Rainwater Catchment

[ernstwa]

I am interested in installing a rainwater catchment system to irrigate my garden. However, I am concerned about what might be contained in the runoff from my roof. My house has asphalt shingles. Does anybody know if it is safe to use this water for garden irrigation?

[Karl]

It depends on what you use the water for, i.e. type of garden, whether lawn/ornamental or if you are planting any vegetables for home consumption, etc. The overall toxicity/risk will depend on the structure and properties of the asphalt shingles (and to some extent their age). For example, whether it is a composite material with any potentially toxic particles; further to this, some hydrocarbons may leach out.

This site on these links may have some useful information on the safety of using run-off water:

A Few Important Points about Rain Barrels
Rain Barrel Project
Constructing a Rain Barrel in Minneapolis and Surrounding Communities
Rain Barrel FAQs
Rain Barrel FAQs

QUOTE:
"When NOT to use a rain barrel for watering If you have certain kinds of roofing material you shouldn’t use rain barrels for watering plants. If your roof is made of wood shingles or shakes that have been treated with any chemical (usually chromated copper arsenate—CCA) to make them resistant to rot and moss, lichen and algae growth, don’t water your plants from a rain barrel. Water collected from copper roofs or copper gutters also should not be used. Zinc (galvanized metal) anti-moss strips—usually mounted at the roof peak—also produce toxic chemicals you don’t want in your garden. Don’t use rain barrels if you have these strips (you may want to remove them), or if you have had your roof treated with moss-, lichen or algae-killing chemicals within the last several years. Note that nowadays there are asphalt shingles on the market which have zinc particles imbedded in the surface. Check your shingle specifications if you have recently re-roofed.

In addition, general practice is to avoid watering vegetables and other edible plants, such as herbs you plan to use in cooking, with rain barrel water collected from asphalt-shingle roofs. These kinds of roofs may leach various complex hydrocarbon compounds, so most people avoid using water from asphalt-shingle roofs or flat tar roofs on plants meant for human consumption. To date there is no definitive research on the amounts and types of hydrocarbon compounds which may leach from such roofs, though it is common practice to use water collected from asphalt-shingle roofs for watering ornamental plants and shrubs. Enameled steel and glazed tile roofs generate little or no contamination and rainwater harvested from them is commonly used to water vegetables."

[ernstwa]

Karl, thanks for your detailed response. I was thinking of using the rainwater for a vegetable garden. However, I think I will just use it for non-food watering based on that information.