These dips are notorious and are nationwide.Mostly in your older homes.I have seen them on homes from the 1890’s up to the 90’s and maybe newer.
Lots of things contribute to this structural anomaly.
Foundation settling,multiple layers exceeding the structures intended max weight load,multiple layers over shake roofs are also notorious for having truss/support issues…Improper bracing.,twisted or busted trusses maybe due to the home having had a tree or large branch fall on it.
It is not out of the normal for roof structures to be different measurements from rake to rake and in the center.
Meaning on a gable roof the rake on the left might be 20’ and the right might be 19’/10"
but the center of the roof is 19’.6".,so the center would be sunken on the ridge 6" from center rake to rake.
I have also seen this with dormers too.Steep roofs also have this problem.Generally from out dated structure reinforcement techniques.These occur mostly in the center of the roof.(Field)
These dips are generally simple to correct.I usually find the source of the problem then correct that.Then I will reset/reinforce the truss.Not finding the original culprit for the problem might have the same thing happen in the future.
Do I think it is a huge structural issue?.,Not a clue.It will depend on what actually caused the dip in the first place.But I can say.,the house is still standing. :badgrin: