Help! How to delete single floor tile?

I don't have the definitive answer, but I figured out how to fix the open area around a staircase. Place your railing (fence) where you want it first, and then use the sledgehammer tool to delete the whole "room" of flooring. The fence will act as a wall and put a stopper to the amount of floor being removed.

Along this lines, I can't figure out how to place a floor (whole or tile) after it's been removed from any non-ground-level floor. The older games simply would focus on the grid and place flooring there. This doesn't seem to happen for me. This discussion would likely be another forum thread.