Check the shingles to ensure that they are still firmly installed. Check for loose or damaged shingles. If the loose or damaged shingles occur in an area less than five square feet, you won't need to remove the existing shingles. Another common problem with roof shingles is that they will curl up or develop frayed edges. If your shingles are worn and frayed to the point that voids exist in the roof, then you will need to remove the shingles.
If you discover damaged shingles in an area less than five feet square, then repair the damaged shingles. Just tear up the damaged area and replace the shingles. It isn't necessary to color match the shingles. The point is to replace the shingles so that the new metal will have a level surface on which to sit.
Remove the entire roof of shingles if the area of damage is greater than a five-square-foot area. Use a shovel to scrape the old shingles away. Cover the plywood decking with felt paper to protect the wood until you install the metal. Make sure that all nails are removed before you install the felt and metal.
Metal Over Shingles
Cover the old shingles with vapor barrier to prevent moisture buildup beneath the new metal. Moisture can build up beneath a metal roof when it is exposed to freezing temperatures and direct sunlight in the same day. Freezing temperatures on the surface of the metal can also come in contact with warmer air from within the attic that is just on the other side of the roof decking, thus causing moisture as well.