They make a steel SHINGLE roofing that goes over existing roofing. Neighbor has it on his home. Looks somewhat like conventional shingles. Is applied somewhat the same way.... Really looks pretty good, not like an old barn roof on the house. He installed it himself with apparently minimum trouble. If I remember right, a 50 year warranty.
Now, a note about warranties - got this from a long time roofing contractor. Apparently many roofing manufacturers sell out, reorganize every few years. Change brand names. That way, when you have warranty issues a few years down the pike, they"re no longer around, and there"s no one to back the warranty.