The Dallas Green Zine

I’ve been using 3 rain barrels to collect water off my composite roof via a regular roof gutter system. Soaker hoses are connected to 2 of the barrels to distribute water along my house’s foundation.

Then I bought some  drip irrigation supplies recently with the intent to use the 3rd barrel to water my vegetable garden. However, through further investigation, I discovered that not all roofs are the same and I had to change my plans.

What’s On Your Roof

Some roofing materials contain chemicals that kill plants (and could make you sick too if you eat the plants). If your roofing contains questionable materials, use the water runoff only for landscaping and not for vegetables or herbs.

Although most newer composite roofs no longer use asbestos, many asphalt shingles have zinc particles embedded in their surfaces, and these types of roofs can leach complex hydrocarbon compounds. Although zinc has low-toxicity and there hasn’t been much testing on how much hydrocarbon compounds these roofs leak out, they are generally avoided for rainwater collection where the water is intended for vegetables and herbs.

Avoid using water from copper roofs or gutters because the acidity of rainwater can eventually cause copper leaching.

Some roofs have galvanized metal strips mounted at the roof peak that are used to discourage moss and mold. These can also add toxicity to water runoff.
Also, wood shingles, which fortunately are not common here in the Dallas area, are often treated with chromated copper arsenate (CCA) to make them resistant to rot, moss, algae, etc.

Finally, watch out for lead. Lead is sometimes used in flashing and in gutter solder.

A Simpler Approach

I’ve decided for the time being to take a simple approach and use the 3^rd barrel to water a decorative bed that is in the front of my yard, which will also enable be to reduce the amount of water that my sprinkler system currently delivers there. Then, I’m going to use city water on my vegetables.

Because the spigot for the tap water is further from my garden, I’m going to run a hose from the spigot to the edge of my garden and have it feed the drip irrigation system. However, this hose will be a marine/camper hose and not a regular garden hose, as almost all garden hoses contains lead in them. See The Lead In Your Garden Hose.    

A Slightly More Complex Approach

With more time and resources however, there are other ways to get rainwater from one’s roof. For example, enameled steel and glazed tile roofs generate little or no contamination, and you can use their rainwater run-off to water vegetables.

Some of the most suitable roofing materials for collecting potable water are:

• Colorbond steel sheets or tiles
• Zincalume steel sheets or tiles
• Glazed tiles that are well fired
• Concrete/cement tiles
• Clay tiles
• Fibro – newer type
• Composite tiles – bitumen based

One rainwater harvesting site offers some caution on cement and clay tiles however: “Cement and clay tile roofs can create some difficulties when it comes to collecting potable water. There are a lot of various minerals in the clay and cement used for making tiles and there is no guarantee the clay pit was not contaminated. The temperature tiles are usually fired at is not sufficient to fuse all minerals together to prevent them from leeching, thus it could never be guaranteed that a cement or clay tile roof would be as safe as some other types of roofing material.”

Also, there is an example where someone used rainwater harvesting with a composite roof, but added a FloTrue Smart Valve to divert the first 10-60 gallons of water during  rainfall (braewater.com) and then used a Katydyn Gravydyn 2 ½” gallon drip ceramic and carbon filter from REI.com. He claims that when he sent the water to a Ft. Worth testing company, it came back with:

• Lead .02 mg/liter
• Sulfates 1.3 mg/liter
• Nitrates - .02 mg/liter
• Fecal coliform – 0% none

So anything is possible. If you have successful rainwater harvesting examples, please post them here at The Dallas Green Zine.

References

Texas Manual on Rainwater Harvesting

Health Issues Involving Roofing Materials

More on Roof Comparisons