In the beginning of the 21st century, (December 2003) the chemicals in pressure-treated lumber changed. The wood preservative manufacturers—the chemical companies—worked with EPA to voluntarily restrict the arsenical chemicals. This is part of an overall initiative to reduce the amount of arsenic exposure in the environment, due in no small part to the fact that arsenic is a known human carcinogen. CCA-treated lumber is still approved for industrial purposes.
New growth vs old growth. Old growth wood is wood grown naturally in a virgin forest and not a lumber farm. To classify as old growth wood, the wood must contain heartwood which forms after at least 50 years of growth.
Old growth wood is far superior to new growth wood. Comparatively, new growth wood reaches maturity in 10 to 20 years. The goal with new wood is to grow the wood as quickly as possible. For construction, you now need larger wood sizes to achieve the same level of structural soundness due to the lower density.
The biggest benefit of old growth wood is the higher density. Older trees that grow naturally develop tighter growth rings which creates more robust lumber. The tighter growth rings allow the wood to have greater capacities. Also, old growth wood has natural rot-resistant properties and more firmness, meaning it doesn’t shrink and expand as much as new growth wood.
Example of how 4x4 posts are effected by the environment. All the posts on the South side of the fence shrank rapidly from exposure to sunlight, while the North end was protected by the shade of a nearby tree. This level of movement occured over a time period of 6 months.
Due to the pressure process treated wood is shipped and sold with a very high moisture content. As moisture leaves the wood it can cause shrinking. If the wood dries unevenly the drier areas will contract and warp the wood permanently. New growths lower density makes it more prone to warping as the shrinkable area between the grain structure is spread over a wider area. The lower density paired with unpredictable evaporation of moisture makes traditional treated wood posts inferior to modern alternatives like steel posts with a price point on average of only few dollars more when purchased by a contractor at wholesale pricing.