The sagging is likely caused by the original builder building "to Code". The Building Code allowed 3/8 OSB sheathing if clips were placed mid span. In my experience, 3/8 OSB is just not a good sheathing for exactly the reason to are seeing. I prefer a plywood sheathing 1/2 thick. Its a little overkill, but 25 years and my roof is flat while my neightbors all have some amount of sag.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags