Terminology

• Catchment area: footprint of the collecting surface, the vertical projection of the roof, not the sloped roof area
• Conveyance: gutters, leaders, and piping that carry runoff to storage
• First flush: the first and dirtiest runoff of a storm, diverted away from storage
• Roof washer: device that diverts or flushes the first runoff; sometimes combined with a first-flush diverter
• Cistern: tank that stores harvested rainwater
• Collection efficiency: fraction of rainfall that reaches storage after losses; about 0.8 on average and 0.95 for metal roofs
• Make-up water: supplemental potable or reclaimed water that refills a tank when rainwater runs short
• Nonpotable: water for uses other than drinking, such as irrigation, flushing, and washing

Standards

System standards

• ARCSA/ASPE/ANSI 63-2020: rainwater catchment systems, design and installation; supersedes the 2013 edition; covers rooftop and other hard, impervious surfaces
• ARCSA/ASPE/ANSI 78: stormwater harvesting, for runoff from sidewalks, plazas, and parking lots
• CSA B805/ICC 805: design, installation, and maintenance of rainwater collection systems for potable and nonpotable uses
• ASTM E2727: assessing site conditions that affect raw rainwater quality
• NSF/ANSI standards: referenced by ARCSA/ASPE 63 for roofing and collection materials and for potable treatment devices
• GSA P100: requires major capital projects over 5,000 sq ft to use rooftop rainwater catchment designed to ARCSA/ASPE/ANSI standards
• EN 16941-1:2024: European standard for nonpotable on-site systems
• Code provisions for nonpotable rainwater systems are collected in the plumbing code reference

Sizing

• Volume collected: V = R x A x k x e, with R rainfall depth, A catchment area, k unit conversion (7.48 gal per cu ft and 12 in. per ft), and e collection efficiency
• Efficiency values: 0.75 for soil surfaces, 0.8 average, 0.9 for concrete in one worked example, 0.95 for metal
• Rule of thumb: about 0.5 gal per inch of rain per sq ft of catchment at 0.8 efficiency; the theoretical maximum is 0.623 gal
• Worked example: a 1,900 sq ft shingle roof with 2.64 in. of March rain at 0.8 efficiency yields about 2,500 gal
• Storage: compare monthly collection with monthly demand, starting from a full tank, and carry storage forward month by month; in one 541 sq ft roof example, an 800 gal tank never ran empty and its lowest level was 98 gal
• Reducing storage need: lower usage or increase the collection area
• Sizing methods: a simple water budget model across several tank sizes is the preferred method in one state manual; another program uses curves of drawdown time versus volume for toilet-flushing demand, based on 1 acre of impervious area

Components

• Catchment surface: roof or other approved impervious surface
• Conveyance: gutters, leaders, and collection piping to the tank
• Screens and first-flush devices: remove leaves, debris, and the dirtiest runoff before storage
• Storage: cistern above or below ground with overflow, vent, access, and drain
• Treatment: filters and, where needed, disinfection
• Pump and distribution: pump, controls, and piping to the points of use
• Make-up supply: potable or reclaimed water with backflow protection
• Controls: float switches or level sensors for the pump and make-up valve

Detailing

• Match treatment to use: ornamental watering needs none, flushing and laundry need filtration, and potable use needs full treatment and approval
• Separate from potable: nonpotable lines, outlets, and tank openings are labeled and protected from cross connection
• Plan the overflow: discharge away from the foundation and neighboring property to a safe drain or a landscape feature
• Locate the tank early: weight, access, and the route for collection piping affect the foundation and site plan
• Test the system: gutters, first-flush diverters, tank, and piping are tested before use
• Operate and maintain: clean screens and filters, empty first-flush chambers, inspect the tank, and replace UV lamps and filter cartridges on a schedule

Uses

Specific uses

• Landscape and garden irrigation
• Toilet and urinal flushing
• Cold water laundry and vehicle washing
• Industrial and air-conditioning uses
• Drinking and domestic water with treatment
• Livestock and wildlife water
• Groundwater recharge and stormwater management
• Fire protection storage, where permitted

Pros and cons

Pros

• Reduces demand on mains supply; homes in one UK source cut mains use by 20 to 30 percent typically and 50 percent or more at best
• Reduces roof runoff to storm systems
• Works with green roofs, rain gardens, and other stormwater measures
• Sales tax and incentive programs exist in some states

Cons

• Needs a tank, pump, filtration, and maintenance
• Supply depends on rainfall; dry seasons need make-up water
• Roof material and air quality affect water quality
• Permits, inspections, and local rules vary by state
• Cross-connection control and signage add cost and coordination

Manufacturing and cost

Construction

• Designed from roof area, rainfall, and demand; built from standard roof drainage, screens, a first-flush diverter, a tank, a pump set, filters, and labeled piping
• Packaged residential systems and engineered commercial systems differ mainly in tank size, treatment, controls, and monitoring

Main manufacturers and suppliers

• Rainwater Management Solutions
• Graf USA
• Norwesco
• Snyder Industries

Trade association: ARCSA, American Rainwater Catchment Systems Association.

Typical costs

• Polyethylene or fiberglass tank, 5,000 gal, above ground: about $0.40 to $0.80 per gal ($2,000 to $4,000)
• Same tank below ground: about $0.80 to $1.60 per gal
• Cistern 10,000 gal, concrete or polyethylene: about $0.50 to $1.00 per gal
• Tank costs rise with size, burial, and fittings; see the cisterns note

Sources

• American Society of Plumbing Engineers: ARCSA/ASPE/ANSI 63-2020 rainwater catchment systems, https://aspe.org/?p=6144
• Contractor: collaborative effort helps rainwater harvesting reach pivotal benchmark, https://www.contractormag.com/green/article/55128839/collaborative-effort-helps-rainwater-harvesting-reach-pivotal-benchmark
• Grafman, L.: To Catch the Rain (LibreTexts), https://eng.libretexts.org/Bookshelves/Environmental_Engineering_(Sustainability_and_Conservation)/Book%3A_To_Catch_the_Rain_(Grafman)
• Wikipedia: rainwater harvesting, https://en.wikipedia.org/wiki/Rainwater_harvesting
• Whole Building Design Guide: 33 16 20 rainwater harvesting (federal green specification), https://www.wbdg.org/FFC/EPA/FEDGREEN/fgs_331620.pdf
• North Carolina Department of Agriculture: cistern design manual, https://ncagr.gov/soil-water/swcccap-cistern-design-manual/open
• Santa Clara Valley Urban Runoff Pollution Prevention Program: Appendix I, https://scvurppp.org/wp-content/uploads/2024/07/Appendix-I-2024.pdf
• Texas A&M AgriLife: rainwater harvesting, https://rainwaterharvesting.tamu.edu/