You could use two Walls: one would be the above and outside of Roof portion, the other the inside and under the Roof portion. You would have to trim both to the Roof and they may cause display issues in the Floor Plan because they are on top of each other.
So, instead of that, I would just place a Morph in front of the Wall. I would go to the Section you posted and create a Morph there, make it, let us say, 10 mm thick. It will be created in the vertical plane of the Section. Then you can move the Morph in 3D to its proper position in from of the Wall, and modify it as needed.
Not the best solution as you would have to update the Morph if the Wall geometry changes or moves, but achieves the purpose.
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