Disclaimer: I get a commission for purchases made through some of the links in this post.
Here in the high desert of northern Arizona, rain is not as frequent a visitor as it is in other parts of the country, making every drop a precious resource. At our off-grid home, we've set things up to collect the often sporadic rainfall to meet our home's daily and annual water needs. Join me as I walk you through building a potable rainwater harvesting system and how you can replicate and modify it for your own home.
Speaking of raindrops and codes, our county has recognized the value and increasing popularity of rainwater harvesting by implementing a dedicated building code. What you'll see here complies with those guidelines, but you'll want to check with your own local building department to see if they have a rainwater harvesting code of their own, especially if you're integrating this type of system in new construction and as your primary (or only) water supply.
Calculating Rainwater Collection Potential
First things first, decide on how much storage capacity you need based on how much liquid gold you estimate you can collect from the sky where you live. The magic numbers come from the square footage (the area) of the roof serviced by your gutter system and the annual precipitation—rain and snow—that falls on that roof.
To simplify this calculation, you can turn to various rainwater harvesting calculators available online, including this one courtesy of WaterCache.
For our rainwater collection, we have two main roof areas, on the house and the workshop, with six-inch seamless gutters all around, totaling about 3,000 square feet. Inputting that figure into the rainwater harvesting calculator, it comes up with 1,869 gallons per inch of rain that our downspouts send into three interconnected potable water tanks.
That said, we don't often get a full inch of rain from a single storm, so inputting the average annual rainfall for our area unveils the potential liquid bounty we could amass in a year.
Where we live, between Williams, Arizona, and the Grand Canyon, the current annual average is 22 inches of rain. Add to that an average of 65 inches of snow, which is equivalent to approximately 6.5 inches of water (thank you, NASA, for the 10-to-1 snow-to-liquid ratio). At a grand total of 28.5 inches of precip per year, with our roof coverage, that's a whopping 52,332 gallons we might collect!
However, given the nature of Mother Nature, tempering expectations is crucial. Averages are just that, and we've gotten less than that average—sometimes much less—for the past few years. Yet, armed with our own historical average water usage of 1,500 gallons per month, or 18,000 gallons annually, a figure we got from our less conservation-minded water bills of yesteryears, we know we could comfortably subsist on less than half of that 52,000-gallon figure.
If you want to look up the figures for your own area, USClimateData provides data on average annual rain and snowfall. And on the practical side, measuring rainfall need not be an elaborate affair. A simple, inexpensive rain gauge, like the one we use, is an easy way to compile your own numbers over time. Each time it rains (or snows), we see how much we get, put the number in the rainwater calculator, and record it in our dedicated notebook.
A Note on Snow and Water Collection Potential
Keep in mind that if the snow just gets blown off your roof, like it often does here, or it doesn't sit up there long enough to melt into your collection system, it basically won't count, right?
We recommend putting snow breaks on your roof, not only to protect your gutters from heavy snow and ice but also to help prevent the snow and ice from sliding off, to instead melt beneath or through the guards and into your collection system.
There are different types of snow breaks. We used and installed these commercial snow guards (pictured below) on our workshop, and our roofer put up breaks similar to this one on our house.
Deciding on Rainwater Storage Capacity
How much rainwater tank capacity is too little, and how much is too much? That's tough to answer because, for one thing, you're continuously using the water you collect, drawing down your tank(s). But if you can't collect and store it all when it comes down, you'll obviously lose some of that potential.
And, while you don't need enough storage capacity for an entire year's water consumption, neither do you want your tank(s) frequently overflowing—especially if most of the rain happens (or snow melts) during one season or within a few months of the year as it does here during the monsoon.
While adding more storage capacity or switching to a larger cistern at a later date is an option, that would cost more and add logistical issues compared to installing all the storage you'll need at once.
Based on our historical usage, our collection potential calculations, and some neighbors' experiences with rainwater harvesting, we decided on a system with three 5,000-gallon cisterns plus two 275-gallon cubes. We also added a series of 50-gallon overflow barrels, especially for outdoor use in the garden, cleaning, and more.
Just before the 2022 monsoon season started, we had the gutters installed on our newly constructed house. It rained the next day for the first time in about two months.
One month later, all three of our tanks, including the one by the workshop, the cubes, and the overflow barrels were full. Needless to say, now we wish we had more storage!
Equalizing a Multi-Tank Rainwater System
When the architect drew up our building plans, including the rainwater harvesting system, he explained the need to equalize our tanks, meaning that once in place, the overflows should all be at the same elevation.
So we did a little experiment in our kitchen to prove to ourselves that what he said was true. (Not that we doubted him—he has a similar system.)
I don't have a picture of that experiment, but we took three plastic bottles (ours were the same size, but you can do this with different-sized bottles) and connected them near the bottom with snuggly fitted straws. This mimicked how our real tanks would be connected by PVC pipe between their outflows, located inches above the base. We then slowly poured water into one of the bottles and watched how it moved from that bottle, through the straw to the second and into the third, until the water level had equalized among the three.
Next, we put a block under one bottle to raise it and repeated the process. The result: the elevated bottle ended up with less water than the other two once the three bottles had equalized.
So, in real life, if your tanks are connected, but their overflows are at different elevations, one will overflow before the others fill completely. That's why it's important that the overflows are all at the same elevation if you have multiple tanks, whether they're the same size or dimensions or not. This means you may have to use a transit or another tool or method to get the tanks as close to level, both on or in the ground and with each other, as possible.
This is made all the more tricky if your tanks are spread out at different locations around your home, the ground is sloped (even a little), or both. If you're required by code, as we are, or by necessity to partially bury your tanks, you'll probably need to do so at different depths in order to make them all level at the overflow. Even if the area around your home appears flat, there's often at least some difference that can make a difference to your rainwater harvesting system. And, of course, the more the slope between tanks, the bigger the difference in how much digging you'll need to do.
A Note on Burying Water Tanks
If you want to completely bury a rainwater cistern, you'll need one specifically designed for that purpose.
The polyethylene tanks we have are not designed to be buried. In fact, the manufacturers of these tanks suggest you don't bury them at all—but if you do, do not bury them more than halfway up the straight sides.
There are reinforced tanks available that can be buried deeper, so be sure you're getting the right kind of cistern for your situation.
Installing Underground Water Pipes
Once our three cisterns were partially buried to a minimum depth of 31 inches per county code and equalized, it was time to find a backhoe operator to dig trenches—about 150 feet of them—for the PVC pipe that would connect the tanks and bring the water into the house.
We also installed a shut-off valve on each tank's outflow below grade so we'd always be able to isolate any one of them for things like cleaning, repair, water management, and so forth. We placed a vertical access tube around each valve so that, once the tanks were backfilled, the shut-offs would still be... well, accessible to open or close it, just as you would use a water key to shut off a municipal water supply.
The PVC pipes pictured here connect three polyethylene tanks, one of the workshop and two others on either end of the house, coming together to then enter the house below grade. We air-tested the system per county code before we backfilled the water pipe trenches. For a non-pressurized system like ours, we tested it to 30 psi, making sure it held the air for at least 15 minutes. It actually held at that level for days.
Adding Screens and First Flush Diverters
It's not only required by our county code in our case, but it's also a good idea to screen the rainwater as it makes its way from the roof to your tanks.
A first flush, also called a roof washer, is a simple device that removes the initial flow of water in a potable rainwater collection system. The first pass of stormwater washes your roof of all the sediment and other "stuff" that's built up since the last time it rained or snowed so that cleaner water goes into your tanks and then into your house.
With a first flush, the water heading from the gutter into the downspout first passes through a "leaf eater," which is a screen that catches the bigger things, like leaves or pine needles, bugs, a field mouse dropped by a passing bird (happened to us, anyway), or whatever else might end up on your roof. After passing through the screen, the water drops into the first flush downpipe.
As the downpipe fills, a ball inside rises. Once it reaches the top of the vertical pipe, the water then goes into the pipe that continues to your tank. A drip valve at the bottom of the first flush downpipe slowly releases the diverted water (which you can collect for other uses, too).
While now required by code in our county and others, not everyone agrees that first flush rainwater diverters are such a good idea, and we know some folks end up removing them down the line. Here's a good article about the pros and cons of first flush systems: To First Flush or Not to First Flush.
A Note on Optional Gutter Guards
In addition to the leaf eaters, we added these simple gutter guards at the top of our downspouts. Why? Well, for one, we once found that dead mouse I mentioned above in one of our gutters, and we really don't want a mouse carcass going any further into our system. We know the leaf eater would have stopped that mouse, but we decided this inexpensive extra protection wouldn't hurt. It goes without saying, though, if you do have gutter guards or any other screen in your system, it's a good idea to get up there and check them periodically to clean out whatever may have been stopped by the screen so it doesn't dam things up.
Filtering and Purifying Rainwater in a Potable System
Is it really necessary to filter and purify rainwater, especially if it's already been through a first flush or roof washer system? According to the county where we live, it is, and we agree. After all, we do get bird poop on the roof, and other goodies float around in the air and get mixed up in rain and snow, so may as well not drink it. So, we purchased a 10-gallon-per-minute (GPM) Pulsar Quantum Disinfection System, which requires no electricity, and a BBF Series 2 Whole House Filtration System with a 5-micron pleated filter and carbon block filter from US Water Systems (shown below). When the water enters the house, it's drawn through the pump, located in a sump," a recessed part of the floor below the level of the foundation. We use a Grundfos Scala2 pump.
Then the water goes through the filter and disinfection unit before moving on to an on-demand water heater and the rest of the house.
In the photo above, you can see we have (red) shut-off valves before and after the (blue) filter and (black) disinfection units. These shut-offs are required by code and are also necessary to turn the water off when we need to change the cartridges in either unit, which for us is every one or two years.
Testing Your Potable Rainwater Supply
Whether it's required where you live or not, we think it's a good idea to have your water tested. Our county building code does require a water quality test, so we had one done through a local laboratory to present to the inspector once the whole system was in place.
While the code doesn't specify what exactly should be tested for, we requested a bacteria test and another for zinc. Zinc is present in metal roofs, particularly in galvanized metal, which ours is not, but we wanted to show that to the inspector.
The water test cost us $25. We passed with an "A" on all counts.
Adding and Raising Overflows
Once our system was completed and inspected and the tanks started filling, we directed any overflow into a series of smaller barrels and cubes for use in our raised beds, for watering some fruit and nut trees, sharing with the birds and other local wildlife, and so forth. And we've had plenty of overflow water since we added those barrels and cubes.
As you can see in the photo below, a pipe from the bulkhead sends any overflow away from the tank (rather than letting the water shoot out or run down the side) and into a 50-gallon rainwater barrel, which itself has an overflow with a pipe to another tank. There's a spigot near the bottom of each barrel, so we can easily use that bonus water from there.
The overflow pipe from the cistern is raised a little to allow the tank to fill to the actual 5,000-gallon line and beyond.
A Satisfying Part of Sustainable Living: Using a Potable Rainwater Harvesting System
In an area where the word "drought" is often part of the local lexicon. it feels good to get the water we need for our home and garden directly from the sky, even during extended dry stretches.
It's a fact that the upfront cost of installing a potable rainwater collection system can be significant—with our three large polyethylene cisterns, seamless gutters and first flush diverters, 150+ feet of underground pipe, filtration and disinfection system, and overflows, ours cost about $18,000 in 2021/22—but it's nice to no longer pay for municipal water, which isn't available where we now live, or for hauling or delivery, not to mention eliminate the fuel involved in the latter.
And living a more self-sufficient, sustainable lifestyle simply feels good, too.
Do you plan to install a potable rainwater harvesting system or already have one? Do you have any questions about rainwater collection? Let us know in the comments below.
Comments