Treated lumber is a popular building material that plays a key role in the construction of many of our structures in the Pacific Northwest. In this series, our good friend and expert in pressure treated wood, Dennis McWhirter of Exterior Wood, answers common questions about treated wood.

Treated wood: a local, sustainable choice

Using treated lumber is one of the best things we can do to help preserve our greatest renewable resource: wood. When lumber is treated, it’s more resistant to water, fungal, and insect damage. This extends the life of the lumber and reduces the amount of trees that would need to be cut down to replace wood structures that experience rot.

The manufacturing process for pressure-treated wood

The majority of lumber that gets treated is “2x” in thickness, and that typically is available kiln-dried from the mills. Kiln-dried lumber can be treated right away. Bigger timbers (4x, 6x, etc.) almost always start off as “green” lumber (not KD). This lumber is put onto “sticks” so it can air-dry, helping to lower the level of moisture to around 25 to 30 percent moisture content before it’s treated. It’s important for the moisture content of the wood to be within this specific range before moisture can but put back into it.

The wood is then placed on a breakdown hoist and run through a lumber incisor, which prepares the wood for treatment by cutting hundreds of small holes in the surface. This is where the little marks seen on pressure-treated lumber comes from. After the incisor, the wood is stained with a clay-based stain. Next, the wood is inspected and graded. Each board is flipped during inspection, and up to 17 percent of the boards are removed. The lumber is then repackaged and transferred to the treatment pad for pressure-treating. During pressure treatment, the lumber is placed into a tube and treated with 160 pounds of pressure. The pressure-treated lumber then sits on the pad until it stops dripping. Once dry, the lumber is shipped to the distribution center where it becomes available for purchase.

Interested in learning more about pressure-treated wood? Check out the first part of our series on how pressure-treated wood is made. There's more to come—stay tuned for the rest of this series, as Dennis answers all your pressure-treated wood questions and more.