Curved windows, or arched windows: read and follow the links in this thread

If you need horizontal window, you may follow these steps
- choose the window you want to insert
- export it, then import it again
- while importing, rotate the window 90 degrees to make it horizontal
- while importing, make sure to check Staircase
- you can now insert the window in a floor/ceiling, and it will make a cut-out, just as windows in walls do

Another way to do this:
- draw a wall that has the size and shape of your roof/ceiling
- insert the window you need and make sure it is in the correct place
- export wall and window
- import wall and window and rotate it 90 degrees

If this is not the correct answer, share a picture of what it is that you need.

H