I'm trying to make the floor in my attic a loft for an extra bedroom. I was told I should sister the joists for support. The problem is, the attic ceiling is vaulted, so I can't get the new joists above the existing plate on both sides. There's not enough room to hoist them up and get them into place.

Is there a solution for this? How can I get the sister joints into place on both sides without cutting a hole in the roof?