Metal roofs are an investment. They can last for 50 years, providing a high level of insulation and roof strength for the building beneath. Installing a new roof however is no simple task, and requires some serious construction and planning work.
If you want to install a steel roof over asphalt its recommended that a steel shingle system is used. These are less expensive than standing seam steel, which is a common alternative. With standing seam steel, single panels run vertically from the eave to the ridge of a roof. Steel shingles however use small panels installed in horizontal rows, creating a flat and straight surface which strengthens the roof against heavy winds.
If a roof has two layers it may require being completely removed in order to be replaced with steel shingles. Although this can vary depending on the roof, so it can help to ask a local building department if you are unsure.