Is the roof a floor?
The roof is neither a building nor a floor. You can build 3 tiles and when the 3rd one is finished with all cubes placed you get a roof.

This is covered on page 6 under the rules for finishing a tile:


  If the [finished] tile is on the 3rd floor, take the appropriate Roof tile (Small or Large), turn it over, place it on top and score +4 Reputation points.


Additionally, the example turn on page 8 illustrates this. The text and picture for step 4 shows a roof being added to a three-story building.