Leaking Roof-Roofer Offers Metal Roof Over Existing Shingles?

[John Thedford]

Got that call we all love...
"Hello John, there is water coming in all over the house. The ceiling is falling. There is water rolling down the walls. it has only been going on a couple of days so..I decided to call you". 

I met a roofer yesterday. He states the roof has more years left. The subject of putting a metal roof over an existing shingle roof was discussed. There appear to be some sagging areas and I wonder if anyone has thoughts on NOT replacing decking, etc and just going over some questionable areas. Would you do a complete tear off, fix any decking that needs replacement, and then put on a metal roof? I have never had a property with a metal roof. This potentially could be my first.