There are differing opinions on this subject. Rainwater collected from barrels under copper roofs or from roofs where wooden shingles or shakes have been treated with chromated copper arsenate to prevent moss or algae growth shouldn’t be used on edible plants. If you’ve treated your roof with chemicals or installed zinc strips to prevent moss or algae growth, you shouldn’t use your rain barrel water in your vegetable garden. Also, water flowing from your roof can contain deposited air pollutants, though this water may end up in your vegetable garden anyway.