How much sag on a roof is acceptable?
Courtesy of @lsiunsuex in chat: http://www.tecotested.com/techtips/pdf/tt_plywooddesigncapacities

Max Live Load Deflection = span/240,
Max Total Deflection = span/180.

So for a 24 inch span, the deflection should be &lt;  ~1/8 of an inch. (2.1333 sixteenths)