Wood members that are protected from direct moisture in building construction reach an equilibrium moisture content below 19%, which is considered dry service conditions. Design values assigned to lumber and engineered wood products in the AWC National Design Specification® for Wood Construction (ANSI/AWC NDS®) and the NDS Supplement are based on the assumption of dry service conditions. When wood structural members will be regularly exposed to rain or other sources of direct water, an adjustment for wet service conditions, the “Wet Service Factor,” must be applied to the design values because the presence of water affects the moisture content and strength properties of the wood. The Wet Service Factor is not the only adjustment factor that designers consider and use for specific service conditions--others include a Load Duration Factor, Temperature Factor, Flat Use Factor, and Incising Factor, just to name a few.
The new span tables for residential wood decks in the 2018 IRC, and similar tables in AWC’s Design for Code Acceptance No. 6 – Prescriptive Residential Wood Deck Construction Guide (AWC DCA-6), are based on the assumption of wet service conditions. That is, the Wet Service Factor has already been applied in the development of the tables, for conservatism. The designer, in consultation with the code official, has final responsibility for deciding whether the deck should be considered a wet service or dry service application.
View-only downloads of the AWC NDS and Supplement are available here: http://www.awc.org/codes-standards/publications. In addition, AWC also has a free span calculator for determining allowable joist spans with an option to include the wet service factor: http://www.awc.org/codes-standards/calculators-software