Pressure Treated Fence Posts Failure
The wood fence surrounding my property was built 18 years ago, at the time it was built, the contractor told me he used pressure treated lumber and gave me the impression the fence would last for decades. I definitely thought the posts would never rot or be chewed up by insects. When it began to fail after about 12 years, I was very disappointed to say the least. The first post that failed we blamed
on a vehicle bumping the post, but then over the next few years several more posts began to lean precariously. I now know that I did not ask the contractor the right questions and made several erroneous assumptions about what he did tell me. I should have asked, what does resistant mean, what insects are we talking about, a few questions about moisture and fungus, etc; hind sight is 20-20.
One of the first lessons I learned was that carpenter bees love to drill big holes and tunnels in pressure treated lumber.
In an attempt to extend the life of the fence, we drove a metal fence post down into the ground next to the wood post and secured the metal post to the wood posts with long wood screws. The first post repair went very well, but work on the second post quickly came to a halt. The metal post hit something and couldn't be driven into the ground far enough to be of any value. Then in trying to extract the metal post, we ended up pulling the entire three feet of the old wood post out of the ground. To our dismay, the bottom of the post was in new condition with no sign of rot or insect damage.
The only part of the post that was rotten was right at and a few inches below ground level. The post above the ground appeared fine. As you can see in the photo, the post is in good shape below the rotted area. About a foot of the post at ground level had rotted. I 'googled' this problem and learned that wood needs 4 things to decay: water, oxygen, food (wood) and favorable temperature (40F - 105F). Wood can be too wet to decay. Waterlogged wood will not allow oxygen in to support the growth of fungi. Marine pilings kept fully submerged may never rot. And wood can be too dry to decay. Keep wood below 22% MC and you are generally safe. This rot is worse near the surface of the ground since that is where the maximum moisture and oxygen exist ... clearly this was our problem.
The following explanation of the mechanics of decay is from the PostSaverUSA site.
The area 2'' above the ground to the top of the post is exposed to sunlight and good air circulation. This is a condition suitable for long life. However, at this level, the type of wood used is subject to its own natural aging process.
The area 2'' above the ground to 6'' through 16'' below ground level is the most vulnerable to decay. At ground-level, high moisture content in the post is combined with both oxygen and soil nutrients. It is in this area that the post is most susceptible to decay.
The area 6'' through 16'' below the ground to the base of the post. This area is the least susceptible to deterioration even though moisture levels can be very high. Oxygen levels are restricted at this depth, naturally resisting deterioration of the post.
It seems there is much misinformation, and, in some cases, disinformation concerning pressure-treated wood, its maintenance requirements, and its safety in common use.
Since my fence was installed, the chemical treatment for wood has changed, but the same problem still exists. I found several solutions on the internet to work around this problem of posts rotting at grade level.
Post Protector II
For more information on pressure treated lumber see Wood Myths a publication of Building Materials and Wood Technology; 126 Holdsworth Natural Resources Center;
University of Massachusetts, Amherst, MA.