GeneralWater ConservationWater Harvesting

Rainwater Catchment Systems

Of the many problems that our civilization faces, one of the most worrying problems is water scarcity. Without a safe and abundant supply of fresh, potable water, our species is vulnerable to many crises. Our current industrial society depends almost exclusively on groundwater sources for our water needs, while many of those sources of water are slowly being depleted, contaminated, and require more energy to access. The rain that falls on our roofs, however, offers a steady and constant source of fresh water for ourselves and our homes. Why don’t more of us take advantage of this abundant source of fresh water?

The Importance of Harvesting Rainwater

Imagine that you are in your home on a rainy afternoon. It has been raining for two days straight now and your front yard is full of puddles. The water is rushing along the drain ways on the side of the road and the local news is talking about the stress on the local sewer system that this extra rain is causing.

When you turn on the faucet in your kitchen sink to wash up the dishes from lunch, however, the water that you use may very well be coming from hundreds of miles away in an area that may very well be experiencing a drought.

Our conventional, industrial water supply has very little connection to local watersheds or local ecosystems. Rather, the focus has been on taking water from areas where water is apparently abundant and moving it to areas with high population densities or areas where water is scarce. To do this, we depend on huge, energy dependent pumping systems that most likely depend on the continued availability of cheap fossil fuels to fuel these pumps.

Learning how to redesign our homes and our lands to take advantage of the water that falls naturally from the sky is the easiest way to supply ourselves with all the water we will ever need. But what if you live in an arid region like Tucson, Arizona or other places where rain of any sort of is an oddity. How much rainwater can you potentially save?

How Much Rainwater Can You Catch?

Even in extremely arid regions, a couple of rainstorms per year will give you more than enough water for your household use. You won´t have to be running around outside with buckets to try and catch the water falling from the heavens because unless you live in a cave, the roof over your head is the ideal catchment platform for rainwater.

To calculate how much rain you may be able to catch, follow this simple formula:
• Measure the square footage of the collection area of your roof (Length x Width)
• Multiply that area by the amount of rain in inches (yearly or per rainstorm if you prefer).
• Multiply that number by 0.623 which is the quantity of water in gallons one inch deep in one square foot of space.
• That will give you the number of gallons that can be collected from your roof area.

Even if you live in a place like Phoenix, Arizona that only gets an average of 8 inches of rainfall per year, you still can harvest close to 10,000 gallons of water each year. You might not be able to take 20-minute showers or water your lawn every other day, but that would be more than enough water to survive on.

How to Design a Rainwater Catchment System?

The first and most important part of any rainwater catchment system is a good roof and gutter system. Metal roofing is often considered the best type of roofing for rainwater catchment. Metals used in roofing are generally non-toxic and very little water is lost during the catchment process. Shingles are not a great option since they are made with toxic ingredients that may leach out over time and contaminate your water supply. Other options can include terra cotta tiles, cedar shingles or thatched roofing, though those options may cause you to lose some quantities of water during the catchment process.

Gutters and downspouts are similarly best made from non-galvanized metal or PVC (though PVC does have some toxicity concerns). If you´re interested in becoming ultra-sustainable, split bamboo makes an excellent gutter system and can last for years before being needed to be replaced.
Your downspouts will need to lead to some sort of cistern where your water can be stored. The size of your cistern will depend on the amount of rainfall your region averages and whether you experience regular extended dry seasons. If, for example, the longest average drought for your region is 1 month, and your family averages a use of 250 gallons of water per day, you´d need a 7,500-gallon cistern to assure your water supply during peak drought season.

There are many options when it comes to cisterns. Plastic cisterns are relatively affordable and will last for several years. Cisterns made from rock or concrete will last longer, though they may require a larger upfront investment. Ferro-cement cisterns are the best option that combines long term durability with cheaper cost. Ferro cement is made by making a structure from rebar and metal wire or mesh and then plastering both sides with cement. While the cement is structurally sound and holds water well, Ferro cement is much thinner than rock or cinder block tanks and thus uses much less cement. Another benefit is that they can be built above or below ground.

Since leaves, dust, and dirt often accumulate on your roof, incorporating a “first-flush” system may help to keep the water in your cistern clean and free from debris. During the first minutes of a heavy rain, the downspouts send the water (and accumulated debris) to the ground. Once the water is running clean from your rooftop, the downspout is reincorporated into the cistern.

You can do this manually, though this requires you to be at home during every rainstorm. A better option is to install an automatic first flush diverter that uses a simple ball and seat system. Every time it rains, this diverter will automatically divert the first few gallons away from your cistern before allowing the cistern to fill with cleaner water.

Catching Rainwater for a More Sustainable Water Supply

Though we may not think of rainwater as a product to be “harvested”, water should be our number one priority on any landscape. The rainwater that falls on our roofs is one of the most untapped reserves of free and abundant resources. Instead of depending on pumping water in from rivers hundreds of miles away or pulling groundwater up from aquifers faster than they can replenish, the rainwater that falls above us offers more than enough abundance for all our water needs.

Tobias Roberts

After working in the development industry for over a decade, Tobias decided it was time to stop advising Central American farmers how to do things if he didn´t have a piece of land to live coherently with what he taught. Together with his family he runs a small agro-forestry farm, tourism cooperative, and natural building collective in the mountains of El Salvador.


  1. interesting article. i have been working on solving my water storage situation for a long time. the main problem is frost in the winter. an outside tank would freeze solid, even if covered by soil and compost, since the ground freezes to at least 3 feet down. i do not have very many options for sheltered, heated water storage. anyone with any ideas to submit would be welcomed.

  2. Very interesting article!
    I’m about to build 3 ferrocement water tanks. Since I need to be very well informed, before startting the job. I have seen many Youtube videos, but didn’t find a quite clear one. Can somebody recomend a good instructive video?…Thanks, beforehand!

  3. We can’t afford to replace our roof. I have heard of filters you put in your rain gutters. We also put barrels out to fill with rain and place netting in the openings to catch any debri. Is there anything more we could do?

  4. I recently came across a blog from Favourite homes stating some of the basic tips to be followed for saving water. As global changes in climate and extreme weather conditions hitting most parts of the world, drought is an increasing threat looming over the horizon. Natural water resources are also becoming increasingly polluted, thereby restricting potable water fit for human consumption. This makes water conservation more important than ever – even if you aren’t living in a drought-prone area. To know more visit:

  5. have you looked at insulating your tank with hempcrete? or trying a hempcrete water tank? would act as its own water filter system too I’d think? be worth a look anyway!

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