The roof of my chalet slopes at 45 degrees.  I am adding a mud room.  My roof is 2×10’s at 24″ centers, sheathed with 5/8″ T &G plywood, shingled with 25 year asphalt shingles.  My new rafters, at 22.5 degrees, will be at 16″ centers, attached 8 feet above the eaves.  I am planning to strip the shingles and place a 16″ wide, 3/4″ plywood as a base to attach the new rafters to.  Do I really need to remove the shingles, or can I just nail the 3/4″ plywood onto the shingles and affix the rafters to it?