Pressure-Treated Plywood

Pressure-treated wood is the wood of choice when building things that need to be protected from the elements, moisture, insects, rot, and decay, but for projects where paint, stain, or other protective finishes can’t be used.

When we talk about “pressure-treated wood products,” we’re talking about a whole family of lumber products, including dimensional lumber, timbers, railings, and plywood.

Many people like using pressure-treated lumber and wood for making decks, outdoor furniture, fences, retaining walls, decking, and other outdoor backyard projects.

The long life and low maintenance you can get out of pressure-treated lumber and wood products make it an ideal choice when building your retaining walls, fence projects, decking projects, or building projects that will remain outdoors.

Pressure-treated softwood plywood is as durable and valuable as pine and other pressure-treated wood products, even though you don’t encounter it often.

Yet there are many outdoor projects, like garden sheds, where pressure-treated hardwood plywood is an ideal choice because it is durable and doesn’t have to be painted or treated.

The pressure-treated plywood is just as durable and robust as pine or any other softwood plywood, and the chemicals injected into it ensure it stays stable and strong.

I built a garden shed in my backyard several years ago. ‘The wet grass, dropping off my lawnmower, had provided the necessary moisture to cause the hardwood plywood and floor to decay. I was replacing the floor four years later because I hadn’t made it using pressure-treated plywood. Once the hardwood with pressure-treated plywood was replaced, there was no risk of the new floor decaying.

What does “pressure-treated” mean?

When we talk about pressure-treated plywood, we are talking about the way the plywood is treated, over and above the normal process of making the pressure-treated plywood itself. “Pressure-treated” refers to a further chemical preservative process that makes the pressure-treated plywood more durable.

Chemical preservatives are mechanically injected into the wood, which helps to make it less vulnerable to termite and insect damage, insect damage, fungal damage, rot, bacteria weather damage, and more.

While pressure-treated plywood can still be damaged, it won’t decay. The chemicals injected into the pressure-treated plywood don’t make it more durable or robust. They make the pressure-treated plywood wood impervious and naturally resistant to the pests above, primarily insects, many of which have natural oils that cause insect damage and decay.

By repelling those natural oils and naturally resistant insects and pests, the same wood manufacturers pressure treat wood to last considerably longer.

When and how did this process begin?

In the early 1900s, Karl Wolman discovered that infusing heat, chemical compounds, and preservatives into plywood could help protect wood against the elements of heat and the abovementioned moisture conditions. Since then, the process of heat and the treatment and pressure-treating of plywood has grown into a massive industry.

First, the plywood material is placed in contact with a holding tank. This tank is then depressurized to remove all of the air in the tank. After removing the air, the chemical wood preservative is placed into contact with the wood material in the tank under high pressure.

This high pressure forces the chemical preservatives deep into the wood to seal it. The pressure-treated seal wood material is removed, and leftover chemicals can be reused to pressure-treat more wood.

Until the early 2000s, Chromated Copper Arsenate (CCA) was the preservative used on pine. However, this chemical preservative is highly toxic and can damage the wood and the health of individuals who come into contact with it. For this reason, the industry has moved to Amine Copper Quat (ACQ) and Copper Azole (CA) to pressure-treat wood.

These chemically chemical compounds of copper and arsenic are much safer. They cannot be absorbed into the human body as poison, unlike Aresenic-type chemicals of copper, chromate copper arsenate, and arsenic, chromate arsenic, chromate copper arsenic, or copper and arsenic (ex. CCA).

Where can pressure-treated plywood be used?

Depending on the specific type, pressure-treated plywood can be used in various building codes and projects almost anywhere. To further explain the difference, different chemicals work better for outdoor applications in different environments.

The same principle applies to wood manufacturers with differences in the specific type and process of pressure-treating wood, meaning that certain types work better for building codes and outdoor applications in different environments.

Various categories have been established to help consumers determine where and when to use plywood materials. Upon purchase, the plywood materials should be labeled according to the following:

• UC1- meant for interior, dry purposes
• UC2- interior can be damp
• UC3A- exterior, above ground, coated (rapid water runoff)
• UC3B- exterior, above ground, uncoated (poor water runoff)
• UC4A- ground contact, general use
• UC4B- ground contact, heavy-duty use
• UC4C- ground contact, ultra heavy-duty use
• UC5A- marine use, Northern waters
• UC5B- marine use, Central waters
• UC5C- marine use, Southern waters
• UCFA- interior, above ground, fire protection
• UCFB- exterior, above ground, fire protection

What safety precautions should be taken with pressure-treated plywood?

Even though modern advances in treatment technology and health knowledge have helped to make this plywood treatment much safer than it was in the past, safety and environmental protection agencies and precautions during treatment should still be taken. The following tips will cover general bases to help protect against any health concerns that may come up while working with chemically treated plywood materials.

• Wear gloves and wash your hands thoroughly. Pressure-treated wood and plywood products are often still wet from the preserving process when you buy them. You will be better off if you can keep this off your hands. 
• Wear safety goggles to avoid eye contact.
• Always cut pressure-treated wood outdoors and in open spaces so that fumes can dissipate. 
• Do not burn chemically treated wood; the chemicals produce toxic, dangerous fumes. 
• Be sure the wood is completely dry before painting or staining.
• Keep up with general wood maintenance, such as checking for cracks, sinking, etc.

3/4″ pressure-treated oak plywood floor, ShedBuilder

Painting and Finishing Pressure-Treated Plywood

As it comes from the lumberyard, pressure-treated plywood furniture can’t be painted. Paint products don’t stick well to the treated wood without manufacturers’ chemicals, and the surface of the treated timber is usually damp.

Were you to paint a freshly finished project made out of pressure-treated plywood, the paint job would flake off, giving the project and you a show job and a poor base for the project and future repainting jobs.

The secret to painting this pressure-treated plywood is to wait about six months after making your project. That will allow plenty of time for the chemicals and the wood to dry out completely, even though most wood dries slowly.

Once you have given it this time, it can be primed and painted, just like any other other plywood sheathing. Be sure to use heavy coats, as the surface of pressure-treated plywood sheathing is not a sanded finish.

Therefore, heavy coats of paint are necessary to ensure that the paint bridges micro-cracks in the surface of the pressure-treated plywood, sheathing, and wood.

Using exterior stains on pressure-treated lumber or other treated wood or plywood walls is more or less impossible. The various treatment chemicals injected into the wood fill the pores that the stain typically fills.

So, any stain product applied will not soak into the pressure-treated wood, plywood, or wood well, giving a very slight amount of actual staining and a splotchy appearance.

Likewise, you can’t use oil-based wood preservatives on pressure-treated oak plywood furniture for the same reasons. However, they aren’t necessary, so that shouldn’t be an issue.