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Rainwater harvesting is a practical way to capture water that would otherwise run off your roof and store it for later use. A simple rain barrel can provide supplemental water for landscaping, while larger systems can incorporate storage tanks, pumps, filtration, and dedicated plumbing.
But collecting rainwater involves more than putting a barrel under a downspout. Your roof, gutters, storage tank, intended use, local regulations, and water treatment requirements all affect how a system should be designed.
This guide explains how rainwater harvesting works, how much water you may be able to collect, the equipment you’ll need, common mistakes to avoid, and important considerations before using harvested rainwater around your home.
What Is Rainwater Harvesting?
Rainwater harvesting is the collection and storage of precipitation for later use. In a typical residential system, rain falls onto a roof, travels through gutters and downspouts, and is directed into a rain barrel or larger storage tank.
Harvested rainwater is commonly used for non-potable purposes such as landscape irrigation and outdoor cleaning. Depending on local regulations and system design, appropriately treated rainwater may also be used for certain indoor applications.
However, rainwater should not automatically be considered clean simply because it came from the sky. Water can pick up dust, chemicals, microorganisms, bird droppings, and contaminants from roofing materials, gutters, piping, and storage equipment.
How Does a Rainwater Harvesting System Work?
A basic system follows a relatively simple path:
- Rain falls onto a collection surface, usually a roof.
- Gutters carry the water toward a downspout.
- Screens or debris filters help keep leaves and larger material out.
- A first-flush diverter can redirect the initial roof runoff away from the storage tank.
- The remaining water enters a rain barrel, cistern, or storage tank.
- The stored water is delivered by gravity or a pump when needed.
More advanced systems may incorporate additional filtration, disinfection, pumps, pressure tanks, overflow management, monitoring equipment, and dedicated plumbing.
Benefits of Rainwater Harvesting
The potential benefits depend on your rainfall, storage capacity, water use, and local climate, but residential rainwater harvesting can provide several advantages.
Reduce Municipal Water Use
Using captured rainwater for appropriate non-potable purposes can reduce the amount of treated municipal water used for tasks such as landscape irrigation.
Supplement Garden Irrigation
Stored rainwater can provide an additional source of irrigation water between storms. This can be particularly useful for homeowners with gardens or landscaping that require regular watering.
Manage Roof Runoff
Capturing roof runoff can reduce the amount of stormwater immediately leaving a property. Properly designed overflow can then direct excess water toward an appropriate drainage area, rain garden, or other approved location.
Rain barrels and similar practices are among the approaches used to manage stormwater runoff.
Provide Stored Water for Certain Emergencies
A stored supply of harvested rainwater may be useful for appropriate non-potable purposes during interruptions to normal water service.
Do not assume stored rainwater is safe for drinking during an emergency unless it has been appropriately treated and its safety is established.
How Much Rainwater Can You Collect?
A useful rule of thumb is that 1 inch of rain falling on 1,000 square feet of roof represents about 623 gallons of water before accounting for system losses.
You can estimate theoretical collection using this formula:
Roof area (sq. ft.) × rainfall (inches) × 0.623 = potential gallons
For example, 1 inch of rainfall on a 2,000-square-foot collection area represents approximately 1,246 gallons before accounting for losses.
Actual collection will be lower because some water is lost through first-flush diversion, splash, evaporation, gutter overflow, roof wetting, leaks, and system inefficiencies.
How to Build a Rainwater Harvesting System
Step 1: Decide How You Will Use the Water
Start with the intended use rather than the equipment.
Water collected solely for landscape irrigation generally requires a much simpler system than water intended for indoor plumbing or human consumption.
Your intended use determines how much storage, filtration, treatment, plumbing, and monitoring may be necessary.
Step 2: Check Local Requirements
Rainwater collection and reuse requirements vary by state and locality. Rules can also differ depending on whether the water will be used outdoors, indoors for non-potable purposes, or as drinking water.
Check applicable plumbing codes, health-department guidance, building requirements, and local water regulations before installing a large or permanently plumbed system.
Step 3: Evaluate Your Roof and Collection Surface
Your roof is part of the water collection system, so its condition and materials matter.
Leaves, dust, bird and animal waste, and chemicals from roofing materials can enter harvested water. CDC notes that roofing materials, gutters, piping, and storage materials can all contribute contaminants.
If the water will be used for anything beyond basic non-potable purposes, determine whether the collection surface and associated materials are appropriate for the intended use.
Step 4: Prepare Gutters and Downspouts
Keep gutters clean and in good condition. Screens or debris excluders can help prevent leaves, twigs, insects, and other large debris from entering the storage system.
Downspouts should direct water securely toward the tank without creating unnecessary leak points or allowing contaminated surface water to enter the system.
Step 5: Add a First-Flush Diverter
The beginning of a rainfall event can wash accumulated dirt, animal waste, and other contaminants from the collection surface.
A first-flush diverter redirects a portion of this initial runoff away from the storage tank. CDC specifically recommends considering first-flush diversion as one way to improve collected rainwater quality.
A first-flush diverter can improve water quality, but it does not by itself make harvested rainwater safe to drink.
Step 6: Choose the Right Storage Tank
Residential storage can range from small rain barrels to large above-ground or underground cisterns.
When choosing a tank, consider:
- Expected rainfall
- Roof collection area
- Expected water use
- Available installation space
- Tank material
- UV exposure
- Access for inspection and cleaning
- Overflow management
- Mosquito and pest protection
Remember that water is heavy. One gallon weighs a little more than eight pounds, so even a relatively small storage tank requires a stable, properly prepared base.
Step 7: Protect the Tank From Light and Pests
Opaque storage helps limit sunlight exposure that can encourage algae growth.
Tank openings should also be screened or otherwise protected from mosquitoes, insects, rodents, and debris. CDC recommends screening rain-barrel openings or regularly emptying barrels as measures to reduce mosquito breeding.
Step 8: Plan the Overflow
Your tank will eventually fill during sufficiently large storms. Overflow needs somewhere safe to go.
Depending on the property, overflow might be directed toward an approved drainage area, rain garden, swale, or another suitable location where it will not damage the home’s foundation or neighboring property.
Step 9: Add a Pump if Necessary
Simple elevated rain barrels can sometimes supply water through gravity.
Larger systems may require a pump when water must travel farther, move uphill, operate irrigation equipment, or provide consistent pressure.
Pump sizing should be based on the required flow rate, pressure, elevation change, and intended application.
Can You Drink Harvested Rainwater?
This is where rainwater harvesting becomes significantly more complicated.
Collected rainwater is not necessarily safe to drink. Even water that looks clear can contain disease-causing microorganisms or chemicals. CDC recommends regularly testing rainwater for germs and chemicals when it is used for drinking, cooking, or bathing.
Making harvested rainwater potable may require multiple treatment barriers selected according to the actual contaminants present, along with appropriate system design, monitoring, maintenance, and testing.
A sediment filter, carbon filter, ceramic element, UV system, or other single treatment method should not automatically be assumed to make unknown rainwater safe to drink.
If you’re considering potable rainwater use, consult applicable local requirements and qualified water-treatment professionals. Requirements can be substantially stricter than those for irrigation systems.
Rainwater Filtration vs. Household Water Filtration
Rainwater treatment and conventional household filtration solve different problems.
A municipal-water filter typically treats water that has already passed through a regulated public treatment system. Harvested rainwater comes from an independently managed collection system, so the homeowner may be responsible for controlling contamination from collection through storage and final use.
If you’re comparing treatment technologies for your regular household water supply, start with our guide to choosing the best water filter for your home.
For treatment throughout the house, see our guide to the benefits of whole house water filtration systems.
What About Hard Water?
Rainwater generally contains fewer hardness minerals than many groundwater supplies, but that doesn’t eliminate the need to evaluate water quality for its intended use.
If the water already entering your home has high levels of calcium and magnesium, that’s a separate treatment issue. Our water filter for hard water guide explains the differences among filtration, softening, conditioning, and scale-control technologies.
Common Rainwater Harvesting Problems
Mosquitoes
Prevent mosquitoes and other insects from entering storage tanks by screening openings and maintaining the system.