EBMUD states on its website regarding the use of rainwater harvested from a roof covered with asphalt shingles:

“….While any material is usually acceptable, asphalt shingles may leach toxins and should not be used to collect water for vegetable gardens. ”

I collect rainwater from my garage roof, which is a flat roof covered with a seamless membrane, self-adhesive surface, base layer -oxidized bituminous coal; fiberglass mesh with resin and sand layer.  The brand, Mod Bit Cap Sheet, is made by Mule-hide and the MSDS can be found at https://www.mulehide.com/Roofing-Products/p/ModBitCapSheet

Can you tell me if water collected from my garage roof can be used to water vegetable gardens?  I use a first-flush diverter.


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Sorry for the late reply to this question.

Generally speaking, asphalt shingles roofs are considered safe for irrigation of any plants. Of coarse you can’t know the actual water quality off your own roof water unless you were to test it, so each person has to make the decision for themselves about what feels safe. Also, there is a big difference in risk from drinking rainwater to irrigating with it.

There are lots of studies on roof water quality from various roof types. You may want to read this one that looked at roof types and compared the water quality to drinking water standards. In this study, asphalt shingle roofs were not worse overall than other roof types.

Click to access Effect-of-Roof-Material-on-Water-Quality-for-Rainwater-Harvesting-Systems.pdf

Here are a few quotes from the conclusion.

Conclusions and recommendations

This study investigated the quality of rainwater harvested from pilot-scale roofs (asphalt fiberglass shingle, Galvalume® metal, concrete tile, cool, and green) and full-scale roofs (asphalt fiberglass shingle and Galvalume® metal). Data from three rain events were collected for the pilot- and full-scale roofs; from these limited data, none of the roofing materials emerged as clearly superior to the others in terms of the quality of the rainwater harvested after the first flush. As discussed below, several conclusions can be drawn and recommendations can be made despite the limited number of rain events sampled.

First, the quality of the rooftop-harvested rainwater generally increased with roof flushing as the rain event progressed, indicating the importance of an effective first-flush diverter. This was observed for both the pilot-scale and full-scale roofs. However, the rainwater harvested after the first flush did contain some contaminants at levels above USEPA drinking water standards (i.e., turbidity, TC, FC, Fe, and Al). This indicates that harvested rainwater must be treated prior to potable use. Thus, we recommend the use of a first-flush diverter and additional treatment prior to potable use.

Second, roofing material is just one factor that affects harvested rainwater quality. The full-scale study showed that the quality of the rooftop-harvested rainwater varied between the two shingle roofs of similar age. In some cases, one of the full-scale shingle roofs showed the highest concentration of a particular contaminant while the other shingle roof showed the lowest concentration of the same contaminant (i.e., as compared to Shingle 1, Shingle 2 consistently showed higher TSS, FC, and TC concentrations), suggesting that geographical location affects harvested rainwater quality.

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