My 80 acres was part of an original homestead. Dating back to 1890. My father purchased this property in 1939. I can remember, as a kid, the old barn and other outbuildings. They are just piles of scrap - out in the weeds - now.
Anyhow - I have a four stall carport. It has some 15 - 4x4 treated posts. Built in 1982 when we moved down here from Alaska. This structure has withstood the test of time.
However - one of the posts has been replaced two times. It will rot out. Finally I dug down about five and a half feet - put a foot and a half of pea gravel on the bottom and then a new post. It show no signs of rotting now after being there eleven years. My guess - had something to do with water and lack of adequate drainage at that spot.