Rainwater Harvesting — Collection and Storage

Rainwater harvesting captures precipitation before it contacts the ground — meaning it picks up far fewer contaminants than surface water or shallow groundwater. In moderate rainfall climates, a well-designed system on a 1,000 square foot (93 sq m) roof can collect enough water to meet a family's non-potable needs year-round and supplement drinking water with appropriate treatment.

Building a permanent cistern?

For sizing calculations, material comparisons, and climate-zone yield planning, see Cistern systems for permanent rainwater storage.

Before you start - Skills: Verify state law before installing any permanent system — Colorado caps collection at 110 gal (416 L) per household; Nevada permits non-potable use only; Utah requires registration above 2,500 gal (9,463 L); most US states are unrestricted. Assess your roof material before targeting potable use (asphalt shingles leach PAHs and zinc; lead-flashed or treated-wood surfaces require testing or exclusion). Know the catchment formula: 1 in (25 mm) of rain on 1 ft² (0.09 m²) of roof yields ~0.62 gal (2.3 L). See Water Testing for potability verification after setup and Cistern systems for permanent storage sizing. - Materials: Gutters and downspouts in good repair — 6-in (150 mm) K-style minimum for high-rainfall areas. First-flush diverter sized at 0.013–0.025 gal (0.05–0.10 L) per 1 ft² (0.09 m²) of roof area per event. Food-grade storage cistern (poly, ferrocement, or stainless; 250–10,000+ gal / 950–37,800+ L). 100-mesh inlet screen to exclude debris and mosquitoes. Overflow plumbed to a soakaway or rain garden — not to septic or municipal stormwater. Treatment chain for potable use: settling → 5 µm sediment filter → activated carbon → UV or chlorine disinfection. - Conditions: State and local law confirmed for your intended use (non-potable irrigation, livestock, indoor non-potable, or full potable). Roof material assessed as safe for the target use. Site geometry allows gravity feed from roof to cistern (minimum 1:50 grade on horizontal pipe runs). Cistern location permits access for annual inspection and cleaning every 5–10 years. - Time: Backyard-scale install (50–500 gal (190–1,890 L)): 1–3 days. Residential underground cistern (1,000+ gal / 3,800+ L with excavation): 1–2 weeks. First-flush diverter self-empties between rain events (24–48 hours). Cistern cleaning and water testing: once per year.

Educational use only

This page is educational. Treat any water source you suspect is contaminated as life-threatening until tested or treated correctly. Where municipal water service is intact, follow your local utility's guidance. CDC and EPA emergency-water guidance supersedes anything on this page.

How Much Water Can You Collect?

The collection formula is straightforward:

Gallons collected = Roof area (sq ft) × Rainfall (inches) × 0.623

The 0.623 factor accounts for unit conversion and a 10–15% loss to evaporation and splash. Metric equivalent:

Liters collected = Roof area (sq m) × Rainfall (mm) × 0.8

Worked Example

A 1,000 sq ft (93 sq m) roof in an area receiving 1 inch (25 mm) of rain:

  • Imperial: 1,000 × 1 × 0.623 = 623 gallons per rain event
  • Metric: 93 × 25 × 0.8 = 1,860 liters per rain event

For an area with 20 inches (508 mm) of annual rainfall, that same roof yields roughly 12,460 gallons (47,170 L) per year — about 34 gallons (129 L) per day on average, though actual distribution is uneven.

Roof Area 1 in (25 mm) rain 2 in (51 mm) rain
500 sq ft / 46 sq m 312 gal (1,181 L) 623 gal (2,358 L)
1,000 sq ft / 93 sq m 623 gal (2,358 L) 1,246 gal (4,716 L)
2,000 sq ft / 186 sq m 1,246 gal (4,716 L) 2,491 gal (9,430 L)

Rainwater harvesting is legal in most U.S. states. A handful of Western states retain volume caps or use-restrictions rooted in prior-appropriation water law. State law changes periodically — verify current requirements with your state's water resources agency before building a permanent system.

Warning

The information below reflects statutes current as of early 2026 (sources: Pacific Northwest National Laboratory report PNNL-24347; worldwaterreserve.com). This is a general reference, not legal advice. Your state water-resources agency is the authoritative source.

States with restrictions or special conditions

State Rule (2026)
Colorado Two rain barrels per household; 110 US gallons (~416 L) total capacity cap; outdoor non-potable use only. Statute enacted 2016, still in force.
Nevada Domestic rooftop harvesting permitted for non-potable use only; codified 2017. Potable harvesting not authorized under state law.
Utah Up to 2,500 gallons (~9,463 L) stored on-site without a permit. Systems exceeding that volume require registration with the State Engineer.
Washington Rooftop collection permitted; registration may be required in water-rights adjudication areas — check with the Department of Ecology for your specific watershed.
Oregon Rooftop catchment permitted. Surface water collection (ponds, ground runoff) is not allowed under the same authority and requires a separate water right.

States with incentive programs

Arkansas, Georgia, Illinois, Ohio, Rhode Island, Texas, and Virginia offer tax credits or rebates for rainwater harvesting systems. Contact your state's revenue or environmental agency for current program details and eligibility.

All other states

Rainwater collection is unrestricted. Most eastern and southern states impose no volume limits and no permit requirements for residential systems.

The long-term trend in Western states has been toward relaxing restrictions — Colorado's 2016 change and Nevada's 2017 codification are examples. Even so, always confirm current law before committing to a permanent installation.

Roof Material Safety

Not all roof materials are equally safe for collecting drinking water.

Roof Material Potable Use Notes
Unpainted galvanized metal Good May leach zinc at very low levels; not a health concern at typical concentrations
Painted metal (acrylic/polyester) Good Check paint type; avoid lead-based paints (pre-1978 buildings)
Slate or clay tile Excellent Best choice; no leaching
Concrete or cement tile Good May raise pH slightly; test before use
Asphalt shingles Acceptable with caution Leaches some polycyclic aromatic hydrocarbons (PAHs), zinc, and asphalt compounds; adequate for non-potable uses; if used for drinking water, use a carbon filter and test for volatile organic compounds (VOCs)
Cedar shingles Avoid for potable Preservatives and tannins leach readily
Older metal with lead solder or lead flashings Avoid for potable Lead contamination risk; test before any drinking use
Green (vegetated) roofs Avoid for potable Soil and plant material contaminate heavily
Roofs with HVAC discharge, pigeon infestation Avoid until cleaned Fecal contamination risk

Best practice: Metal roofing with a bare or food-safe painted finish is the ideal collection surface. Asphalt shingles are acceptable for non-potable uses and, with proper treatment, for drinking water with annual testing.

First-Flush Diverter: Purpose and Installation

The first flush of rain after a dry period carries the highest concentration of bird droppings, dust, pollen, insect material, and atmospheric particulates from the roof surface. A first-flush diverter automatically discards this contaminated initial flow before routing cleaner water to storage.

How It Works

The diverter is a vertical chamber (typically 3–4 inch (7.6–10 cm) diameter PVC pipe) installed inline on the downspout. The chamber fills during the first flush and holds that contaminated water while redirecting subsequent, cleaner rain to the cistern. A small drain hole at the bottom of the chamber empties it slowly (over 24–48 hours) between rain events.

Sizing Rule

Discard 1 liter (0.26 gal) per 10 square meters (108 sq ft) of roof collection area.

For a 100 sq m (1,076 sq ft) collection area: divert the first 10 liters (2.6 gallons) per downspout.

A 4-inch (10 cm) Schedule 40 PVC pipe holds approximately 0.66 gallons per foot (8.2 L/m) of pipe length. For a 10-liter first-flush chamber: you need roughly 1.2 meters (4 ft) of 4-inch pipe.

First-Flush Diverter Step-by-Step Installation

Materials needed: - 3–4 inch (7.6–10 cm) PVC pipe (length per sizing calculation) - PVC tee fitting and cap - 1/8 inch (3 mm) drill bit and drill (for slow-drain hole) - PVC cement and primer - Downspout adapter fittings

Steps:

  1. Cut the downspout at the desired installation height (typically 3–4 feet (0.9–1.2 m) above the ground or cistern inlet level).
  2. Install a PVC tee on the downspout cut. One outlet runs down to the first-flush chamber (vertical pipe going down); the other side-outlet routes to the cistern.
  3. Glue the first-flush chamber pipe (vertical, pointing down) onto the tee bottom.
  4. Drill a 1/8-inch (3 mm) hole through the bottom cap — this is the slow-drain hole that empties the chamber between rain events.
  5. Cement the cap onto the bottom of the chamber pipe.
  6. Route the tee's side outlet to the cistern inlet via gutter or pipe.

Testing: Pour a measured bucket of water into the top of the downspout. The first-flush chamber should fill before water routes to the cistern side. Time how long the chamber takes to drain (target: 24–48 hours). Adjust drain hole size to control drainage rate.

Field note

In areas with heavy bird activity (pigeons, starlings), add a mesh bird guard over all gutter openings and install a downspout screen above the first-flush tee. A single pigeon roosting on a roof edge over a collection season can add measurable coliform bacteria to the first flush that a standard carbon filter does not fully remove. The bird guard is inexpensive and eliminates the problem entirely.

Cistern Selection and Sizing

Sizing Approach

Size your cistern to bridge the longest typical dry spell at your target daily demand.

Formula: Cistern capacity (gallons) = Daily demand (gal/day) × Dry period (days)

For a household using 50 gallons/day (189 L/day) with a 30-day dry period: 50 × 30 = 1,500 gallons (5,678 L) minimum cistern size.

Cistern Options

  • Size: 275–330 gallons (1,041–1,249 L) each
  • Cost: $100–$300 new; $50–$150 used (food-grade only)
  • Material: HDPE food-grade plastic in metal cage frame
  • Best for: Budget systems; multiple totes can be linked in series
  • Warning: Only use IBC totes that previously held food-grade materials. Totes that held industrial chemicals can leach even after washing.
  • Size: 500–10,000 gallons (1,893–37,854 L)
  • Cost: $300–$2,500 depending on size
  • Material: UV-stabilized HDPE or LLDPE; food-grade versions available
  • Best for: Dedicated cistern use; durable, long-lasting, available in sizes suited to single-cistern setups
  • Installation: Requires level, compacted base; larger tanks need concrete pad
  • Size: Custom; typically 500–5,000 gallons (1,893–18,927 L)
  • Cost: $400–$1,200 in materials; significant labor
  • Material: Reinforced cement plaster over mesh armature
  • Best for: Permanent in-ground or semi-buried installations; excellent thermal stability keeps water cool
  • Lifespan: 30–50+ years with proper construction
  • Size: 500–50,000+ gallons (1,893–189,271 L)
  • Cost: $2,000–$10,000+ installed
  • Best for: Large permanent installations; buried systems are freeze-proof and protected from UV degradation
  • Installation: Requires excavation; must account for water table to prevent floatation

Cistern Location Rules

  • Locate at least 10 feet (3 m) from any septic system components
  • Locate downhill or at grade from collection surface for gravity feed; if uphill, a pump is required
  • If above-ground, insulate or shade in climates with freezing temperatures to prevent frost damage
  • Vent the cistern to prevent vacuum lock; screen the vent to exclude insects and rodents

Treatment Train for Potable Use

Collected rainwater requires treatment before drinking. The standard treatment train:

1. Leaf screen and first-flush diverter (passive; catches the grossest contamination)

2. Sediment pre-filter — 50–100 micron mesh screen or wye strainer at the cistern inlet; removes fine particulates before storage. Inexpensive component available at hardware stores.

3. Sediment filter — 5–10 micron cartridge filter at point of use. Removes turbidity. Replace every 3–6 months depending on use. Affordable for the housing; inexpensive replacement cartridges.

4. Activated carbon filter — Removes chlorine taste, some organics, and improves color/odor. Does not remove biological contamination. Affordable for housing and cartridges.

5. Disinfection — Choose one: - Boiling: Full rolling boil 1 minute (3 minutes above 6,500 ft (1,981 m) elevation); 100% effective against biological threats; no ongoing supply cost - UV treatment: 254 nm UV lamp; effective against all biologicals; requires electricity; affordable point-of-use units available - Chemical treatment: Unscented bleach (5.25–8.25%), 8 drops per gallon (2 drops/L) of clear water; effective against most bacteria and viruses; lowest cost

Optional 6th stage: Reverse osmosis membrane, if removing dissolved solids or heavy metals is required. Affordable to moderate investment for a countertop unit.

See Filtration for detailed filter selection guidance.

System Cost Ranges

System Type Description Approximate Cost
Basic non-potable IBC tote + first-flush diverter + gutter connection $150–$400
Basic potable (small scale) Above + sediment/carbon filters + UV $500–$900
Mid-range residential Poly tank 1,000 gal + full treatment train $1,000–$2,000
Whole-house system Underground cistern 5,000 gal + pump + full treatment $3,000–$8,000+

Gutter and System Maintenance

Interval Task
After every major storm Check first-flush chamber; ensure drain hole is clear
Monthly Inspect gutter screens; remove debris
Every 3–6 months Replace sediment and carbon filter cartridges
Annually Clean cistern interior (remove sediment from bottom)
Annually Test water for coliform bacteria and pH
Every 2–3 years Inspect cistern for cracks, seal integrity

Cistern cleaning procedure: Drain tank to 10% full. Mix 1 cup (237 mL) of unscented bleach in 5 gallons (19 L) of water. Scrub interior walls with a long-handled brush using the bleach solution. Rinse thoroughly with fresh water before refilling.

Cross-References

Failure Modes

Even a well-designed system fails at specific points. Knowing how to recognize and recover from each failure matters most when the system is your primary water source.

First-flush diverter clogged or drain hole blocked Recognition: During a rain event, watch the first 1–2 gal (3.8–7.6 L) — if no water diverts into the chamber, or if the chamber fails to empty within 1–4 hours after the storm, the drain orifice is blocked. The cistern is then receiving contaminated first-flush water that should have been discarded. Remedy: Open the inspection cap and clear debris from the drain hole. Check the ball float for stuck-open or stuck-closed condition. Replace the 0.05–0.13 in (1.3–3.2 mm) drain orifice gasket if cracked. Inspect and clean every 3 months, and after major debris events such as windstorms or pollen seasons.

Cistern contamination event Recognition: Rotten-egg smell when opening the cistern; visible biofilm on inner walls; positive coliform test after a period of clean results; recent animal carcass or bird die-off near the catchment area. Remedy: Stop using the water immediately. Pump the cistern to empty. Pressure-wash the interior. Chlorinate with 1 gal (3.8 L) of 5–6% unscented bleach per 1,000 gal (3,785 L) of capacity; let sit 24 hours; pump out and rinse. Refill with rain or potable water. Require 2 consecutive negative coliform results before returning to potable use. See Cistern systems for the full cleaning and chlorination procedure.

Treatment train bypass or saturation Recognition: Post-filter water tests positive for turbidity, coliform, or elevated chlorine demand. Sediment filter pressure drop exceeds 15 PSI differential (normal operating range: under 5 PSI). Carbon filter has passed its rated gallon-life — typically 10,000–20,000 gal (37,800–75,700 L) for residential cartridges. Remedy: Isolate each filter stage to identify the failure point. Replace the cartridge, media, or UV lamp per manufacturer schedule. See Water Testing for how to interpret test results after a treatment failure.

Field note

Never bypass a filter stage to maintain flow. A clogged sediment filter is an inconvenience; drinking water that bypassed it is a health risk. Re-establish the full treatment chain before issuing water for potable use — even for a single day's supply.

Mosquito breeding in cistern or first-flush chamber Recognition: Visible larvae in standing water; mosquito activity around the overflow pipe or vent; local West Nile virus or eastern equine encephalitis risk in your county. Any standing water accessible through a gap larger than 1 mm (~0.04 in) is a potential breeding site. Remedy: Verify all openings are screened with ≤1 mm (~0.04 in) mesh. Replace torn screens immediately. Treat accessible standing water that cannot be eliminated with Bti dunks (Bacillus thuringiensis israelensis) — selective for mosquito larvae, does not affect humans, livestock, or aquatic life. Drain any low-volume reservoir holding less than 1 in (25 mm) of water for more than 7 days.

Roof contamination from animal activity or industrial deposition Recognition: Bird droppings or nesting visible near downspouts; wildfire ash accumulation on the catchment surface; industrial facility within 1 mi (1.6 km) upwind. Asphalt shingles in these conditions can compound contamination with PAH leaching. Remedy: Close the cistern inlet valve and divert harvest away from storage during active contamination events. Allow the first 0.1 in (2.5 mm) of rain post-event to flush the catchment surface before reopening the inlet. After any wildfire ash event, test for heavy metals before returning to potable use.

For specific test-result thresholds that trigger "do not use" decisions, see Water Testing — decision tree.