If a deck is under a roof or other covering, do I still need to apply the wet service factor for the structural members?

The NDS requires the use of the CM factor for any member subject to moisture contents in excess of 19% for an extended period of time. If there is doubt regarding the moisture content, it is recommended to assume the worst conditions. Since an outdoor deck would be subject to moisture from rain, lawn sprinklers, etc., it would be appropriate to apply the factor.