I think you will find that the building materials (or at least one of them) used in your wall composite are stronger that the building materials in your roof.
The roof needs to be stronger than the walls.
The greyed button you mention is I think for 'cropping' walls which is different to trimming walls.
Cropping is permanent and stays as is when you delete or adjust the roof.
Trimming is interactive so as you adjust the roof the wall will re-trim automatically.
Use the little blue 'connect' box (has 4 little squares in it) to cancel the trims - can be seen in plan or 3D (and elevations) when ever you select the wall (or roof).
Barry.
One of the forum moderators.
Versions 6.5 to 27
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