Pressure Treated Decks
Pressure-treated deck wood is truly a “wood for all seasons”, a rugged exterior building product that’s rot and insect resistant.
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.
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.
What is pressure-treated wood?
Over 70 years ago, Dr. Wolman invented the process of infusing preservative deeply into wood products.
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 deep into the wood.
Needless to say, this process makes the wood quite unappetizing to all vermin, insects, and fungus, which accounts for its 20 years plus lifespan under the harshest conditions!
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.
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 as 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 the 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.
We now use amine copper quat (ACQ) and copper azole (CA)… though you may find other chemical combinations in specific areas.
Is the copper-based preservative as good as the arsenic-based preservative?
Yes, the replacements are equivalent.
Are there any special considerations when using wood with a copper-based preservative?
Following the safety rules 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.
Why does pressure-treated wood need to be coated with a preservative?
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.
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.
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?
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.
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.