Texas Rainwater Harvesting Manual:
For potable systems, a plain galvanized
roof or a metal roof with epoxy or latex
paint is recommended. Composite or
asphalt shingles are not advisable, as
toxic components can be leached out by
rainwater. See Chapter 2 for more
information on roofing material.
On the other hand, this research makes me question galvanized because of metals in the water: Link
The Texas Manual also has costs starting on page 50.