As a natural material, wood exposed to the outdoors faces significant threats from
predators such as mold, decay fungi and insects like termites and wood borers. These
organisms seek to break down the wood fiber, reducing the structural soundness and
serviceability of the wood.
Preservative treating creates a chemical barrier that protects the wood from
these threats. The preservatives are not just on the surface, they are infused
deep into the wood to provide long-lasting protection. As a result, the service
life of the wood is extended from just a few years to as much as 50 years or more.
Click on the video to see how wood is preserved
The treating process utilizes pressure that forces the preservatives into the wood
fiber. While there are a variety of preservatives used today, the manufacturing
process is virtually the same for all.
Preparing the Wood
Preserved wood manufacturers start the process by purchasing kiln-dried lumber
from sawmills or specifically selected round poles from forest companies. To
meet national standards for use in applications where the wood will be in contact
with the ground, Western species such as Douglas fir and Hem-Fir must be incised
on all four sides prior to treating. Incising consists of making small incisions
in the wood to allow proper penetration and retention of the preservative. See
PreserveTech - Incising for more information.
The untreated wood is then loaded onto carts, which are moved into a specially made
cylinder called a retort. The retort, which can be up to 150 feet in length, is
sealed and a vacuum is applied to remove the air.
Next, the preservative solution is pumped from storage tanks, completely filling
the retort. Pressure is continuously applied within the retort to force the
preservative into the cells of the wood. Processing time depends on the wood
species, the size of the products treated and the preservative used, as well
as the standard for the intended exposure of the final product.
The preservative is drained from the retort and a vacuum is applied to return the
excess solution to storage tanks for future use. The carts are moved out of
the retort onto a sealed drip pad to reclaim any residual preservative on the wood.
Representative core samples are taken and analyzed to ensure that quality and
durability standards are met. Third-party inspection agencies also take core
samples during regular visits to confirm the plant's quality control practices.
The finished preserved wood products are typically covered with a top wrap and
moved to the plant's storage area for shipping.