A gable-end wall of my 1-story house forms the wall of two approx. 14' wide bedrooms; that wall in each bedroom has two 14' sheets of drywall applied horizontally (using screws, naturally). The studding is 2x6's. A hairline crack at the horizontal seam has appeared in both rooms. The seams were taped with fiberglass tape and mudded in the usual fashion.
I can't think of a mechanism which would cause a horizontal seam to open-it would have to require extension of the 2x6 studs, wouldn't it? I can also see it happening if the studs could bow inward sufficiently, but they're pretty stiff.
I think it may somehow be related to the fact that this wall faces the main wind direction (westerly), and the gable does not provide as much stiffness as a loadbearing wall on which the trusses sit--the wall may flex from windload?
Any ideas? No other wall has done this, but the other gable is in the garage and I used vertical sheets there because it was easier to do--also it is comparatively sheltered from wind. Besides, why would you get cracking in the garage where nobody cares?