Thanks Erwin, this is a direction. This makes the openings disappear, but my ridge lines too.
And I get additional double lines of the cutplane on top. Normally I have to show the ridge to the top and no virtual cutplane-lines. Wrong floor plan representation. Less worse than skylights, which are not part of the current sory, but no satisfying solution.
This shows a problem beside my skylight problem: I may not draw the complete overhead lines. But if the edge of a roof is not vertical, I get a whole bunch of overhead lines more or less parallel, which may not be there.
I can understand the problem of GS in their effort to derive the 2d representation out of the 3D model. But in case of roofs and skylights this does even work worse than the combination of walls and openings (reveal problem).
Some time ago I had to write a skylight object to calculate and draw level lines of the roof, the skylight was placed in.
Finally I had to implement a vectorial charset in 3D to label the lines out of the object itself in floor plan because TEXT2 does not work ind skylight objects. Very annoying. And there is no way (as far as I could determine) to check in the script the context in 2D, to be able to create a context sensitive 3D model. So the lines are visible in perspectives too! The home and the current story are not available in the object. Maybe thjere is a global I do not know, but I guess there isn't one.
This seems to me like a very unfinished end of development. And I remember, that GS promised to fix all this problems in future, when multiple roofs have been introduced.