Cathedral ceiling drywall sag?


We have a home in the Denver, Colorado area that was built about 30 years ago. I'm seeing what appears to be a drywall seam separating on the cathedral ceiling in our living room. I also see "dimples" above and below the seam that appear to be drywall nail/screws trying to pull through. They are along what appears to be a single joist or truss. The drywall appears to have been installed longways across the joists (trusses?). I don't see any evidence of water damage and the painted surface has not broken anywhere yet.
I am not sure how much space exists between the drywall and roof deck, but I'm guessing it is around 10" to allow for the insulation. Also, the ceiling pitch appears to be about the same as the roof pitch. The roof is shingled with wood shakes and there are vents installed about 2/3 of the way up the opposite or back side of the roof--not near the sagging area.
I'm wondering what kind of contractor I should contact to evaluate the situation. I don't want to assume it is simply a drywall problem and just call in the "drywall guy". Nor do I want to assume it is a roof problem and call in a roofer. I'm guessing a general contractor, but is there someone else that might be a better "fit"?
Thanks, Dale
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snipped-for-privacy@gmail.com wrote:

A forensic architect or engineer should be able to tell you what the problem is and give a comprehensive solution. The fee should insure that the work done would be coordinated and directed toward solving the problem. TB
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snipped-for-privacy@gmail.com wrote:

How high is the location? I'd try to at least get up there and see if it is just a piece of drywall hanging before I got too carried away. Might simply be one joist/rafter was off by an inch or so and they missed or nearly missed it originally and time is showing the effect of a poor connection.
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I can reach it with a step ladder. However, I'm not sure how I could tell if it is just a piece of drywall hanging without starting to tear into the drywall. Of course, I guess no matter what, I'm going to have to tear into it at some point, but I'm a bit nervous about doing it myself.
Dale
dpb wrote:

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