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?

Our roofs, of course, are another impermeable surface that prevents rainwater from going where it should go–to our edible landscape. We can minimize the surface area by living in as small a house as possible and trying to maximize open ground. At our own compound we’ve even gone so far as to remove some previous resident’s bad addition and reduce the footprint of our house. So called green roofs, which have soil and plants growing on them our an option for the wealthy, but at present are still rare in the US. Most of us will still be dealing with conventional roofs.

Now assuming you’ve got a roof we will hope that you have gutters. Gutters have two purposes, to channel water away from your fragile and very expensive to repair foundation, and to channel it to where that water can be useful, to your edible landscaping. As much as we support self-sufficiency, putting on gutters is a job we think is best left to professionals, specifically professionals who produce seamless gutters with a machine like the one pictured above. Putting up gutters yourself can often be a frustrating experience that you must perform while balanced high atop a ladder. Gutters must have the correct slope or water will stagnate and create the perfect breeding ground for mosquitoes. If they leak you run the risk of rotting out your fascia boards. By all means, if you can afford it, hire a professional for this job. If you can’t afford it go sell some crack on the street corner until you have enough money. The do-it-yourself gutter systems available at home centers, especially the plastic kind, look like crap even in the off chance that you install them correctly.

While we’re big believers in sloth and idleness, one task that absolutely must be performed at least once a year is to get up on a ladder and clean out the gutters. Otherwise, you will be attempting this task, as we have on several occasions, during a downpour at midnight, after the downspouts have clogged up sending a cascade of water over the damned up gutters.

Two simple bits of technology can make gutter cleaning easier. We have inexpensive strainer baskets made out of 1/4 inch hardware cloth in each of the downspouts to keep them from clogging up. Some may wish to consider leaf guards which run along the top of the gutter to keep out leaves and other debris. The problem with leaf guards is that in order to clean out your gutters you must tediously remove the guards along the whole length of the gutter while, once again, balanced precariously on a ladder. While leaf guards catch large leaves, smaller stuff can still get through, so you might as well just stick with the strainer baskets in most situations.

Gutters are just the first step, however. Downspouts must be incorporated to carry that water from your gutters to where you can put that rainwater to use.