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The Best Roofing Material for Harvesting Rainwater

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Rainwater harvesting has really been catching on as environmental consciousness has become more prevalent. For example, you might notice a few homes in your neighborhood that have barrels under their downspouts for the purpose of catching the rain that runs off the roof. If you are interested in doing the same, there are things you need to know about your roof and contamination before you use the rainwater you collect.

Harvesting rainwater is a great source for graywater that you would use to water your plants in the summer or wash your car. But, because the water quality can be affected by the season, surrounding conditions and your roofing material, you should definitely think twice before you drink it or cook with it. There are some roofing systems that are far better than others in terms of contamination.

There will be biological and chemical contaminants in rainwater collected from any type of roof, in the same way as any water in nature. Studies show that cool roofs, concrete tile and galvanized metal roofs produce the best quality of rainwater, especially for indoor, domestic use.

Rainwater from asphalt fiberglass shingle and green roofs contain dissolved organic carbon (DOC). Green roofs are considerably lower in other pollutants such as aluminum and turbidity, but the DOCs are significant.

The age of the asphalt fiberglass shingle roof makes a difference in the quality of rainwater it produces. New roofs have high DOC concentrations, but after one year, the concentrations of DOC were about the same as galvanized metal, concrete tile and cool roofs. However, other pollutants such as particulate matter, zinc and iron were increased after the shingles aged.

Roofing that was manufactured before 1980 may contain asbestos, and cedar shake roofing has toxic runoff. Some roofing has been embedded with zinc or copper as a moss killer. If moss is growing on the shingles, there is no moss killer present.

With the high concentrations of DOC coming from a green roofs it would take high concentrations of chlorine to disinfect the water. This alone would be harmful to humans, however in combination, DOC and chlorine can become carcinogenic. The same is true for asphalt fiberglass shingles. Therefore, it is not advisable to disinfect the water from these roofs with chlorine.

Green roofs need to be carefully watched for measurable concentrations of lead and arsenic if commercial growing media is used. Green roofs also show the highest microbial diversity of all the five main roof types.

The water collected from galvanized metal has lower concentrations of fecal bacteria than the other roofing materials. This is an advantage over other roofing material because it produces water that is lower in microorganism that can cause disease in humans.

Cement and Clay tiles could be contaminated if the clay source was contaminated. The firing temperature is often not high enough to fuse all the minerals to prevent them from leeching. There are companies that will clean the roof, seal it with an approved sealer and put a decorative coat that is also approved for potable water. Otherwise, the water could be safe for washing the car or gardening.

All of the roof types have human pathogens such as Bacillus and Staphylococcus as well as other types of things that are associated with soil. This indicates that all rainwater harvested needs to be disinfected to reach potable standards.

For those building a new house, it is a good idea to ask the manufacturer of the roofing material considered if it is acceptable for rainwater harvesting. If so, all the pipes and collection system should also be compatible.

On all roofs, the pollutants are reduced by a first-flush diverter, but there are still enough left to be too high for the United States Environmental Protection Agency (USEPA) potable water standards.

 Photo By C. Frank Starmer

 

Sara Thompson writes about roofing and green building in partnership with the experts at Pacific West Roofing, LLC in Portland, Oregon.


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