We are moving forward with having a metal roof placed on top of existing shingle roof. Country Manor Shake was our selection for the look we were after.  This type of roof looks like a tile roof from a distance.  An additional protective layer called Titanium UDL-25 plus was laid over the existing shingles to not only provide a water barrier but also because of the expansion and contraction of metal, it ensures that holes are not created over a period of time.  Then the Country Manor Shake Shingles were screwed down on top of Titanium UDL-25+, according to code for the Treasure Coast city of Port Saint Lucie (9 stainless steel screws were used per 4′ panel).  The roof looks beautiful and it should outlast our lifetime.