Question: Can rainwater collected from an asphalt shingle roof be harmful to vegetables and other edible plants?
Answer: The water flowing from a roof does pick up a small quantity of contaminants, including chemicals from shingles and gutters, possibly bird and animal waste, tannins and other compounds from leaves in the gutter, etc. Also, micro-organisms can proliferate in the reservoir itself. So, roof runoff is not considered safe for human consumption in most countries, at least not without filtration. (Curiously, in other countries, the inhabitants are not so squeamish and roof runoff water is their main source of drinking water!)
On the other hand, the water entering vegetables grown using roof water is filtered in various ways: by their roots, by soil microbes, etc. Plant roots, after all, are very selective about what they let in! And moreover, plants have the ability to use certain products that we consider pollutants by converting them into useful products.
Normally, therefore, the plants that use roof water are considered safe enough for human consumption … as long as certain precautions are followed.
First, it’s best to apply the water to the roots, not the foliage. By means of a soaker hose or drip irrigation, for example. And if runoff water is applied to the foliage, it would be wise to rinse the vegetables with clean water before consumption.