The standard recommendation is to avoid shingle roofs for rainwater collection, and use a non-toxic metal or tile roof instead. Ignore the advice saying plant

The standard recommendation is to avoid shingle roofs for rainwater collection, and use a non-toxic metal or tile roof instead. Ignore the advice saying plant cells or fruiting bodies can "keep out" pollutants; this isn't true across all classes of pollutants, so best to err on the safe side.