Asphalt shingles can leach chemicals such as zinc, lead, chromium, arsenic, and polyaromatic hydrocarbons.
- It is not exact as to what the levels are or what levels would be safe for watering edible vegetation.
- It's recommended that this collected water is only used for flowers and lawns, rather than herbs, vegetables and fruits.
An alternate roof material that would be suitable for reclaiming water for food plants would be a glazed tile or enameled steel.
For more information:
Read "Is it feasible to reuse greywater as irrigation water?", a Q&A answered by Randy Potter.