Add coincident constraints
Fix the edges of the slopes to the walls and to each other using coincident constraints.
- Select
Constraints > 
Coincidence. - Select the exterior surface line of the wall (A).
- Select the line of the roof (B).
- Select the exterior surface line of the wall (C).
- Select the line of the roof (D).
- Select the line of the roof (E).
- Select the line of the roof (F).
- Quit by pressing the Esc key.
