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Not all wood is the same; even 4x4s that come from the same mill and have the same treatment can have dramatically different life spans when put into a demanding application such as holding up your privacy fence. There are two main factors that will determine the life of your post: the strength of the wood and how well it accepts the pressure treatment chemical.
Of course, an untreated oak post will last longer than an untreated pine post, but we are not talking about the species of wood here. When you walk up to the stack of lumber, which post do you pick out? First, eliminate any posts that have excessive knots – especially if they have included bark. The knots lower the lateral strength of the wood and the included bark diminish the integrity by allowing easy access for moisture and insects to the interior of the post.
Most importantly check the end grain of the post. You want to select rings that are close together. The denser the rings, the stronger the post and the longer it will last. However, you want to avoid the center of the tree – it contains the pith, which is the youngest wood in the tree that has two big disadvantages. The pith does not accept the pressure treatment well and will rot earlier. The pith is also weak wood, its job is to grow vertically very quickly and be flexible, it is the heartwood that provides the strength.
It is common for many of the 4×4 posts to be made from ‘peeler’ cores where the mill will turn the log to make plywood veneers until it gets too small. The peeler cores contain the pith, which we just learned is the weakest part of the wood that doesn’t accept the pressure treatment chemical.
Examine the picture – both of the top posts are peeler cores and should be avoided. The bottom right post is a little better, but you can tell that is still close to the center of the tree. The bottom left post is the best of the group, it is from farther out from the center of the tree away from the pith. Generally, the flatter rings that are spaced close together will give the strongest post and the best value for your money.
What about weight? Won’t the heaviest post be the strongest? The pattern is for pressure treated lumber to be sold very wet, almost to the point of dripping. Most of the weight will be water weight. If you buy a wet post and then let it dry for a year, you will find that it will only weigh 20-40% compared to when you purchased the wood.