Roof trusses left out in rain, are they still good?
I believe this article should give you some answers:

http://www.sbcmag.info/article/2012/truss-storage

In short, it all depends on how the trusses look after the time. The article says that they may be left out for a week max and then should be covered, but any time after that just comes down to what the current weather conditions have done to them.