Fix the edges to the wall lines in the reference drawing
- Select
Constraints > 
Coincidence. - Zoom close to the intersection of garage's interior wall and exterior wall.
- Select the edge line (A) of the ceiling and the wall line (B).
- Select the edge line (C) of the ceiling and the wall line (D).
- Select the edge line (E) of the ceiling and the wall line (F).
- Select the edge line (G) of the ceiling and the wall line (H).
