As far as I know you can only align the top surface of a roof.
I would model this as it is built.
Make the roof a thin plane for the surface and trim the wall to the underside of it.
Or maybe you could make the roof a composite if you want the thickness of the entire roof structure.
Then make sure the wall building materials are stronger than the roof building materials except for the very top surface (which needs to be the strongest of all) because you don't want the wall to extend through the roof.
Then 'Connect' and trim the wall to the roof.
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