Rainwater Frequently Asked Questions

 

Rainwater harvesting FAQs

Can I drink rainwater?

All over the world rainwater is used for washing, cleaning, and drinking, without filters or disinfection. However, in the U.S. and Canada most potable systems do include filtration and disinfection since the water can become contaminated, for example if birds or other animals poop on the roof. Potable rainwater harvesting systems filter and disinfect water just as is done with ground or surface water.

Can I harvest rainwater in a freezing climate?

Many people successfully harvest rainwater in freezing climates, though these systems require extra precautions so the pipes and components are not damaged from freezes. Systems can include whole house, indoor systems, with rainwater being the sole supply of water, to simple outdoor irrigation systems used only during the growing season. If you live in a freezing climate find installers in a similar climate to yours and ask them about their recommendations for freeze protection. Also consult resources such as the Alaskan Building Research Series HCM-01557, Water Cistern Construction for Small Houses and the Alberta Guidelines for Residential Rainwater Harvesting Systems (see the resource page for links).

Can I install a gravity-fed drip irrigation system?

If your yard is flat or downwards sloped (in relationship to your storage tank or barrels) you may be able to install a gravity-fed drip irrigation system. The system will be similar to a standard drip irrigation system, but requires the use of low or zero-gravity system components (emitters and timers). Standard emitters, timers, and controllers will not work properly under low pressure. See the resource section for more info.

Can I use roofwater on my vegetables if I live in a city?

Excerpt from © The Water Wise Home: How to Conserve, Capture, and Reuse Water in Your Home and Landscape (by Laura Allen)

City Rain: Clean or Not?
Rain is distilled water, free of pollutants. As raindrops fall through the atmosphere, they dissolve the carbon dioxide in the air, forming a weak acid with a pH of 5.7 or so. When rain falls through polluted air, it picks up particles of soot, dust, and smoke. If the air is polluted where you live, using rainwater catchment for irrigation is of no more concern than the rain falling directly from the sky and landing on your garden; the same pollutants will end up in the garden either way. However, with a roofwater catchment system, you should use a first-flush diverter (page 148) to minimize the introduction of additional pollutants from the roof to the garden. Any drinking water system requires additional filters to remove particulates.

How much rain can I collect from my roof?

Calculate how much rainwater you can collect based on the square footage of the collection surface (the roof) and your annual rainfall.

Find the square footage of your roof by measuring the footprint of the roof, the slope of the roof doesn’t affect the catchment potential. For a square or rectangular house, measure the length and width and multiply the numbers together. If you have multiple roof sections do this for each one and add the results together.

After you’ve found the square footage multiply it by 0.56 to determine how many gallons you can collect per inch of rain. This calculation assumes a 90% efficiency. To find how much rain you can collect in an average rain year multiply this number by the average inches of rain.

For example, on a 2,000 square foot roof, you can collect 2,000 x 0.56 = 1,120 gallons/inch of rain. If your average rainfall was 25 inches/year, your annual collection potential is 1,120 x 25= 28,000 gallons/year.

What types of roofing can be used for rainwater collection?

Most roofs materials are suitable for collecting rainwater, especially when the water is used for irrigation (and not for drinking). Make sure to avoid any materials that could leach toxins into the water, like lead (in flashing) or fungicides in wood shingles. There are a few products certified for potable-water that can be painted over less desirable materials, such as asphalt. Weather Barrier Raincoat 2000 is one such product.

So long as your roof doesn’t contain toxins, you can safely use the water for irrigation without any treatment. Studies that have compared the quality of rainwater flowing over common roofing materials types have found that all types would require the water to be treated to meet potable water standards.

Where can I find a rainwater installer?

The American Rainwater Catchment Systems Association (ARCSA) lists accredited rainwater installers on their website here.

Where can I get barrels to make rain barrels?

Rain barrels can be constructed from any food-grade 55 gallon drum. Plastic barrels are easier to work with and last longer than metal barrels. In the SF bay area unfitted (no hose attachments) you can get free barrels from Keith Wilson (keith@lotusfoods.com805-680-3666) in Richmond. (These are closed top, food grade plastic, either white or blue.)You can often find plastic food grade drums from breweries, bakeries, or food import stores.

Plastic rainbarrels can be purchased new with hose fittings from many hardware stores. These usually cost over $100. There are also on-line rainbarrel sellers, but shipping costs are high (not to mention the environmental impact!) so buying local is prefered.