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 C_M 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.

This FAQ is based on the presentation of BCD303 – Design for Code Acceptance No. 6 – Prescriptive Residential Deck Construction Guide.

Also, see Page 12 of Part II: Design Values for Structural Members for background on the Wet Service Factor.