As explained in the help, Sweet Home 3D computes slopping ceilings only at the last level, mainly for backward compatibility reasons and because this feature didn't work that well for complicated ceiling shapes. In your case, I suggest you to create a room for the floor part where you'll hide the ceiling in its room modification dialog, and a smaller room for the ceiling part for which you'll hide the floor. For the slopping part, you can add a roof 3D model, or import a box and orientate it precisely at the second step of the furniture import wizard.