Most roofs materials are suitable for collecting rainwater, especially when the water is used for irrigation (and not for drinking). Make sure to avoid any materials that could leach toxins into the water, like lead (in flashing) or fungicides in wood shingles. There are a few products certified for potable-water that can be painted over less desirable materials, such as asphalt. Weather Barrier Raincoat 2000 is one such product.
So long as your roof doesn’t contain toxins, you can safely use the water for irrigation without any treatment. Studies that have compared the quality of rainwater flowing over common roofing materials types have found that all types would require the water to be treated to meet potable water standards.