I do these frequently. In section I draw in polysolids for ceiling and fascia elements. I draw one side and then mirror. I don't model roof framing so this cheat is just to minimize time spent to get the needed geometry. No arm wrestling the program or head scratching required.
Where in the world are you building Homes in Milimeters? I am used to using mm when I design for 3D printing stuff. Is the only way to go for small stuff.
imodel's post in Open roof over a deck was marked as the answer