I have a small cabin in a camper community and the camp ground handyman is suggesting that I install a metal roof over the existing shingles. The cabin desperately needs a new roof and there are two or three shingle layers on the property currently. My question: Is it ok to install a metal roof over the existing asphalt shingles? If yes, should some type of underlay material be used between the existing roof and the new metal one? Just making sure this guy does a proper install for me, so any and all suggestions are appreciated. Thanks.