KoDA86:
Two Options:
1. The Roof's Building Material
Intersection Priority must be stronger than the Wall's.
2. Use
Solid Element Operation >
Subtraction with upward extrusion to remove the upper part of the Wall.
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
