Rok:
There is no simple way to do this, the easiest solution is to have two walls, one below the roof and another above. There are ways using SEO where the target (wall) picks up the Surface (Material) of the operator, but this is a more complex solution.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David