We have demolished most of the original bathroom and are a little confused about how to continue. We had a 60" shower pan, but have reduced the new one to a 48" pan. As a consequence, we have to reframe new front and back walls for the shower stall. The previous ceiling for the shower is still there. The problem lies in the framing for the shower stall walls. The existing upper plates on the front and back walls of the shower are not securely attached to any joists in the ceiling (ie. joists that exist for the upstairs floor), but are merely attached by nails into an existing stud. Because of that, the other end of the plate is merely hanging in the air, now that the wall below it has been removed. So, it was the studs below that supported the upper plate.
Our question is: How do we put in a new upper plate when there is no joist handy to which to attach the 2x4?