Baz on our retirement house there was one layer of standard 3 tab shingles when we bought it last year . We had no leaks and the shingles were laying flat ( not curled ) . I wanted to replace and wanted metal . Had a local metal roof manufacturer come out and give a quote . They recommended to leave the shingles on as an extra layer of protection , this also helped with the overall price . The installers went with a membrane over the shingles which adds another layer of protection AND protects the back of the metal panels from the grains on the shingles .
Install went perfect and we have had many aggressive rain storms and no issues to date . Went with a light gray which helps with heat build up . Full length ridge vents just like what was on the house before . I'm 60 years old and I figure this is the last roof I will have to deal with
. Also have a metal roof on the pole barn/man cave/two car detached garage . I just like the way they look/perform and in our country setting it adds to the look of the house . Another benefit for me is we have lots of woods/forest around us and IF we ever get raging wild fires a metal roof is more forgiving in regards to igniting . Like anything else a good install makes or breaks the job .