Pressure treated in-ground shrinkage

[Ronald Blue]

Here's a permanent solution if available there. My shop building is built with the pole barn version.
https://permacolumn.com/products/deck-posts.html

I suppose you could set these in concrete also but I certainly wouldn't. I am a firm believer in gravel in the bottom of the hole and using gravel to set the post. If one hasn't set a post using gravel as the filler I highly recommend you try it. There is no faster way and it's very solid. Just wiggle the post as you add the gravel. Make sure it's plumb and square because it will be to solid to move if you don't have it where it needs to be.

[John Goodin]

I have never lived in a place where ground froze but here most 4x4s last fifteen or twenty years in concrete and everyone uses concrete for fence posts. However, many people are replacing 4x4s or landscape timber posts with 2 1/2” metal posts. I just did that and I am pretty sure the metal ones will outlast me.

[Maurice Mcmurry]

The critical area is at ground level and the first foot of underground. I buy the highest retention treated I can find. Retention continues to vary from store to store and even day to day. Back when you could get CCA .60 a treated 4x4 was a great thing for lasting. They were not so good for the environment. I try to let posts dry as long as posable and then brush on several coats of Jasco or Woodlife copper green. I have used the plastic post savers. I will pull one out in a few years and see what's going on. I also use peel and stick bituminous rolled roofing and stainless steel staples to wrap posts.