While you can't save the actual roof, we did save the plywood sheathing and the trusses. That saved us over a thousand dollars. It was a pain but worth it. We had to build half the second story floor then move the trusses from the top of the garage wall (where we left them after tear down) onto the new floor, then finish the rest of the floor. Started the wall framing (half), fliped the trusses up, then finished the rest of the wall framing. We also had to cut into the concrete floor in the garage and add two posts to carry the extra weight load of the addition. It took us 3 months to complete and I cleaned drywall dust out of the rest of the house for months after. It cost us $5K to build from start to finish. Some friends of ours were building an addition at the same time, ground level, hired a contractor and it cost them $15K. (built in 1978)
On another note, I hope that bc knows I did not post that response to his/her post, the one just above this with my name attached. I have not and would not belittle anyone here for their comments, good or bad and bc had some valid points.