Roof:
The roof will be inserted on the current active floor, so you possibly need to set your 2nd floor as your current floor. However you must remember that the attic also has a floor, so create another floor and call it the Attic and place a roof on that.
Use the Building Wizard to quickly see what that creates for a normal 2 floor house + attic.
Wall Placement:
I prefer to set out the outte dimensions using guidelines before placing any walls.
This is
demonstrated in this video.
The tool is the numeric parallel guide line. There is a similat tool in the wall placement tools, which allows you to place walls at a distance without guidelines. Remember to use Ctrl + W to set what the walls reference point is.
