Constructing a cistern large enough to supply potable indoor water is an ambitious project involving pumps, filters, UV purifiers, and very large and expensive tanks. We don’t want to discourage anyone from making an attempt at it, but for most urban homesteaders it won’t be economical or practical given the space requirements and weight of thousands of gallons of stored water. Thankfully, there are simpler strategies for harvesting rainwater.

Rainwater used for irrigating plants does not need filtering or purification, and since outdoor watering accounts for the number one household water use, you’ll be using that water where it is most needed. Now, once again at the risk of sounding like a broken record, our strict rule around the SurviveLA homestead is that all irrigated plants must be useful, i.e. you gotta be able to eat it or make tea with it. Priority in our plantings goes to useful plants that don’t need additional water once established. But it’s still nice to have citrus trees, salad greens, rapini, beets, and other fruits and vegetables that do need supplemental irrigation. For these types of plants it’s possible to supplement municipal water with rainwater collected in barrels.

You can purchase commercial barrels made of this purpose, but it’s also possible to construct your own using surplus barrels with the same improvised bulkhead fitting we described in our post on washing machine greywater harvesting. In fact your rainwater collection barrel is pretty much the same as your washing machine greywater surge tank, but instead of being connected to the washing machine, it’s connected to your gutter downspouts. Depending on how much rain you get and how many loads of laundry you do, it may be possible to double up and use the same barrel. And, just like your greywater surge tank, it’s best to elevate the rain barrel to let gravity do the work of getting the water where you need it and thereby not have to deal with the cost and complexity of pumping water.

Some precautions–your rainwater barrel should have screens over the inlets to prevent mosquitos from breeding and it’s wise to rig an overflow pipe at the top of the barrel to channel water away from your homestead’s foundations. You will also need to clean the barrel periodically, and where it freezes it may be necessary to turn the barrel upside down for the winter.

Sizing your rain barrel system, or determining if using a rain barrel is worth the effort, requires going back to the formula we discussed earlier. But instead of figuring out how much water per year your roof can collect, you need to consider the amount per month. Monthly rain data for cities in the US can be found here. Rain barrel harvesting makes the most sense in places where it rains throughout the growing season, with the barrels providing additional water to bridge gaps between rains. In a place like Los Angeles where all of the rain is concentrated at one time of the year, rainwater barrels may not be practical.

To size up your rain barrel collection system it’s possible to daisy chain several barrels together using yet another bulkhead fitting towards the top of the barrel and pvc pipe to connect them together. There is also a commercially available kit for this purpose. Remember that bulkhead fittings are notorious for leaking, so be precise with your improvised plumbing work.

SurviveLA believes that all greywater and rainwater harvesting systems should be kept as simple as possible since most folks are lousy at doing any kind of maintenance (when was the last time you drained your water heater?). And, with the low cost of municipal water, even a simple system built with surplus parts probably won’t make sense from an economic standpoint. But, aside from being just generally groovy, rain barrel water collection could be an insurance policy for the uncertainties of global warming. Someday water may be more valuable than oil . . .

Our next step in designing a rainwater harvesting system is to figure out how much water your rooftop will provide.

To do this measure the outside perimeter of your roof–you need not take into account the pitch or slant of the roof, since this does not affect the amount of water collected. Next, use the following formula:

collection area (square feet) x 0.6 x collection efficiency factor x rainfall (inches) = gallons per year

The collection efficiency factor is basically how good your roof is at shedding water. Metal roofs are the best and have the additional benefit of reflecting the heat of the sun and being less prone to leaks. Figure on a efficiency factor of .95 for a metal roof. Pitched asphalt shingle roofs are next with a efficiency factor of .9. Tar and gravel roofs are the least efficient with a factor of .8. Both tar and gravel and asphalt shingle roofs retain a fair amount of water after a rain, but they will still work fine for rainwater collection.

Data on both yearly and monthly average rainfall can be found on the National Oceanographic and Atmospheric Administration website.

For our house here in Los Angeles, we have a collection area of 992 square feet, a pitched asphalt shingle roof and an average of 15.06 inches of rain a year. So the average amount of rain we could collect in a year would be:

992 x 0.6 x 0.9 x 15.06 = 8,067.34 gallons.

If we lived in Atlanta, where the average annual precipitation is 48.6 inches per year we could theoretically collect 26,034 gallons of water a year.

With these numbers in mind we can begin to answer a few questions. Does rainwater collection make sense for your climate and living circumstances? If so, what kind of system will work?