How should I build the second floor?

[FFATE]

I have already built the four walls and the floor of the first floor, and I also built the stairs. However, no matter what I do, I can't place the floor for the second floor,it always appears on the ground of the first floor.

[mirta000]

So, the first thing that happens, is the game applies a ceiling on the 2nd floor. There should be like a "stairs up" button. Put your floor on top of the ceiling there.

[Nullifier]

Go into Build Mode > Use the Room Selection Tool / Create Surface (R) tool (It's the little cursor icon with a square around it. > Select your room that you already have built on the first floor > raise the height using the controls > move the room a store above the previously built