Floors and ceilings are defined by the rooms (doubleclick the room icon within the wall.)
For roofs:
- Either use one of the roofs from the download page.
- Or draw a wall the size of your roof, export it as .obj file, and re-import turning it to the correct angle.

More about roofs in Hansmex' tutorial: http://www.sweethome3d.com/slopingCeilingGuide.pdf

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