Constructional Carcassing Timber is generally used for structural purposes . C16/24, for example, can be used for construction projects such as studding or roof rafters AND EXTERNAL STRUCTURES . 47 x 125 is treated to UC3 INTERNAL AND EXTERNAL  USE. Ref WPA timber association. 

Product will require regular additional treatment if used externally for best service life.

UC2 = USE CLASS 2 - Internal use only, above ground & within building envelope. (Occasional risk of wetting).

UC3 = USE CLASS 3 - For external use but not in contact with the ground.

UC4 = USE CLASS 4 - External use suitable for contact with the ground.

5x2, five by two