 Am I able to sister the underlying boards with 2x4s or something to build up the base, or am I better off replacing all of the substructure along with the deck? So if it's 20 years old and it sounds like it's not pressure treated, and if the existing floor joists have decayed to the point where they won't even hold a screw or a nail, I think it's time to replace that deck structure and all. Because the clock is ticking now, and it's going to be potentially very unsafe in a very short period of time.