Create a structural floor by selecting support framing, walls, or by sketching the extent of the floor.
 (Structural Floor).
 (Structural Floor). Where is it?
 Build panel
Build panel Floor drop-down
Floor drop-down
 (Floor: Structural)
 (Floor: Structural)  Structure panel
Structure panel Floor drop-down
Floor drop-down
 (Floor: Structural)
 (Floor: Structural)  (Boundary Line).
 (Boundary Line).  (Pick Walls) and select the bounding walls.
 (Pick Walls) and select the bounding walls.  Span Direction, and click the desired edge or line.
 Span Direction, and click the desired edge or line. 
 (Finish Edit Mode).
 (Finish Edit Mode).