If you want to create a sloping roof, you have to draw it in two parts.
1 - the horizontal part,
2 - the sloping part.

Strictly speaking, in this example you are drawing a ceiling, not a roof.
Floors and ceilings are drawn at the same time.
Ceilings can only be seen
- from below - using the virtual visitor
- from above - using the virtual visitor and making a photo.

Hans