All framing timber is stress graded and tanalised. Usually usage class 3 but posts which might be subjected to standing water could be usage class 4. Deck planks are also tanalised, and guaranteed against rot and insect infestation for 15 years. By default, I use the “York” profile by “Q-Deck” which is the thickest at 33mm, although I can supply and fit any of the profiles from the Q-Deck range.
Posted in: 2. Questions about deck construction