@dorin, first of all, thanks for the answer.
I probably make a mistake, I still don't manage to activate the roof generator.

I did the following:
1. 2nd floor selected
2. All walls marked
3. -> Tools -> Autodimension
4. Roofenerator is still gray ??

Would it be possible to explain the step by step to me?
I hope it is not too much.

Regards Klaus