethylene or fiberglass tanks from 500 to 5,000 gallons with a pump, pressure tank, and basic filtration for irrigation and non-potable indoor use. Professional installation costs $1,500 to $5,000.
Underground cistern systems bury large tanks below grade for aesthetic and space reasons, requiring excavation and connection to the building’s plumbing through a dedicated non-potable distribution system with purple pipe identification per IPC Appendix L.
Potable rainwater systems add a complete treatment train including sediment filtration, activated carbon, and UV disinfection to produce drinking-quality water, costing $10,000 to $30,000 for whole-house supply.
Dual Plumbing, Backflow Prevention & Municipal Supply Integration
Rainwater harvesting system installation is part of Bonded Plumbworks’ outdoor and specialty plumbing services. Systems connected to indoor fixtures require dual plumbing with clearly marked non-potable lines separated from the potable water supply.
Any rainwater harvesting system connected to a building also served by a public water supply requires a reduced pressure zone backflow preventer to protect the municipal water system from cross-contamination. Bonded Plumbworks’ plumbers install and test these assemblies as part of backflow prevention services. For properties with existing water line services infrastructure, Bonded Plumbworks designs hybrid systems that automatically switch between rainwater and municipal supply when tank levels drop.
State Rainwater Laws, IPC Appendix L & ARCSA/ASPE 63 Standards
Many states prohibit local governments and homeowner associations from banning rainwater harvesting systems, though installations must still comply with local building codes. IPC Appendix L establishes requirements for rainwater collection systems connected to buildings, including purple pipe identification, signage at non-potable outlets, cross-connection prevention, and minimum treatment standards for indoor non-potable use. ARCSA/ASPE 63 provides design and installation standards for residential rainwater catchment systems.
RainFlo, Bushman & Rain Harvesting Pty Collection Systems
RainFlo manufactures complete residential rainwater harvesting packages including above-ground tanks, first flush diverters, pump systems, and UV treatment modules designed for plumber installation. Bushman produces rotationally molded polyethylene storage tanks from 265 to 2,825 gallons in slim-profile designs that fit against building walls. Rain Harvesting Pty manufactures the Maelstrom filter, leaf eaters, and first flush diverters used as standard components in professionally installed collection systems.