This may have been caused by any number of factors.
Sagging roof ridge beam.
If the beam is not properly nailed it can also pull loose and this contributes to.
Straighten out your old house s sagging roof and get rid of those cracks in the wall with these easy to follow steps.
The weight of the roof will create pressure and then the hangers may pull away from their beam causing the walls on the exterior to come outwards to cause a visibly sagging roof line on the top of your roof.
Check the ceiling for sagging by attaching a string on one end of the beam and run it along the bottom edge of the beam.
Step 1 the first step is to the basement first.
The beam is attached to both exterior walls and to the bottom of each sloping rafter to give the roof strength.