Is it safe to use rainwater harvested from an asphalt shingled roof to water my raspberry garden?


Is it safe to use rainwater harvested from an asphalt shingled roof to water my raspberry garden? I am not so much concerned about the bacteria, but wondering about the chemicals from the shingles.


Jonathan Foster, Home Horticulture Outreach Professional

The short answer is that it’s complicated and different credible sources I found advise different levels of caution–there has been some test data, the potential contaminants include animal waste and petroleum-based products from asphalt, and there are reasons to mitigate the introduction of these things into your garden. However, it is also true that the amounts detected seem to be quite small, so the overall risk is tough to gauge and will probably rely on common sense and your own tolerance. Mitigating strategies include avoiding watering the foliage, using only on fruit-bearing plants and not leafy green vegetables (many contaminants do not travel/distribute easily through plant vessels and are mostly risks for direct ingestion from surface residue), washing produce thoroughly before eating, and considering a bleach treatment of the water in its reservoir (effective for pathogens, less so for inorganics).

I will refer you to this excellent discussion of the topic from the UConn Extension. A takeaway quote: “In summary, there are risks to using collected rain water for irrigation of food crops. In most cases, the risk appears to be low, and using the above sanitation practices can reduce risk.” Note that we cannot say there is zero risk, merely that these approaches should lower the risk that is there. If you’d like to get deeper into the weeds of specific test results, you can peruse this WA State Dept of Ecology “Roofing Materials Assessment” (highly technical) and NC State Univ Extension’s “Water Quality of Rooftop Runoff.” You can also reach out to your local Maine Soil and Water Conservation District office to see if they have additional information.

In the end, you’ll need to make an informed decision about your risk tolerance. I’m sorry I can’t give you a more definitive answer.

Happy gardening.