Rainfall Harvesting Math

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?

In the Gutter

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.

Water Conservation

“the price we paid was the price men have always paid for achieving a paradise in this life — we went soft, we lost our edge.”
-Frank Herbert Dune

SurviveLA was planning on discussing rainwater collection today, but we realized that we would be getting ahead of ourselves without first discussing what we call BOC, or boring old conservation. So before delving into greywater and rainwater harvesting it’s time to assess where your household is in terms of water consumption and see if that level can be reduced. It’s generally far easier and less expensive to conserve resources than it is to resort to the latest techno-eco-gadgetry.

So let’s count down the major sources of domestic water use (according to percentages calculated by the American Water Works Association Research Foundation) and consider how to conserve:

58.7% Outdoor Water
Clearly, the place to start with water conservation is outdoors, in designing a landscape that doesn’t need supplemental irrigation. Our rule around the SurviveLA compound is, once again, if you gotta water it you gotta be able to eat it. The vegetables that we grow are irrigated with a water-saving drip irrigation system that we’ll describe in detail in a later post. We have no lawn, and other than the vegetable garden, all the other plants are either natives or from the Mediterranean, many of which are also edible. Reusing greywater from your shower and washing machine can also reduce the amount of outdoor water usage. Above all remember that lawns are the wasteful and evil thought-spawn of generations of golf-obsessed Republicans. Replace them with edible landscaping, or decomposed granite.

10.8% Toilet
All toilets should be low-flush, and todays low-flush toilets are considerably better than the first generation. A low-flush toilet uses less than 1.6 gallons of water compared to 3.5 to 5 gallons for an old toilet, which many municipalities will give you a substantial rebate for getting rid of. If you live in an apartment or don’t have the energy to dump the old crapper, it’s possible to fill a plastic water bottle with stones and put it in the tank to displace and thereby reduce the amount of water used to flush. Don’t use a brick for this purpose since it can kick around and damage the flushing mechanism.

And remember the charming slogan, if it’s yellow let it mellow, if it’s brown flush it down. The mellow yellow strategy can save hundreds of gallons a year. This idea can be automated with a so-called dual flush toilet. These toilets, common in Europe, have two buttons–one for pee-pee and one for poop. Most dual flush toilets use the standard low-flush rate of 1.6 gallons when you hit the #2 button, but use only .8 gallons when you hit the #1 button.

And don’t use the toilet as a trash can. Not only does this waste water, but it puts an additional burden on sewage treatment plants. Having visited a sewage plant in person, SurviveLA can personally attest to the depressing sight of witnessing hundreds of plastic tampon applicators afloat in a vast sea of poo.

8.7% Clothes Washer
Wash only a full load. Run the waste water outside to irrigate your landscaping using either a surge tank, or directly.

Adventurous homesteaders, willing to put in the extra labor, may wish to consider a James Washer, an electricity free hand washer, commonly found in off-grid households, where you put some hot water in and agitate the clothes for a few minutes with a lever on the side. A hand wringer is used to dry the clothes before hanging them on a clothesline. A simple drain line can be hooked up to a hose and run out to the garden.

6.8% Shower
Shower less and celebrate your body odor–pretend you are the type of person Andy Warhol used to lovingly refer to as Eurotrash. Barring lifestyle changes, install a low-flow shower head. Also consider a shower head with a cutoff valve to allow stopping the water while soaping up. Use a bucket to catch the water that flows before it heats up. Use this water for plants or to “bucket flush” the toilet.

It’s cheating somewhat, but take your showers at the gym and let someone else pay for the water.

6.3% faucet
Turn off the water until needed when brushing teeth or shaving.

5.5% leaks
This is a no-brainer, but something everyone seems to forget. Fix leaks immediately! The knowledgeable folks at your local hardware store (not the idiots at Home Depot!) can tell you how.

.6% dishwasher
Run your energy efficient dishwasher only with full loads. And incidentally, a study conducted by the University of Bonn in Germany concluded that dishwashers use half the energy, one sixth the water, and less soap than hand washing.

One last sobering statistic to remember–the average American uses between 80 and 100 gallons per person per day, with the higher consumption coming from folks living in the dry Southwest. The average African uses 5 gallons a day.

More Washing Machine Greywater Fun

One of the problems with the washing machine surge tank greywater strategy is that what you are watering has to be downhill of the machine. This is a common problem with greywater systems, and trying to use pumps to get the water uphill is simply not economical, nor is it environmentally hip since those pumps have a lot of plastic and other icky industrial materials in them.

Fortunately washing machines have a pump built-in and it’s possible to use your washing machine’s pump to get water both uphill and a reasonable distance horizontally. Unfortunately, washing machine pumps are not designed with this in mind, so you run a slight risk of burning out or decreasing the life of your washing machine’s pump should you attempt to move the waste water out to your garden. There are, however, ways to minimize the risk of pump burnout.

The guru of greywater, Art Ludwig, suggests the following methods for using your washing machine’s pump to irrigate plants:

1. Use only 1 inch HDPE or either rigid or flexible 1 inch PVC pipe. Smaller pipe may burn out the washing machine’s pump and larger pipe will allow water to accumulate and get stinky. Standard garden hose is not a good idea because of the risk that it will kink and burn out the pump, however flexible 1 inch PVC can be easily substituted.

2. Every 50 horizontal feet of run equals the same amount of pressure as running 10 inches vertically. Ludwig suggests limiting the combined horizontal (using the 50 feet = 10 inches formula) and vertical run to the equivalent height of where the water would normally empty out into the standpipe near the top of the machine.

3. Install a three way valve at the standpipe to allow diverting the water to the sewer.

4. The pipe may need to be vented (either by running a vent pipe up or using a swing check valve as a vent) if the end of the pipe is lower than the water level in the washing machine. This is to prevent an unintentional siphon that could empty the washing machine before its cycles have completed. This is apparently not a problem with all washing machines.

5. It’s best to get the pipe up to its highest point early and then run down from there though it is possible, since the water is under pressure, to run pipe up, down and up again. Having sections of pipe where water will stagnate, however, runs a slight risk of the aforementioned stinkiness, and burst pipe in places where it freezes.

6. Installing a swing check valve close to the washing machine will prevent water from flowing back into the machine should you be moving a flexible hose around and lift that hose above the level of the machine.

7. Rig up a bypass to somewhere the water can flow if you live in a place where it freezes. This is not a problem for us in sunny LA.

8. Since the water is under pressure it’s possible to rig up fancy distribution systems in the garden. For ideas on this see Ludwig’s book, Create an Oasis with Greywater.

9. Lastly, a note to our rentin’ homesteaders–this is a greywater strategy you may be able to get away with. And if you burn out the pump, and the machine is owned by the landlord, you’ll be sticking it to the Man!

Build a Washing Machine Greywater Surge Tank

After the shower, the next best source for greywater is the washing machine. Constructing what is called a surge tank with a fifty gallon plastic drum is the simplest way to reuse your washing machine waste water. Perfectly good water that would ordinarily just go down the sewer will instead water your plants after first spending a short time in the fifty gallon drum.

Temporarily draining your washing machine into a fifty gallon drum has two advantages. First, it allows hot water to cool and secondly it prevents siphoning mishaps and washing machine pump burnouts that can happen if you try to move the water directly to your garden through a pipe. Here’s how to create a surge tank:

1. Get ahold of a fifty gallon plastic drum. Most big cities, Los Angeles included, have businesses that resell used drums. Make sure that you get a food quality drum and not something that held toxic materials. The best kind of drum for this purpose is one that has a lid, both so that you can clean it out periodically, and to make it easier to fit the hose connection at the bottom.

2. Drill a 1 inch hole in the side of the tank at the bottom.

3. You will need to improvise what is called a “bulkhead” fitting in order to hook up the tank to a regular garden hose. Instructions for doing this can be found here. Seal the fitting with silicon. Connect this fitting with a standard garden hose and use a ball valve if you want to be able to hold the water in the tank temporarily. Remember that greywater quickly turns into black water if allowed to sit around for more than 24 hours, so use this water quickly.

4. Direct your washing machine’s drain hose into the tank. The hose must first go above the top of the machine before going down into the tank in order to prevent the machine from draining accidentally. Also, don’t make this connection airtight–the washing machine needs an air gap, normally provided by the loose connection to the standpipe to prevent waste water from siphoning back into the machine.

5. For a deluxe installation, use a three way valve so that waste water can be easily shifted back to the sewer line should the need arise.

6. Place the tank on bricks to increase water pressure.

7. Remember not to use washing machine waste water if you are washing diapers.

As always, for more detailed information on how to do this get Art Ludwig’s excellent book Create an Oasis with Greywater.