Compare these two instructions from the Green Grocers and Pet Shop modular building and you may notice (other than the background colour) that the top of the walls of the buildings are both arranged differently:
Tiles are raised at certain lengths of the wall to ensure the floor above will stay in place.
The floor above is secure using the relatively new 92593 modified plate. The new plates occasionally 'lock' into the floor above when it's removed, leaving an impression of where the plate once was. However few pieces are required.
I'm building a multi-story police station with a large area (32X62) and I'm looking for a suitable way of connecting floors with as few bricks as possible.