Take a look at this tutorial:
http://www.sweethome3d.com/slopingCeilingGuide.pdf
In rough, you have to compose (or re-import) the object vertically, give the inclination as a rotation over Z axis (which is the only allowed in the plan editing), then export the rotated object and re-import it again rotating it so it's horizontal again.
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If a long walk every day were healthy, mailmen would be immortal.
