Q. What makes “ground contact” PT lumber more durable than lumber specified for “above ground” use?
A. Pressure-treated lumber has been immersed in a liquid preservative and placed in a pressure chamber, which forces the chemical into the wood fibers. The pressure ensures that the chemical makes it to the core of each piece of wood. The amount of protection provided by the chemical depends on the amount of chemical the wood absorbs. In the United States, the amount of chemical is measured in pounds of chemical per cubic foot of wood. For above-ground rated lumber, 0.25 pounds per cubic foot is often needed when treated with ACQ preservative. Ground-contact lumber may have 0.40 pounds. For foundations, 0.60 pounds per cubic foot may be required. The amount of preservative also depends on the particular chemical being used. Despite the type of preservative, the ground-contact variety of PT lumber simply contains more chemical preservative than the above-ground products.
Special thanks to EHT reader Michael Tucker for adding some clarification to this topic: “In reference to the June 2012 Extreme-How-To Q&A on page 12. The response identifiing the retention levels of above ground vs. ground contact … There are multiple chemicals currently available with different required retention levels for the intended use. Please look at the end tag of the product for application. Retention levels include but not limited to .05,.06,.014,.015,0.23 lbs. per cubic feet for various applications. Look for the wording above ground or ground contact.”
