I know this isn't timberframing, but I also know y'all have a lot of general knowledge.

I have a friend who is looking to build a new 8/12 roof over an existing 4/12, then removing the lower roof to reveal the higher. His family is living in the house and he is hoping to keep a leakfree roof overhead at all times. Is this feasible, and if not, how can he minimize the unroofed exposure time?