Add a coincident constraint to the gable eaves of the crossing roof
Fix the gable eaves to each other using a coincident constraint.
- Select
Constraints > 
Coincidence. - Select the line of the roof (A).
- Select the line of the roof (B).
It is not necessary to fix the eaves on the other end with a coincident constraint, as the crossing roof will later be cut with the slope of the main roof.
