We have removed some interior non-load bearing walls (old closet walls) and opened up our bathroom floor plan. Now we would like to construct new walls for the stand up shower for which we have purchased a premade shower pan and intend to tile the walls. We have already exposed all the studs and most of the ceiling. The current walls are framed with two 2x4's running horizontally on top, attached to the joists or joists braces, (see picture below) and one 2x4 on the bottom running horizontally attached to the subfloor (plywood/ 2nd story) with studs running vertically 16 inches apart in between.
My question is this: Why are there two 2x4's on the top of the frame? Is it because there needs to be a lam beam there or because the studs are too short to reach the ceiling joists, or something else?
I have posted a picture showing an earlier stage of this remodel (before we took the old fiberglass shower out). You can see in the picture that the wall that contains the plumbing is framed out as I have described. We have since torn this wall down, removed the shower, moved the plumbing and waste pipes, and would now like to reconstruct the wall. We figured we would just copy the way the walls are already but I would really like to know the reasoning behind the two 2x4's on top and if it is really necessary. Along a similar line of questions: I noticed at the store there were two different lengths of studs (96" and 92 1/2"). The shorter one was cheaper. Why the differences? The people at the store had no clue.
It is just part of normal construction. Bearing walls need the double plate for weight distribution, and locking the walls together. For the most part it is easier to construct the non-bearing walls the same height where they can tied together with the other walls in the same fashion.