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

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 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.

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 four levels of pressure treatment, regardless of the chemical used as a preservative. These are based on the intended use of the product and the measurement is in pounds of preservative per cubic foot of wood product. The ratings and suggested uses are:

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.  Your local home store or lumberyard is now selling lumber treated with (hopefully) less toxic alternatives... amine copper quat (ACQ) and copper azone (CA)... though you may find other chemical combinations in specific areas.  CCA is still being used in certain marine and industrial applications since it is still the best preservative available at the present time.

Whether these new chemicals will turn out to be less hazardous in the long term is anyone's guess.

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.)

Also, 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.

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

• The sawdust from pressure-treated wood can be an irritant to the nose, eyes, and skin.
  Use of a dust mask and eye protection is highly recommended. Prevent contact with the skin as much as possible.
• Try to collect as much of the sawdust as possible for disposal.
  If possible, do most of your cutting in a specified location, and lay a heavy disposable plastic tarp under your saws or sawhorses. Use of a tarp is especially valuable if you must cut in an area that you cannot easily sweep, such as a lawn. Actually, this is a cleanup timesaver you can use even when cutting ordinary boards, especially if you generate large amounts of dust, as in making rip cuts!
• Pressure-treated wood should not be burned under any circumstances.
  Other products that are on the do-not-burn list are plywood, particleboard, chipboard and painted or finished wood. To be fair, there was one government study that did not find any more toxicity in the smoke from pressure-treated wood than from regular wood. The ash, though, was definitely more toxic, because the arsenic does not burn off. So you wouldn't want to add this ash to soil or your compost heap. All of us who have heated their homes with wood know how the ash is everywhere and on everything!
• Do not use pressure-treated wood for making cutting boards, or for any food preparation surface.
  Picnic tables made of pressure-treated wood have become common, and are fine to use for the purpose they were intended... to serve food on (or play cards on), not to prepare food on!  They should be coated as needed with an oil-based wood preservative to seal in the chemical preservative.
• Pressure-treated wood is intended for outdoor use only!
  The only exception I have heard of to this rule (check with your local building inspector on this one) is the use of pressure-treated wood in place of Douglas fir for sill plates in new construction. Sill plates are the lowest framing boards in a wood home. They are bolted to the top of the foundation (so the next windstorm doesn't hurl your house to Oz!), and the house is erected onto them. Dampness in the poured concrete foundation can be absorbed by the sill plate, making it attractive to carpenter ants and termites. pressure-treated wood is resistant to termites and unappetizing to ants, so its use can help convince the little critters to go next door for dinner.

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.

Applying a preservative slows drying, so less shrinkage occurs and the surface of the wood remains smoother longer. The preservative should be applied immediately upon completion of the project. 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 (or paint) as needed.

Tips for working with pressure-treated wood...

• Always install pressure-treated deck flooring with the bark side up.
  Pressure-treated wood tends to cup as it dries. Cupping is the tendency wood to bend along its width away from the bark side. You can tell which side in the bark side by looking at the end grain of the board.
• Predrill any nail or screw holes within an inch of the end of the board.
  This lessens the chance of splitting the board while fastening it. Even if it doesn't split when you initially fasten it down without predrilling, it will very likely do so later as the wood dries out.
• Use the right fasteners.
  Use only screws or nails that are galvanized and designed for pressure-treated wood. Any of the many styles of galvanized metal brackets and reinforcements commonly used in construction today can be used outside with pressure-treated wood.
• Do not allow any space between boards when laying in a deck floor.
  Putting spacers between floor boards is the amateur's biggest mistake. pressure-treated wood shrinks, even if you coat it with a preservative! Years ago when it first came on the market, people installed deck floors like good old untreated Douglas Fir, installing the boards with spacers between them. Wood shrinkage typically caused 3/4" or wider gaps in the floor... just the right width to snap off a high heel!
• Can deck posts be buried?
  Pressure-treated wood will not rot, even if buried, making it ideal for fence, mailbox, and light posts. Check with your local building inspector to see if code allows burial of pressure-treated deck posts in your area, or if the concrete footing must be exposed above ground level.