by Bruce Pooley » Sun Sep 03, 2006 3:15 pm

If any portion is exposed to direct precipitation, the code requires the glulam to be pressure preservatively treated (ppt). If the beam is under a roof and wind blown moisture can not contact the beam, ppt is not required.

See AITC Standard 104 for details showing these conditions.
https://www.aitc-glulam.org/shopcart...=AITC_104-2003

As for the strength difference between normal an ppt glulam, the code or design manuals do not indicate any strength difference. You would want to consider reductions in strength based on wet or dry conditions.