Sunday, August 2, 2009

How Clean is Your Roof?

Thinking about installing a rainwater harvesting system someday that will be part of a system to provide you with ALL your water needs? Thinking about what that raindrop might encounter between the cloud and your throat? I sure do. What's a safe roof surface? What kind of roof (or other catchment surface) should we avoid? Well, there are general thoughts on this. Most feel that rainwater that comes off of asphalt composition shingles on a hot day might pick up some petrochemicals and other yukky stuff. And I suppose that most people who think about these things also assume that a good metal roof is probably the best surface to catch rainwater for drinking and bathing uses. To bring the image home, if you had to drink your water through a straw, would you prefer the straw to be made from an asphalt shingle or a metal tube. Okay, that's not exactly fair because I'm ignoring rain water filters and sterilizing, but you get the idea.

Finally, the folks at the Ladybird Johnson Wildlife Center and other roofs in the Austin area are participating in a study conducted by the University of Texas, and funded by the Texas Water Development Board (the publisher of the Texas Manual of Rainwater Harvesting, available as a free download from The study is tracking the three most common roof types used in Central Texas, composition shingle, Galvalume, and concrete tile. The study includes collecting rainwater samples taken from roofs since earlier this year and making an analysis based on the presence of a variety of chemical and microbial elements, including suspended solids, pH, nitrates, nitrites, heavy metals, turbidity, total and fecal coliform, and synthetic organic compounds. None of these things we want to get past our teeth in any large quantity for sure!

More information about the project is available on TWDB’s rainwater harvesting Web site at You may also contact the TWDB contract manager Sanjeev Kalaswad at

Results are expected by the end of the year, so I'm really looking forward to finding out if our assumptions regarding the relative safety between different catchment surfaces match actual science. My thanks go out to Mike Mecke who gave me a heads-up on this study.

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