How To Build Your Own Rainwater Harvesting System

How To Build Your Own Rainwater Harvesting System

By William L. Seavey

We all have heard the news: California has been declared to be in Exceptional Drought, and a governor has declared a state of emergency. Here’s an idea of how to start help create your own water supply.

Reservoirs nearly everywhere were running very low, mandatory restrictions on outdoor and indoor water use began being imposed, and water bills were going up. In some cases, way up—especially when fines were imposed.

But finally many people were beginning to realize how precious potable water really is in a state where usage averaged 100-150 gallons per person nearly every day prior (and even during) the drought.

In my little town of Cambria (population 6,400) on the Central Coast, the drought just added insult to injury. We had been under a county water moratorium already for 15 years—most building had been suspended until a “permanent” water source could be found (our water comes from two streams, and they were virtually dry in 2014). Rainfall that averaged between 20-30 inches yearly 10 years ago was averaging five to six inches.

In early 2014 we were told we would no longer be able to use potable (purified) water to nurture our plants. About the same time the local water/sewer district began restricting water use to 50 gallons per person per day. To avoid draconian fines, we dropped water use over 40 percentfrom the previous year, a record in California.

But in late 2014 it was revealed in the media that the town would still likely run out of water. Porta-potties were set up around our tourist town near the famous Hearst Castle. The Castle itself closed its restrooms, as its independent springs had fallen from 300 gallons a day to 50.

Then the $9 million brackish water recycling plant, finally approved after years of wrangling, went online in early 2015. And then the rains came—nearly seven inches in a little over a month in late 2014.

The drought is far from over, but there is a hard lesson that running out of water is not an option. The hardships would have been many and our town could have been severely economically impacted.

About four years ago I decided to pre-empt total helplessness and bought an 1,100-gallon poly tank. I knew I could somehow route rainwater from a garage roof to it, and gravity feed it to plants in a worst-case scenario. (That scenario occurred!)

I live on a sloped lot so pumps would not be required. I’m a handy guy so was able to figure out how to capture gutter water and send it via PVC piping and into this giant barrel, along with others I was collecting over time. It didn’t seem to be rocket science and over the course of the next three years I led several rainwater harvesting seminars (free) for members of my community as my sophistication increased.

Since rainwater is not being pressurized to enter the home (though it could be, codes permitting), a gravity system like mine is much simpler than what most plumbers have to contend with. (But some plumbers and general contractors will do this work.)

Can rainwater be used for anything besides plants? This is a common question. Of course!

It is relatively pure. It could be used to flush toilets. It is pure enough to shower, bathe or wash clothes with assuming your roof is washed clean and your storage containers are also sanitary. (Ideal roof: metal.) I’ve drunk rainwater straight with minimal filtration through cheesecloth or a carbon filter. I’ve even drunk water from my giant tank, some of which was three years old—and it tasted fine. (Ed. note: We suggest you purify the water before drinking by adding 1/4 teaspoon of chlorine bleach—without scents, fabric protector, etc.—per one gallon of water. Mix thoroughly and let sit for at least one hour. However this won’t get rid of chemicals that may be in the water.)

Water Storage
A hot tub makes a great water storage vessel.

Right now I can store about 2,000 gallons using various “hybrid” methods that include plastic trash barrels, a stock watering tank, a hot tub, and a 275-gallon, food grade, square poly reinforced container bought from manufacturers. Other options: underground cisterns make sense (but they’re expensive); the cheapest aboveground approach turns out to be a modular swimming pool if you can’t dig and line a small reservoir.

Adequate storage is the weak link in any rainwater harvesting system, as it is simply not cheap. You can figure $1 per gallon set-up costs if you DIY, $2-$3 if you employ others.

But once paid for, it’s the gift that keeps on giving.

What’s amazing is how much water you can collect from rain and a roof (see box above). In the recent rains I was able to collect nearly 1,000 gallons just from one inch on two roof surfaces. Later rains were routed to the parched ground as was practicable. (Even I wished I could have been able to collect more, but 2,000 gallons should be enough to get us through next summer.)

A hose diverts water from a downspout to storage tanks.

Below are some tips on how to save money and avoid stress in building a system:

• Big poly tanks can be delivered from retail outlets. They come in all sizes, configurations and prices. Check out some of them online or in the below recommended book. The hardest thing for me was siting the tank, since I had to level a sloped pad near my street. I dug out as much as I could and also had to reinforce the base with concrete.

• You’ll need 1″ to 3″ PVC piping (easy to cut with a hacksaw). If you don’t have a truck you could buy shorter pieces and link them together. Make sure you have enough slope from the roof(s) to the tanks and install a shut off valve if your tank(s) can’t overflow safely.

• Rain gutter protectors keep leaves away from downspouts and you may find them useful. I’ve experimented with various kinds— they need to shed fine vegetative matter and the screens should not plug up themselves. In most cases I just get on a ladder and use a paint brush to sweep the gutters free when they’re dry. I’ve also gotten on the roof with a leaf blower.

• I’ve often thought someone should come up with some sort of kit to do all this. (I might be interested in investing, if so.) It might involve not only rainwater collecting (the hardware) but possibly water-saving devices for homes such as a toilet dam. (There is a cheap Canadian product available at Home Depot that inserts pretty neatly in a downspout, with a hose attachment—still experimenting with that. It could be part of the kit).

How Much Water Can Your Roof Harvest?

To compute how much roof you need, and how much rain you can get/potentially store: A roof surface of 20 feet by 50 feet would give you 1,000 sq. ft. Convert your area’s average rainfall (say, 18 inches a year) to feet by dividing by 12. This gives you 1.5 feet of rain. Now multiply 15 feet by 1,000 sq. ft. and you get 1,500 cubic ft. There are 7.48 gallons per cubic ft., so the total is 11,220 gallons.


Source: San Luis Obispo County (CA) Coalition for Appropriate Technology’s Guide to Harvesting the Rain.

For Further Information:

• Water Storage (Tanks, Cisterns, Aquifers and Ponds), by Art Ludwig, $19.95,

• Bill Seavey’s “Budget” Roof Rainwater Catchment and Storage Setup (detailed instructions and illustrations), $8 pp, Crisis Response Publishing, P.O. Box 1681, Cambria, CA 93428.

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