In the recently published article Freestanding Decks (July/August 2012), I noticed some images of beautiful deck projects that were potentially flawed. It appears that many of these deck projects included Above Ground Use treated wood in direct contact with the ground, and in some cases these materials were even buried into the ground or backfilled with dirt.
There are many differences in the treated wood products available today as compared to a decade ago, when many suppliers only carried Ground Contact treated wood. Most retailers today stock dimension lumber and deck specialty products that are treated to an Above Ground retention level, and these materials may be exposed to weather. These Above Ground Use treated materials should not be used as stair stringers, joists, or decking where they will touch the ground.
Preservative retention values for Ground Contact standardized uses are typically found in 4x4s, 6x6s and larger structural posts, so they can withstand the higher decay potential and termite attacks when placed into the ground. Both types of treated lumber products are labeled with identification end tags indicating which retention value they have and which use they are suited for.
The American Wood Protection Association (AWPA) standards are the only standards for treated wood directly listed in the IRC and IBC, so the first thing to do is look for "AWPA" and the applicable standard, usually "U1" on the end tag. (Editor’s note: Other industry standards may be acceptable, but that’s up to the building official’s discretion.) You should also look for the appropriate AWPA Use Category for the application, such as UC3B or UC4A. Note that in most cases, Ground Contact treated Posts can be used in an Above Ground Use application.
Below is a brief summary of the AWPA's Use category system for treated wood.
UC3B ABOVE GROUND Exposed -- Treated wood materials used in exterior construction that do not come into contact with the ground. Materials do not require an additional exterior coating, however these materials may be finished to achieve a desired aesthetic appearance. UC3B materials are used for a variety of applications in either horizontal and vertical positions such as decking, sills, walkways, railings and fence pickets.
UC4A GROUND CONTACT General Use -- Treated wood used in contact with the ground, fresh water, or other situations favorable to deterioration. Examples are fence posts, deck posts, guardrail posts, structural lumber, and timbers.
UC4B GROUND CONTACT Heavy Duty -- Wood and wood-based material used in contact with the ground in severe environments, and in climates with a high potential for deterioration including; permanent wood foundations and wood used in salt water splash zones.
With so many beautifully enhanced outdoor living areas being constructed, we all need to make sure that their longevity meets customer expectations. If you are interested in additional information about the AWPA Use Categories, visit the "references" section at AWPA.com.
Director of Marketing