Pressure-Treated Wood - Its Uses,
Limitations and Safety Considerations

Lessons in "Outdoor Wood"

Pressure-treated wood is truly a "wood for all seasons"...  a rugged exterior building product that's rot and insect resistant. If Noah's ark was made from this stuff, it would still be cruising the Mediterranean!

Treated wood is used for decks, mailbox and light posts, swing sets and playscapes, picnic tables, landscape ties, underwater dock pilings, oceanside boardwalks, telephone utility poles and, believe it or not, residential building foundations in some parts of the country!

You can purchase pressure-treated (PT) wood as lumber, boards, posts, and even plywood! Its unique ability to fend off decay makes it ideal in any high moisture and/or ground contact installations.

Yet, there is much misinformation, and, in some cases, disinformation concerning pressure-treated wood, its maintenance requirements, and its safety in common use. Let's explore the story of pressure-treated wood, and together seek out the truth.

If you wonder whether you should use PT wood for a raised garden frame, click HERE.

What is pressure-treated wood?

Over 70 years ago, Dr. Karl Wolman invented the process of infusing preservative deeply into wood products. Today, a giant industry has grown up around his quest to invent a wood that can last forever.

Pressure treating is a process that forces a chemical preservative deep into the wood. The wood product is placed into a humongous cylindrical holding tank, and the tank is depressurized to remove all air. The tank is then filled with the preservative under high pressure, forcing it deeply into the wood. The tank is then drained and the remaining preservative reused. The wood is removed from the tank and prepared for shipment to your local lumberyard.

Needless to say, this process makes the wood quite unappetizing to all vermin, insects, and fungus, which accounts for its 20 year plus lifespan under the harshest conditions!

Currently, there are twelve levels of pressure treatment.  These are based on the intended use of the product.  This chart is courtesy the American Wood-Preserver's Association (AWPA):

Use Category Brief Description
UC1 Interior Dry
UC2 Interior Damp
UC3A Exterior Above Ground, Coated with Rapid Water Runoff
UC3B Exterior Above Ground, Uncoated or Poor Water Runoff
UC4A Ground Contact, General Use
UC4B Ground Contact, Heavy Duty
UC4C Ground Contact, Extreme Duty
UC5A Marine Use, Northern Waters (Salt or Brackish Water)
UC5B Marine Use, Central Waters (Salt or Brackish Water)
UC5C Marine Use, Southern Waters (Salt or Brackish Water)
UCFA Interior Above Ground Fire Protection
UCFB Exterior Above Ground Fire Protection

This information is required to be posted on each board and is either marked with ink on the board or on a plastic tag that is stapled onto the end of each treated board.  Generally speaking, this is not something for you to be concerned with, since your local lumberyards will only carry the types suitable to your climate.  Always let the salesman know the eventual location of the PT lumber... above, on or below grade... so that you can make the best purchase!

What is the chemical preservative used, and is it dangerous?

Until 2003, the preservative most commonly used in residential pressure-treated lumber was chromated copper arsenate (CCA), an extremely toxic chemical. Remember "Arsenic and Old Lace"? How about that old box of rat poison you have lurking in the garage? CCA is so toxic that the Environmental Protection Agency, over 20 years ago, imposed strict guidelines regarding the manufacturing practices of companies using CCA.

However, one must distinguish between the toxicity of the chemical and the toxicity of the wood product in everyday use. Extensive studies were done since the mid 1980's concerning the potential dangers of pressure-treated wood, and rightfully so! Large volumes of CCA were being used, and the treated wood products were beginning to be widely distributed, justifying the need for some hard research.

The research was mixed, but the typical hysteria ensued as attorneys and plaintiffs lined up to claim damages from exposure to CCA.  In the end, the industry agreed to voluntarily eliminate use of CCA for residential use.  CCA is still being used in certain marine and industrial applications since it is still the best preservative available at the present time.

The new alternatives to arsenic-based preservatives.

Your local home store or lumberyard is now selling lumber treated with less toxic alternatives... amine copper quat (ACQ) and copper azole (CA)... though you may find other chemical combinations in specific areas.  The reason these new copper-based alternatives are considered safer than arsenic-based preservatives is based on the human body's inability to absorb these poisons.  Inorganic arsenic is readily absorbable by the body.

Whether these new chemicals will turn out to be less hazardous in the long term is anyone's guess, but all indications are that they will be.  Fingers crossed!

Is the copper-based preservative as good as the arsenic-based preservative?

Yes, from what I've read overall the replacements are equivalent, but copper-based preservatives are mucho more expensive.  To keep PT wood affordable,  manufacturers are lowering the amount of preservative for boards used in less demanding applications.  For example, 5/4" boards typically used for decking will receive less preservative than 2x boards which will be rated for above ground use.  4x4 and 6x6 and larger beams and posts will receive a ground contact rating.

What that means is if your lumberyard does not carry 2x lumber with the proper amount of preservative (for ground contact) and you use it for a raised garden or bury it, you might find the material rotting in short order.  Albeit not as quickly as untreated lumber, but far from the decade plus that the older PT wood could last.

Are there any special considerations when using wood with a copper-based preservative?

Following the safety rules (below) regarding cleanup should be sufficient.  However, these newer products are extremely corrosive to steel and aluminum.  Fasteners and construction hangers/ties should be rated for use with the new wood.  Any aluminum flashing should be covered with an impervious layer of a non-corrosive material such as tar paper or non-permeable plastic sheets to prevent contact with the wood.

If CCA is potentially dangerous, should I tear out my old deck?  It's otherwise in great shape.

Absolutely not.  Existing decks pose no danger and, in fact, tearing them out may release more chemical than leaving them in place!  (See the safety precautions below.)

The EPA has stated that applying a penetrating oil finish as needed to pressure-treated wood surfaces (that have human contact) can lessen or eliminate human and animal exposure to CCA in existing decks.  Studies done to date show a dramatic decrease in the amount of arsenic at the surface of the wood for periods up to two years when compared with unsealed wood.

What are the safety precautions regarding use of pressure-treated wood?

Why does pressure-treated wood need to be coated with a preservative? If it's so dang tough, why bother?

Though the infused preservative prevents rot, it does not inhibit weathering...
The effects of the elements on pressure-treated wood are no different than with ordinary wood. So a preservative is a must, and should be applied as soon as possible after your project is completed.

Rapid drying causes warping, cracking and splintering...
Pressure-treated lumber is shipped to the lumberyard in stacks that are tightly bundled and damp... sometimes even wet. If you go and pick through a bin of pressure-treated lumber, you will see some pieces are straight, and others moderately to wildly warped. The warped pieces are invariably the pieces that were on the outside of the bundle... exposed to the sun and air and dried on one side. Once the bundle is broken they twist like Chubby Checker!

Once installed in your project and subjected to freely moving air and the sun, the same effect occurs. Shrinkage of deck boards can be excessive, in both length and width, and twisting can loosen railings and floor boards. Railings can become cracked and splintery, making them uncomfortable to use.

Health and safety...

A certain amount of PT preservative will leach to the surface of the wood over time.  Applying a coating every year or two (once the rain stops beading) greatly lessens the amount of preservative that leaches to the surface.

Applying a preservative slows drying and inhibits shrinkage and helps maintain a smoother surface to the wood.  The sun takes no prisoners and even pressure treated wood needs protection from it.  Remember, the preservative protects the wood from mold and mildew, not good old Sol!  The preservative should be applied immediately upon completion of the project or within a month or so if the wood is especially wet. Be sure that the preservative you purchase is recommended for use with pressure-treated wood. More about this in the next section...

Can pressure-treated wood be painted or stained?  Definitely!

Stains first... 

 Many manufacturers carry full lines of both oil and latex products that can be used on pressure-treated wood. According to the folks at Cuprinol, you should wait at least one to two months before staining. You may apply a clear preservative immediately, but it must be a product manufactured for use on fresh pressure-treated lumber.  One such product is Wolman Oil-Base RainCoat Clear Water Repellant.

What about painting?  Don't even think about painting fresh pressure-treated wood!

The moisture in it "stacks the deck" against good paint adhesion. Seal your project with a pressure-treated wood preservative immediately. Follow the preservative's instructions regarding future painting, making special note of the amount of time the preservative should weather before painting.

Applying a sealer can protect against CCA exposure...

According to the EPA, studies show that the application of a penetrating oil finish can reduce or eliminate exposure to CCA in older decks and to the chemicals used in newer decks.  So it is recommended that all pressure-treated surfaces that have human contact be coated with an oil finish as needed.  It has been noted in some studies that paints and opaque exterior stains do not offer the protection of stains that are absorbed more deeply into the wood.

Tips for working with pressure-treated wood...

Should I install a pressure treated wood deck with the boards "bark up" or "bark down"?

If you look at the end grain of any board, you'll notice that the rings have a particular curve to them.  "Bark side" refers to the side of the board that the rings curve away from.  In other words,  "bark-side up" would be a deck board that has the visible grain curve downward.  With pressure-treated southern yellow pine (the most common type) you might even see a little bark on some pieces.

There has been lots of debate on this issue because of a process called "cupping".  Cupping refers to the tendency of a board to bend across its width as it dries.  Ideally, you would want all the deck boards to cup downwards so any rain would run off them rather than collect on the wood.

Some people assert that the ONLY way to install deck board is "bark down".  The rationale is that a board in laboratory conditions resting across a couple of sawhorses will tend to cup towards the bark side as it dries out.  This is undisputed fact and this principle used in cabinetmaking when edge-joining boards so the cupping in one board is counteracted by the reverse orientation of the next board and so on and so on.  This would make you think that wood decks should be installed with the bark side down, right?  If it were only that simple.

The truth is not all boards react in the "wild" as they do in the laboratory or the woodworking shop.  For example, unlike redwood or cedar which are installed dry, pressure-treated wood is installed damp or even soaking wet!  The excess moisture from the preservative treatment can cause excess cupping as it dries, whether it is installed with the bark up or down!  In other words, the simple drying action of the sun and wind can cause the top of the board to dry faster than the bottom, causing cupping regardless of the exposed top.

I used to think that bark up was the rule, but over the years I've come to realize that it doesn't matter as much as I thought.  So now I use a couple of general principles to determine which side of the board will be the top.