Quote:
Originally Posted by natethebrown
Seeing this lead me to believe subsoil contact and concrete encasement don't matter. The real issue is top soil contact for the wooden posts.
Here is what I did to install my new fence:
1) Auger-ed out ~24" deep holes.
2) Placed the post into the hole.
3) Poured concrete around the posts, but still keeping the base firm against the bottom of the hole and maintaining direct contact with the subsoil. I read somewhere that this helps the post dry out.
4) Mounded the concrete up around the base of the post to keep as much soil and water away.
5) Painted the bottom 6" of every post with Roofing Tar, sealing it off from any potential top soil contact (which I already have even after step 4).
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Subsoil wood contact with concrete matters (sort of). Wood will only rot in the presence of oxygen, and the further you are in the soil the lower the oxygen content, especially when you have an effective air seal around the post. In other words wood rot is most likely to occur in the first 1' or so of soil anyway. Creating an air barrier moves the rot to where there is ready moisture and oxygen. The contact with the concrete allows capillary action to happen. If the soil stays wet enough you would still see rot where air can get to the wood.
For maximum life of a fence post you would ideally wrap it in a moisture and air barrier plastic, that goes above the soil and under a metal collar (galvanized steel, stainless steel, aluminum etc...) that goes about 4-6" above grade. The Post hole would be cone shaped (larger at the bottom than top) to prevent frost heave and likely filled with concrete.