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Rainwater Harvesting: A Simple Beginner's Guide

A red maple leaf floats in a tin bucket on the surface of the water with falling raindrops

Harvesting rainwater may sound like an antiquated, outdated method of accessing water, but the practice is much more widespread than you might imagine. Common in many underdeveloped countries, it also serves many US households in remote areas and rural communities where access to municipal drinking water is unreliable or even non-existent.

As the number of US cities with water contamination increases, alternate sources of clean drinking water will become more relevant and in demand. Harvesting rainwater may not become the next homeowner’s trend anytime soon, but it’s worth investigating and more practical than you think.

The American Rainwater Catchment Systems Association(ARCSA) is a national organization catering to homeowners or anyone with an interest in harvesting rainwater. Rainwater harvesting systems are available from different brands, but hundreds of the two thousand -plus ARCSA membership built their own custom configurations

Using a few simple low-cost materials and some know-know, it’s possible to learn how to purify rainwater and construct a rainwater harvesting system for your cabin, private wilderness resort, ranch or your home.

Some Facts About Rainwater In The US

  • The average roof surface of the American homeowner is approximately 2400 square feet, or the equivalent of 1500 gallons of rainwater per inch of rain.
  • Did you know that the average 25ft x 40ft roof sheds about 600 gallons of water in 1 hour of moderate rainfall?
  • Rainwater has no chlorine or fluoride and in many cases is healthier than chemically-treated tap water. It also has zero hardness.
  • A full 60 gallon rain barrel weighs approximately 500 pounds.
  • 60 gallons can water approximately 100 square feet of garden or lawn with one inch of water
  • Over the summer, gardens and landscaping can account for 50-70% of your home’s water consumption!

Rainwater System: How Much Is Enough For You?

Determine the square footage of your roof by measuring the footprint of the roof, the slope of the roof doesn’t affect the catchment potential. For a square or rectangular house, measure the length and width and multiply the numbers together. If you have multiple roof sections do this for each one and add the results together.

After you’ve found the square footage multiply it by 0.56 to determine how many gallons you can collect per inch of rain. This calculation assumes a 90% efficiency.

Next, to calculate how much rain you can collect in an average rain year multiply this number by the average inches of rain. You can research this on your city’s official website or local online weather reporting site.

For every 1000 gallons of water you capture, you can expect to harvest 0.550 per cent successfully. Factoring the average rainfall per year in inches, use the following calculation:


  • 5,000 square foot roof
  • 5,000 x 0.56 = 2,800 gallons/inch of rain.
  • Avg rainfall = 25 inches
  • Potential Capacity = 2800 gallons x 25 inches
  • Answer: 70,000 gallons/year.

Factor in your needs and wants, and research the average amount of annual rainfall you can expect so you can buy adequate storage space.

Rainwater Harvesting Setup

Here’s an overview of the basic components of a rainwater system setup:

Catchment Surface: Installed on your roof for collecting either rainwater or stormwater. Rainwater is funnelled from the basin through a water filter, then directed to a storage tank or rain reservoir by pipes.
First Flush Diverter: A water cleaning device specially designed for diverting the first flood of water from your rooftop away from the catchment basin. Preset and automated for a specific time, this allows the dirty water to get flushed so that a steady stream of clean water can continue.
Lead-free pipes: For catching run-off from rooftops, lead piping may be needed to direct the flow of water from the eaves through a downspout into a reservoir.
Overflow Connection: Required for catching the overflow of stormwater
Premium Water Pump: Flow rate and ppsi will depend largely on your calculations. There are many brands to choose from, so review the specifications carefully to be sure you get exactly what you want.
Cartridge Filter: Preferably premium grade carbon media. The best brands require annual replacement and in some cases, even two year replacement. In any case, better quality means lowering the cost of maintenance for you in the long run.
Sediment Filter: Key for the removal of undissolved particulate matter that may have been left behind after the water filtration at the catchment level.
Disinfection Filter: UV light, Chlorinization, Ozonization, RO Membrane Filtration
Storage Tank: Basin or storage tank for storing the rainwater you catch. You’ll need at least 25,000 liters of storage capacity for normal domestic use by a small family. A tank is fairly cost and will require careful planning to install, so consider carefully your water usage, space, budget and needs.

Large tanks can only be installed by means of a crane, which could be a complicated affair for existing homeowners in a quiet neighborhood. There are some rain reservoir retailers who offer a free crane installation, so shop around. A tank can either be buried completely underground or partially.

There are 3 types of rainwater collection types you can look for:

  • Narrow, Rectangular Tank: typically fits under an eavestrough in a narrow space against an exterior wall. Galvanized iron or BPA-free, weather-proof plastic will suffice.
  • Collapable Bladder: A series of tanks and filtration units installed beneath your deck, out of sight.
  • Underground Aquifier: Used particular for groundwater sources and well water, this bed of porous rock typically lines a ground well or dugin water reservoir, effectively filtering out harmful bacteria and as well as a host of harmful pollutants.
Variety of water storage tanks and rainwater harvesting equipment

Catchment Surface Materials To Consider

Rain falling directly from the sky is fairly clean and safe to drink. Once it comes into contact with the surface of your catchment surface, the risk of contamination increases exponentially. Dirt, grime, dust particles, animal feces, and bacteria can quickly breed on an exposed outdoor surface.
With that in mind, maintaining a healthy catchment surface should always be a top priority.
To avoid leaching contaminants into your rainwater, go with a metal surface. Stainless steel, corrugated steel are both suitable options.

How To Purify Rainwater

Understanding how to purify rainwater is vital. Contaminants in water may include algae, air pollution, bird excrement, and leaves, sand, and dust. Local wells have dealt with these problems for decades. Installation of filtration and purification equipment can remove these contaminants at home as well.

Test a sample of your rainwater for contaminants before you invest in a filtration system. You can purchase a water testing kit at almost any hardware chain or order online. They’re very easy to conduct yourself, and the kit comes with all the necessary testing apparatus.

Once you’ve determined which chemicals need to be removed, you can decide on the appropriate treatment method. Either you’ll need a multi-filtration system for removing a wide range of pollutants, or a more straightforward water softener and sediment filter combination.

Look for these 4 basic types of water filtration methods and choose the one appropriate to your needs:

  1. Water Softener: for treating hard water with high levels of calcium, magnesium (not needed for rainwater which has zero chance of hard water)
  2. Water Purifier: Improving taste, removing foul odors, adding fortifying minerals
  3. RO Filtration: Removes a wide range of pollutants
  4. Multi-stage Filtration: Combines several types of filtration (sediment, carbon, UV, prefilters, RO media) to treat and purify water more thoroughly

Carbon filters can improve the taste and remove excess iron and some chemicals. In addition to chemicals, undissolved particulate matter can drift to the bottom of the storage tank. Installing a floating filter in your storage basin can remove floating particles without disturbing the sediment bed and further distributing contaminants.

The addition of in-line filters in the catchment basin, such as carbon filters and UV lighting, will add an additional layer of defense against contaminated rainwater.

Cost of A Rainwater Harvesting System

On average, the cost of a fully functioning domestic system ranges between $3,000 and $4,000. Add another $2000 for the cost of a professional installation.

Other factors that may impact your budget include the amount of piping required from the roof to the storage reservoir, as well as the cost of roof conversion for your catchment basin, and the crane installation of your storage basin.

Benefits Of Harvesting Rainwater

Is there any benefit to harvesting rainwater? Several, in fact. With the right catchment materials and setup, you can enjoy greater self-sufficiency and clean drinking water for a variety of uses.

Some of the key benefits include:

  • Inexpensive and low-cost maintenance
  • Reduce the cost of obtaining clean drinking water
  • Can make your home self sufficient
  • Flexible catchment system can be reconfigured, expanded or relocated
  • Reduces stormwater runoff
  • Better water for gardening due to higher levels of fertilizing nitrogen
  • Direct control of your water supply
  • Chlorine-free
  • Solves drainage issues
  • Can serve as an emergency or alternative source of drinking water
  • Helps conserve water usage

A study published in the Journal of Cleaner Production revealed that the most commonly reported use for harvested rainwater in the US is irrigation [X]. Following close behind, potable uses for rainwater are also a common motivation for installing a catchment system.


Undoubtedly, there’s a lot more to be said on the subject, but this guide makes a useful launching point for your rainwater harvesting project. Whether it’s clean water for gardening, cooking, cleaning or bathing, rainwater is a free, healthy source of water available to everyone!

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