New Wood Preservative Treatments Coming

New wood preservative treatments are being phased in to the market to replace chromated copper arsenate (CCA), which will no longer by used to treat lumber intended for residential construction after the end of this year. As these new treatments become available, designers and builders will encounter a new alphabet soup of chemical acronyms and new proprietary trade names. Depending on the treatment type in use, it may also be necessary to give more attention to the corrosion resistance of framing hardware used with these products than has been necessary in the past.Corrosion Concerns

For an explanation of wood preservative treatments, see Chapter 3 Wood of the the textbook (page 101). The text also discusses the pending phase-out of CCA and the introduction of new preservatives such as alkaline copper quat (ACQ) and copper boron azole (CBA). While differences of opinion persist over the risk from exposure to arsenic in CCA, the industry has nevertheless reached a voluntary agreement with the US EPA to discontinue use of CCA for treatment of lumber intended for residential use as of December 31 of this year.

The Journal of Light Construction reports that new wood preservative treatments may corrode metal framing hardware--such as joist hangers, post bases, and their fasteners--significantly faster than chemicals used previously (JLC, September 2003). At the time of this writing a good technical overview of the new preservative treatments and the corrosion issue is available at Simpson Strong-Tie's Corrosion Information page. This manufacturer provides recommendations for using a range of galvanizing treatments or stainless steel hardware and fastener materials, depending on the particular treatment chemical used to protect the wood.

Identifying Preservative Treated Lumber
Treatment of lumber varies according to its intended use--the more severe the exposure, the greater the level of chemical retention required during the treatment process.
Two labels from CCA-treated lumber are shown here. The upper label indicates a 2x6 treated sufficiently for above ground use only. The lower label indicates a heavier treatment suitable for ground contact, in this case for a 4x4 post that may be partially buried. Typically every piece of treated lumber carries such information.

Voluntary standards for preservative treatment are developed by the American Wood-Preservers Association. The AWPA's Use Category System allows identification of treated lumber according to end use or exposure (such as "above ground" or "ground contact") regardless of the specific wood species and treatment process. The Southern Pine Council provides a good overview of these standards and how they are applied on its Pressure Treated Lumber pages. For those responsible for specifying preservative treated lumber, downloads available from this site's Commodity and Use Category Standards and Preservative Retention Levels pages are also particularly useful.

This image illustrates part of a table listing AWPA requirements for treatment of Sourther Pine lumber, depending on Use Category and treatment chemical. So for example, with the 2x6 discussed above, knowing that the joist is CCA-treated Southern Pine and is intended for above-ground use, the table tells us preservative must be applied at a retention rate of at least 0.25 pounds per cubic foot of lumber. For the post intended for ground contact (also Southern Pine), the required retention rate is at least 0.40 pcf.

This next image shows the front and back of a third treatment label. In this case, the preservative is Copper Azole Type A (CBA-A). Noting that this lumber is labeled for above ground use and assuming that it also is Southern Pine, we can refer to the table above and determine that the required retention rate for this preservative treatment is 0.20 pcf, about 20% less that required to achieve the same degree of resistance to decay as the CCA-treated 2x6 noted above.

Specifying Preservative Treated Lumber
The Use Category System makes it easier for the specifier to set treatment requirements without indicating particular treatment chemicals or retention rates. This is particularly helpful as the number of possible treatment choices increases. However, until concerns regarding differences in corrosivity among treatments are resolved, the specification may need to address this specific issue as well in one of a number of ways, such as:

• Restricting the choice of acceptable treatment chemicals to exclude those that are considered too highly corrosive
• Specifying only the most corrosion-resistant framing hardware, at possible additional cost
• Varying the requirements for framing hardware depending on the treatment processes that may be used with the particular lumber supplied for the project.

More Information:
Canada's Wood Durability web site
US Forest Products Laboratory's Comparative Decay Resistance of Heartwood of Native Species [PDF]