Setting fence posts in concrete/ bad ????

[eee]

I have heard both pro's and con's of setting a fence post in concrete causing a fence post to rot prematurely. Would this not have the same effect on concrete posts on pole barns and decks or porches being set in concrete ?

 

[shvl73]

I think they mostly are set on concrete pads, not in concrete, thus the difference. I could easily be wrong though.

 

[sendero]

I dunno, I think just the opposite.

First off, I'd never use anything but a pressure-treated post. I pour a bit of concrete in the bottom of the hole before I put the post in, "sealing" the post from the dirt. At the top of the posthole, I build up the concrete into a shallow cone shape just above grade to drain water away from the post.

This is how I was taught, guess I'll find out if it works in about 25 years, hopefully /forums/images/graemlins/smile.gif

 

[Cliff_Johns]

Basic idea is that the wood will shrink slightly away from it's concrete sleeve and some water will get in there -- can't avoid that really. Once the water is in there, there isn't any way for it to leave, so the wood slowly rots. Presure treated will help, but it doesn't do the fibers any good to be wet all the time.

Still, I guess it dependes on what stresses are put on a particular post. The posts at gates and the like may be a different issue, but I would make sure the post could get dry, that is, that it could drain.

Cliff

 

[Dargo]

I was told just the opposite by some fence builders and pole barn builders. They said to never put concrete in the bottom. They say to use some gravel in the bottom to get the post to the desired grade, then use concrete around the post. The idea is that the gravel in the bottom will allow moisture to flow away and off of the bottom of the post. With the concrete bottom, they say you will trap moisture against the post permanently and cause premature rotting.

However, I used 9 6X6 treated posts for my dock in my lake. They have been completely submerged in water 100% of the time for over 12 years now, and they are as solid as the day I put them in. When the water was down this last summer, I used my pocket knife to scratch a spot in one of the posts to see if it was getting soft. About 1/16" into the post, it still looked like new. Again, that's after 12 years of being completely under water.

 

[MikePA]

<font color="blue"> They have been completely submerged in water 100% of the time for over 12 years now, and they are as solid as the day I put them in. </font>
I was always under the impression that it was the wet/dry cycle not just getting wet and staying wet, i.e., why fence posts rot most frequently right at ground level. There was a story on one of the educational channels about a guy who made a living of retrieving logs that have been submerged in rivers for over a century. Some of, if not the only, old growth timber left.

Re: concrete cookies. I believe they are used to spread the load of the building over a larger surface area and not to keep the post off the ground, although they do that as well.

 

[beenthere]

Wood in posts submerged underwater will not decay because there is no oxygen. But wood in posts that have both water and oxygen will decay. Problem with many treated posts is that the heartwood does not take treatment, so many posts are only surface treated (the sapwood takes the treatment, and the heartwood just has to survive on some natural resistance to decay).

Posts in concrete will decay faster than those not in concrete.
Groundline decay is usually what happens first. Above and below that groundline area will last longer, but the weakest link is at the groundline. Posts in a pole shed usually are drier because they are under roof and likely in ground that is well drained (animal barns may be an exception to this).

Posts in water will decay fastest at the water line, where there is both oxygen and water. Wood can be protected from decay by submerging it in water. In coastal areas, there are marine borers (toredos) that will bore into wood and destroy it, something like termites, and again mostly at the line between high and low tide. The borers don't survive in fresh water.

There are all kinds of ideas on how to do it the best. Some like concrete and will do it that way. Getting a rotten decayed post out of concrete is not fun, because the decay is right there where you want it to hold together. /forums/images/graemlins/wink.gif

 

[Freds]

Yes roughcut, it would have the same effect, both pros and cons. If it will affect one, it will affect the other.
I've heard arguements both ways on this subject with both sides having plenty of experience and their projects still standing. I built a deck and a porch this past fall and the new inspector our township has wanted the posts set on a solid concrete block in the hole and either dry quikrete added or mixed and poured.
If I was going to set any more fenceposts I would pour dry mix into the hole. It seems no matter how much I pack the dirt around the posts, four months of wind and rain always has me repacking the dirt again. You just don't have that problem if you fill the hole halfway with quikrete.

 

[ThistleKop]

<font color="blue"> Once the water is in there, there isn't any way for it to leave, so the wood slowly rots. </font>

Cliff,
I reckon wood that is submerged in water will not rot until it's exposed to the air(oxygen). Wonder how this concept fits in this discussion?

TK

 

[ThistleKop]

<font color="blue"> You just don't have that problem if you fill the hole halfway with quikrete.
</font>

Freds,
That's the method I used on posts for 50 acres of pasture in 1991. They are still solid after 13 yrs. and I'm expecting many more yrs.

TK