Copy a floor with the project viewer buttons. To access these functions you have to select the building from the tree structure in which the floor you want to copy exists. By selecting the building you get buttons to copy a floor above (the current) or below on the top of the project viewer window. The current floor is marked red.
With the buttons a "New floor ...." dialog apears in which you can make settings for the height and the thickness of ceilings etc.. All these settings can be changed at a later point as well.
Finally you have to define which elements of the current (the source) floor you want to copy to new one. The element selection is the same as in all other categories dialogs, e.g. visibilities of views.