Pressure Treated Wood

Pressure treated wood is a staple of the North American backyard. Decks, fences and play structures will last longer when made from pressure treated wood. Wood exposed to the elements will decay as it is attacked by fungus and insects. Pressure treatment introduces a chemical into the wood that inhibits fungus growth. No fungus - no rot.

For many years, the chemical used was chromated copper arsenate (CCA). CCA contains chromium, copper and arsenic. Chromium and arsenic are toxic to the environment and to people. In 2004, CCA was phased out. It is no longer used to pressure treat wood for most residential uses.

Health Risk
Many people are concerned that their existing CCA pressure treated wood structures pose a health risk. Government bodies agree that there is no significant health risk from existing CCA pressure treated wood. They do not believe there is any reason to remove or replace CCA treated structures including decks and playground equipment. However, prudent avoidance is advised.

Stains and Coatings
The U.S. Environmental Protection Agency (EPA) in conjunction with Consumer Product Safety Commission (CPSC) tested the effect of coating existing CCA treated wood to reducing exposure to arsenic. The results were that application of a penetrating sealant can reduce the exposure to one tenth but the protection is short lived. The penetrating sealant should be re-applied every year or two. Oil based, semi transparent stains that soak into the wood are the most effective. Surface coatings such as paint or varnish were not considered to be as effective.

New Pressure Treated Wood
New pressure treated wood does not contain arsenic or chromium. The two main preservatives used are alkaline copper quat (ACQ) and copper azole (CA). These new preservatives have very low toxicity.

Corrosive
New pressure treated wood is 5X more corrosive to steel than CCA. This means that special fasteners must be used. Stainless steel fasteners and copper flashing are ideal but expensive. Galvanized steel fasteners can be used but they must be a much higher grade, designed for this application. If the wrong fasteners are used they will corrode very quickly and could lead to failure of the structure.

Pressure treated lumber is tagged showing the treatment levels and the type of preservative used. This illustration shows a typical pressure treatment tag. In this case, we are looking at wood designated for above ground use. The treatment chemical is copper azole. The amount of preservative in the wood is 0.1 pounds per cubic foot. This is called the retention level.

Each and every board of pressure treated wood should have its own tag.