The Challenges of Pressure-Treating Engineered Wood Products

"Treating engineered wood is sometimes possible—but it's important to understand when and how to do it. Today, industry expert Dennis McWhirter answers questions about which types of engineered wood can take pressure treatment, and which can't."

Jim Coshow


Treated lumber is a popular building material that plays a key role in the construction of many structures in the Pacific Northwest. Our good friend, Dennis McWhirter of Exterior Wood, is an expert on pressure-treated wood—with nearly three decades of experience in the industry. In this series, Dennis provides answers to some common questions and reminds us why treating lumber is one of the best things we can do to help preserve our greatest renewable resource.

In today’s episode, Dennis shares his insights into treating engineered wood.

What is engineered wood?

Engineered wood is a type of composite wood that is manufactured by binding several fibers, particles, or boards together with adhesives (think plywood, cross-laminated timber, etc.). One of the most popular kinds of engineered wood in construction is glulam (a glulam beam is manufactured from layers of wood laminations that are bonded together with industrial adhesives).

engineered wood

What are the concerns when pressure-treating engineered wood?

The most important thing to know before using pressure-treated engineered lumber is that sometimes the treatment process can void the manufacturer's warranty. For example, pressure-treating glulam beams can create the potential for fractures in the glue line. The cell structure of each piece of wood (lam) is different because each part of the tree is a little different. When you glue several pieces of lumber together, the pieces with bigger growth rings will absorb more of the treatment, causing them to swell at a different rate than pieces with tighter growth rings, creating a potential for the glue line between each piece of wood to fracture or break. Unfortunately, that means that hardly any manufacturer will honor the warranty on their glulam beams after pressure treatment.

pressure treated glulam beams

What types of engineered lumber can be treated?

Plywood and I-joists are some of the most popular engineered products we work with. One thing to be aware of with I-joists is that some are manufactured with a protective wax coating, which hinders their ability to absorb the chemical treatment.

joint wax coating

What types of pressure treatments can be applied to engineered wood?

We can treat engineered lumber with everything from copper azole (a preservative which can be used above-ground, in-ground, and in freshwater), to CCA in non-residential applications (chromated copper arsenate, another type of preservative that protects against termite and fungal damage), to Dricon (a fire retardant treatment). If you're not sure whether your engineered materials can be treated, the best thing to do is to send a sample to the treating facility so they can test the ability of the material to absorb and retain the treatment.

treated engineered lumber

We hope this video equips you with more confidence when working with pressure-treated engineered lumber. To learn more about treated lumber, check out the facts about building with treated, engineered lumber or how to maintain the lifetime warranty of pressure-treated lumber. And be sure to stay tuned for the rest of our series with Dennis.