Thank you @barthbradley and @PijPiwo for answering my question and the file. I am using revit 2016, so I am not able to open it.
Our wall is 42,5cm porotherm block. The slanted part will be out of 24 cm porotherm and styropor insulation as a slant. The opening would be made just for the window frame but the sill would be lengthened to the edge of the slant.
We have same 200 windows in the project, to model each and every in the plan would be a nightmare, and to be honest I don't know any method to create a wall using a wall tool with a slant as given. I thought there is a way to model is as a component maybe?
I will go through your suggestions but I don't know if I really understand what you mean by void as a solid.
I tried for few hours to cut of this part with the void but I am getting this message first: "the void is to thin" and then "openings and cuts not possible in the same basic component". Which shows up as an orange line on the picture. The problem disappears only when I am deleting the opening but the same time the void cuts trough my sill.
And the thing is I am editing the window family I have not build it from scratch as it has to many parameters that are necessary for the project. And my new parameters are very simple the width of the slant is always 70cm and the high same as a window high.
Thank you for your help.