Rainwater Collection System: Design and Installation

System Overview

A complete residential rainwater collection system has four components:

1. Catchment surface — the roof area where rain falls
2. Conveyance — gutters and downspouts
3. First flush diverter — diverts contaminated initial flow
4. Storage — tank, cistern, or barrels

Each component affects water quality. A well-designed system produces water that, after filtration and disinfection, is comparable to municipal supply.

Calculating Your Catchment Potential

The formula:

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

Multiply by 0.75 for realistic collection efficiency (first flush, evaporation, system losses).

Example:

• House footprint: 1,500 sq ft
• Monthly rainfall: 3 inches
• Theoretical: 1,500 × 3 × 0.623 = 2,804 gallons/month
• Realistic (75%): 2,103 gallons/month

For reference, a family of four at 2 gallons/day needs 240 gallons per month for drinking and cooking. A 1,500 sq ft roof in a 3-inch rainfall month can theoretically supply nearly 9 times that.

Look up your local rainfall: NOAA's Climate Data Online provides monthly precipitation averages by location. Knowing your dry months is critical for storage sizing.

Roof Surface Assessment

Assess your roof before designing the system:

Metal roofing (steel, aluminum): Excellent catchment surface. Clean, durable, no chemical leaching concerns with most modern coatings. If galvanized, zinc from the galvanizing dissolves in rainwater at low levels — activated carbon filtration handles this.

Clay or concrete tile: Good. Natural materials with minimal contamination potential.

Asphalt/composition shingles: Acceptable. May leach PAHs — especially from new or recently replaced shingles. If using asphalt roofing, activated carbon filtration is more important. Wait 1-2 years on a new asphalt roof before using water for drinking.

Not suitable:

• Wood shake or cedar shingles (harbor bacteria, moss, lichen)
• Roofs with chemical moss treatments or paint applications
• Roofs under heavy tree cover with organic debris accumulation

Maintenance for catchment: Clean gutters at minimum twice per year. Remove leaf debris, bird nests, and accumulated organic matter. Debris in gutters is debris in your storage water.

First Flush Diverter

The first flush is the most important component for water quality. Rain washing a roof carries everything that's accumulated since the last rainfall: bird feces (pathogens), dust, pollen, atmospheric pollution, and whatever organisms have colonized the gutter.

The diverter concept: A standpipe diverts the first-calculated volume of roof runoff to ground rather than storage. Once the standpipe is full, overflow goes to storage.

Sizing the first flush diverter:

Standard guidance: 1 gallon of diversion per 100 square feet of roof.

• 1,500 sq ft roof: 15-gallon first flush
• 2,000 sq ft roof: 20-gallon first flush
• 2,500 sq ft roof: 25-gallon first flush

Larger is better for roofs with high bird traffic or heavy tree cover.

Diverter components:

• A vertical standpipe of appropriate volume (4-inch PVC at common lengths covers typical volumes)
• A ball float at the bottom of the standpipe — when the pipe fills, the float seals the bottom drain
• A small slow drain at the bottom of the standpipe to empty it between rain events (typically a 1/4-inch hole drilled in the bottom fitting)
• A Tee fitting directing overflow to the storage tank once the standpipe is full

Commercial first flush diverters are available from rainwater harvesting suppliers and simplify installation. DIY versions from 4-inch PVC work equally well.

Storage Tank Options

55-gallon barrels: The accessible entry point. Four barrels connected in series provide 220 gallons. Relatively cheap, available, and replaceable.

250-gallon IBC totes: More cost-effective per gallon, standard for larger residential systems.

Polyethylene cisterns (500-10,000 gallon): Purpose-built rainwater cisterns. Available from agricultural water suppliers in a range of sizes. Professional installations often bury these below grade for temperature stability.

Underground cisterns: Most stable temperature, most aesthetically concealed, requires pump for access. Best for long-term primary water supply installations.

Tank Requirements

Regardless of size, the storage tank must be:

Food-grade or NSF/ANSI 61 certified — the material must be safe for contact with drinking water

Opaque — UV light degrades plastic over time and promotes algal growth. Clear or light-colored tanks placed in sunlight have shorter lifespans and more algal contamination issues

Screened and sealed — mosquito screening on all openings prevents vector breeding. A sealed tank prevents evaporation, debris entry, and animal access

Equipped with overflow — when full, overflow must have a clear path that doesn't flood your foundation or neighboring property

Elevated for gravity distribution — even 2 feet of elevation provides 0.86 psi of pressure, enough to gravity-feed a filter or distribution point

Treatment for Drinking Water

Roof-collected rainwater requires treatment before drinking. The treatment requirement depends on your roof material, local air quality, and the quality of your first flush system.

Minimum treatment stack:

1. Sediment pre-filter (30-50 micron) on the tank outlet — removes large particles
2. Hollow fiber filter (0.1 micron) — removes bacteria and protozoa
3. Chemical disinfection or UV — addresses viruses and any bacterial breakthrough

If asphalt roof:

• Activated carbon filter is important to address PAH leaching

Complete treatment for high confidence:

• First flush diverter
• Sediment pre-filter
• Activated carbon block filter
• Hollow fiber membrane filter
• UV disinfection

This sequence produces drinking water of excellent quality from a properly designed system.

System Maintenance Schedule

After every rain event:

• Inspect first flush diverter operation
• Check tank screen for debris
• Verify first flush standpipe is slowly draining (the small bottom hole shouldn't be clogged)

Quarterly:

• Clean gutters and downspouts
• Inspect and clean screen filters
• Check all connections for leaks

Annually:

• Inspect tank interior for sediment accumulation or biological growth
• Clean inside of tank with dilute bleach solution (1/4 cup bleach per 5 gallons water, swish and rinse)
• Replace carbon filter elements
• Test water quality (basic water test kit)
• Check first flush diverter function by covering it during a rain

Every 5 years:

• Inspect tanks for UV degradation (exterior), cracking, or discoloration
• Inspect gutter and downspout condition

Legality

Rainwater collection is legal in most US states but regulated in some. Several Western states (historically water-rights-sensitive regions) have had restrictions, though most have liberalized collection laws significantly in recent years.

See the dedicated rainwater legality article for current state-by-state status. Colorado in particular has had restrictions that have been progressively loosened. Check your state's current regulations before installing a system designed to capture large volumes from downspouts.