Depends --
Is your county (or city) zoned? Subject to building permits, inspections and other government intrusions (or safeguards, depending on your point of view)? If so, part of the IBC prohibits any structure ever used for housing livestock from ever being used for human habitation. That might also apply to parts of the structure. If your intended use of the trusses is allowed, and engineer sealed drawings are required, said engineer would have to be willing to examine and certify the trusses. Since they're not new, the chances of tracking down the manufacturer to get the certifications are about nil. Maybe the trusses themselves are stamped - maybe not.
Given their heritage, in a permitted area, they may be allowed for a "shed" but not a "garage".
But, let's say best case your county is non-zoned, non-permitted outside of municipalities and your location is unincorporated. What I would do:
1: Set the trusses outdoors in a manner that all sides are accessible and spray liberally with Clorox (wear a mask!!). Let stand an hour or so and hose off. Let dry THOROUGHLY, don't stack.
2: Measure carefully and go to the WWW (wierd, wonderful web). Find the closest match you can for the size and configuration and look up the span. Different span/loads may be specified for different grades of lumber. Assume yours is the lowest grade.
3: Derate yours for the next lowest load/span and go for it. If they're rated at 24" OC, use them at 16" OC.
4: If they are the traditional chicken house trusses with clerestory windows, you've got a crucial decision to make. Point the windows South for maximum sunlight, point them North for minimum heat (insolation), point them into the prevailing winds for maximum ventilation (assuming they open) or point them away from the prevailing winds for hail protection.
Good luck, and keep us posted. Remember, no pix= it didn't happen!
