Circular saw. But you really need to rip off some shingles and some sheathing to tie it in properly. If you just cut the eave off and tie the other to it, you will end up with a hump where the two are joined if all you do is tie in the shingles. Your sheathing boards or plywood or OSB, (whatever is on your roof) needs to be staggered all the way up. That is why you will see a lot of places with the addition dropped down below the existing roof line, so the don't have to mess with the existing roof.