Pressure-treated Plywood

Pressure-treated wood is the wood of choice when building things which need to be protected from moisture and decay, but where paint or other protective finishes can’t be used. Actually, when we talk about “pressure-treated wood products”, we’re talking about a whole family of products, including dimensional lumber, timbers, railings and plywood.

Many people like using pressure treated wood for making decks, outdoor furniture and other backyard projects. The long life and low maintenance you can get out of pressure treated wood products, makes it an ideal choice when building projects which will remain outdoors.

Pressure-treated plywood is just as useful as other pressure-treated wood products, even though you don’t encounter it quite as often. Yet there are many outdoor projects, like garden sheds, where pressure-treated plywood is an ideal choice, because it doesn’t have to be painted or treated in any way. The plywood itself is just as strong as any other softwood plywood and the chemicals injected into it ensure that it will stay as strong.

I had built a garden shed in my backyard a number of years ago. Four years later I was replacing the floor, because I hadn’t made it using pressure-treated plywood. The wet grass, dropping off my lawnmower had provided the necessary moisture to cause the plywood floor to decay. Once replace, there was absolutely 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 plywood. The term “pressure-treated” refers to a chemical process which makes the plywood more durable. Chemical preservatives are mechanically injected into the wood, which helps to make it less vulnerable to termite and insect damage, fungal damage, bacteria weather damage, and more.

While pressure-treated plywood can still be damaged, it won’t decay. The chemicals injected into the plywood don’t make it any stronger. They just make the wood impervious to the aforementioned pests, many of which cause decay. By repelling those pests the wood lasts considerably longer.

When and how did this process begin?

In the early 1900’s, Karl Wolman discovered that infusing chemical preservatives into plywood could help protect against the various conditions listed above. Since then, pressure treating plywood has grown into a massive industry.

First, the plywood is placed into a holding tank. This tank is then depressurized to remove all of the air in the tank. After the air is removed the chemical preservative is placed into the tank under high pressure. This forces the chemical deep into the wood. The pressure-treated wood is then removed and leftover chemicals can be reused to pressure-treat more wood.

Until the early 2000’s, the preservative used was Chromated Copper Aresenate (CCA). However, this chemical is extremely toxic and can lead to damages in both the wood and health of individuals that 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 are much safer to use and cannot be absorbed into the human body as poison, unlike Aresenic-type chemicals (ex. CCA).

Where can pressure-treated plywood be used?

Depending on the specific type, pressure-treated plywood can be used pretty much anywhere. To further explain, different chemicals work better for different environments. The same principle applies to the specific type of wood, meaning that certain types work better for different environments.

Various categories have been established to help consumers determine where and when to use plywood. Upon purchase, the plywood 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 technology and health knowledge have helped to make this plywood much safer than it was in the past, safety precautions 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.

  • Wear gloves and wash hands thoroughly. Pressure-treated wood and plywood products are often still wet from the preserving process, when you buy it. 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, as the chemicals will give off noxious fumes that can be dangerous. 
  • 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,wood,floor
3/4″ pressure treated plywood floor, ShedBuilder

Painting and Finishing Pressure-Treated Plywood

As it comes from the lumberyard, pressure-treated plywood can’t be painted. Paint products don’t stick well to the chemicals and the surface of the wood is usually damp. Were you to paint a freshly finished project made out of pressure-treated plywood, the paint would flake off, giving you a poor base for future repainting.

The secret to painting this 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, even though most wood dries slowly. Once you have given it this amount of time, it can be primed and painted, just like any other plywood. Be sure to use heavy coats, as the surface of pressure-treated plywood 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 wood.

It is more or less impossible to use exterior stains on pressure-treated plywood. The chemicals which are injected into the wood fill the pores which the stain would normally fill. So any stain product applied will not soak into the 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 plywood, for the same reasons. However, they really aren’t necessary, so that shouldn’t be an issue.

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