Wood Protection

Pressure treatment is used to protect wood lumber from decay. The process begins with dry lumber placed inside a large pressure vessel. A vacuum is first applied to remove air from the wood cells. Then a preservative solution is introduced and forced into the wood under high pressure. This ensures deeper penetration compared to surface coatings, and is intended to make the lumber suitable for exterior or even ground-contact use.

From the 1930s until about 2004, chromated copper arsenate (CCA) was widely used for pressure treatment. CCA contains chromium, copper, and arsenic. The copper acted as a fungicide, the arsenic provided insect resistance, and the chromium bound the chemicals to the wood fibers. CCA was superior to the alkaline copper quaternary (ACQ), copper azole (CA), and others used now. The newer ACQ and CA cause steel fasteners to more quickly corrode due to their high copper content. This was never such a problem with CCA. More importantly, CCA lumber lasted much longer. To this day, it is not unusual to pull pristine CCA treated lumber out of the earth. Lumber treated with ACQ, CA, etc. is known to rot away within several years if subjected to harsh exposure conditions.

Creosote is another wood preservative that worked. It is distilled from coal tar, and has been used for centuries. It is an oily, pungent liquid applied directly to wood. However, the U.S. EPA also banned the widespread availability of creosote in the mid 1980s, and banned it altogether for private use in early 2005. Once again, a product that actually worked in preserving wood is no longer available to the average consumer. Power companies, railroad lines, etc. still use cresote to protect their poles and ties.

So what is the average consumer left with? Sadly much less than what should be. However, there are a few things left to say. First, keep wood off the ground. Soil retains moisture, and extended or repeated exposure to moisture in combination with fungi is what cause rot. Second, consider the use of a borate solution such as Penetreat or Tim-bor. This is a white powder that, mixed well with warm water, can be applied with a hand-held wand sprayer. Apply at least every third year to exterior areas of wood more susceptible to repeated or sustained moisture. Apply when the wood is dry. Second, be careful with painting exterior wood, especially when the paint encloses the entire member. Often paint can trap moisture inside wood, accelerating rot. Third, consider using cedar or cypress. Both of these wood species have a natural resistance to decay, although cedar in particular is a very light wood not generally suitable for structural applications. The Japanese had an ancient method of preserving wood that involved burning it, but further information about this would have to be researched elsewhere.

One final note that pertains to wood preservation. Caution should be used when applying vapor barriers, such as poly sheeting, over walls, floors, and other assemblies. Barriers should never be placed on the outside face of a wall in northern climates. This because any moisture driving from the interior to the exterior of the wall during cold weather will be trapped behind the vapor. And wood framing or other members susceptible to moisture should never be encapsulated by the application of a vapor retarder to both sides. This traps moisture and can lead to rapid deterioration. This is discussed here.