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It sounds like you have termites and they have eaten the lumber connections that hold the roof rafters to the floor joists. As these connections weaken, the roof starts to spread away from the walls. Sometimes the termites attack the center of the floor joist and the roof spreads taking the outside walls with them. In both instances you can see a definite sag in the(usually) center of the ridge line.
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General C&ontractor
35 years of troubleshooting construction, remodel, component & material failures. What to do next