Changes in Pressure-Treated Wood: Alternatives for Dimension Lumber

Stan Lebow, USDA, Forest Service, Forest Products Laboratory, Madison WI

Because it is biodegradable, wood used in applications where it may be attacked by decay fungi or insects should be protected by pressure treatment with preservative chemicals. Wood preservatives are broadly classified as either water-based or oil-type, depending on the chemical composition of the preservative and the carrier used during the treating process. The most common oil-type preservatives are creosote, pentachlorophenol, and copper naphthenate. The oil-type preservatives are commonly used for applications such as posts, poles, piles, and glue-laminated members. They are not usually used for applications that involve frequent human skin contact or inside dwellings because they may be visually oily, oily to touch, or have a strong odor. Water-based preservatives have become more widely used in the recent years because the treated wood has a dry, paintable surface and no odor. Until recently, the most common of these preservatives was chromated copper arsenate (CCA). CCA-treated wood, commonly called “green-treated” wood has dominated the residential market for several decades and is sold at lumber yards under a variety of trade names. CCA-treated wood has also been widely used in post frame building applications. However, as the result of the voluntary label changes submitted by the CCA registrants, the EPA labeling of CCA will permit the product to be used primarily for industrial applications. The label change is effective December 31, 2003, although suppliers will be allowed to sell existing stocks of CCA treated wood after that date. This recent development has raised questions about the availability of CCA treated wood and the properties of alternative types of treatments.

What type of Applications are Affected by the CCA Restriction?

The label changes cite specific commodity standards listed in the 2001 edition of the American Wood- Preservers’ Association (AWPA) standards . The changes were made as part of the on-going CCA re-registration process, and in light of the current and anticipated market demand for alternative preservatives for non-industrial applications. CCA treated wood, however, can be used in certain critical structural applications (Table 1). Many applications of sawn lumber and timbers are affected, while CCA will still be allowed for treatment of roundstock (poles, building posts, piles) and sawn structural supports.

Examples of sawn products that may still be treated with CCA include:

• Sawn structural piles used to support residential and commercial structures
  
  
• Sawn posts and poles used in building construction
  
  
• Wood used in highway construction, including lumber and timbers
  
  
• Lumber produced for permanent wood foundations
  
  
• Utility pole cross-arms
  
  
• All dimensions used in salt water and subject to marine borer attack

Engineered wood products that may still be treated with CCA include:

• Glue laminated beams and columns
  
  
• Structural composite lumber
  
  
• Plywood for agricultural use, roof decking, subflooring, and boat construction

Alternatives to CCA for Dimension Lumber

Several arsenic-free preservative formulations have been standardized by the American Wood-Preservers’ Association for use in many of the applications previously dominated by CCA. Currently, these alternatives all rely heavily on copper as their primary active ingredient. Depending on your location and intended use of the wood, one or more of these alternative types of treated wood may be available. Availability is expected to increase rapidly in the coming months. Be aware that the various suppliers may sell the same type of treated wood by different names, and that you may need to ask the supplier for more information to determine the preservative used. Also, some manufacturers incorporate colorants or water-repellents into some of their preservative treatments. These treatments may also have a different trade name. Most importantly, do not assume that all “green-treated” wood is equal. The type of preservative, retention, and quality of treatment are critical to performance. With the CCA alternatives it will be more important to match the type of preservative, and its retention, with the intended application.

To become standardized by the AWPA, preservative treated wood must undergo a series of rigorous tests to ensure its durability. The results of these tests are reviewed by AWPA members who represent government agencies, universities, commercial chemical suppliers and treaters. Be wary of purchasing wood that has been treated with a preservative that has not been standardized for that application by either the AWPA or some other major standard setting body, such as ASTM.

Acid Copper Chromate

Acid copper chromate (ACC) has been used sporadically as a wood preservative in Europe and the United States since the 1920s. In the last few decades, it has been primarily used for treatment of wood used in cooling towers. ACC contains 32 percent copper oxide and 68 percent chromium trioxide. The treated wood has a light greenish-brown color and little noticeable odor. ACC provides acceptable average service life, but wood used in ground contact may suffer occasional early failures from attack by copper tolerant fungi. ACC is listed in AWPA standards for treatment of a wide range of softwood and hardwood species used above ground or in ground contact. However, in critical structural applications such as highway construction, its AWPA listings are limited to sign posts, handrails and guardrails, and glue-laminated beams used above ground. It may be difficult to obtain adequate penetration of ACC in some of the more refractory wood species such as white oak or Douglas-fir. This is because ACC must be used at relatively low treating temperatures and because rapid reactions of chromium in the wood can hinder further penetration during longer pressure periods. The high chromium content of ACC, however, has the benefit of preventing much of the corrosion that might otherwise occur with an acidic copper preservative.

Alkaline Copper Quat

The fungicides and insecticides in ACQ are expressed as copper oxide (67%) and a quaternary ammonium compound (quat 33%). Multiple variations of ACQ have been standardized or are in the process of standardization. ACQ type B is an ammoniacal copper quat formulation. ACQ-B treated wood has a dark greenish-brown color that fades to a lighter brown and may have a slight ammonia odor until the wood dries. ACQ type D is an amine copper quat formulation. Wood treated with ACQ-D has a lighter brown color and little noticeable odor. ACQ type C is formulated with either ammonia or amine, and a slightly different quat compound. Wood treated with ACQ-C varies in appearance between that of ACQ-B and ACQ-D, depending on the formulation.

The ACQ formulations are listed in AWPA standards for a range of applications and many softwood species, although the ACQ-C listings are limited because it is the most recently standardized. Minimum retentions of 4.0 kg/m3 (0.25 lb./ft.3) are specified for wood used above ground and 6.4 kg/m3 (0.4 lb./ft.3) for wood used in ground contact. A retention of 9.6 kg/m3 (0.6 lb./ft.3) is specified for critical structural members placed in ground contact. The multiple formulations of ACQ allow some flexibility in achieving compatibility with a specific wood species and application. When ammonia is used as the carrier, ACQ has improved ability to penetrate into difficult to treat wood species such as Douglas-fir. However, if the wood species is readily treated, such as southern pine, an amine carrier can be used to provide a more uniform surface appearance. The number of pressure treatment facilities using ACQ is increasing. In the western United States, the ACQ-B formulation is used because it allows better penetration in difficult to treat western species. Treating plants in the remainder of the country generally use the ACQ-D formulation or the more recently standardized ACQ-C formulation.

Ammoniacal Copper Citrate

Ammoniacal copper citrate (CC) is a recently developed wood preservative that utilizes copper oxide (62%) as the fungicide and insecticide, and citric acid (38%) to aid in the distribution of copper within the wood structure. The color of the treated wood varies from light green to dark brown. The wood may have a slight ammonia odor until it is thoroughly dried after treatment. CC is listed in AWPA standards for treatment of a range of softwood species and wood products. The minimum CC retention is 4 kg/m3 (0.25 lb/ft.3) for wood used above ground or 6.4 kg/m3 (0.4 lb./ft.3) for wood usedin ground contact. As with other preservatives containing ammonia, CC has an increased ability to penetrate into difficult to treat wood species. Few treating plants currently use CC, and wood treated with this product may not be readily available in most areas.

Ammoniacal Copper Zinc Arsenate

Ammoniacal copper zinc arsenate (ACZA) has been used commercially for two decades, primarily in western North America for treatment of Douglas-fir. ACZA is a refinement of an earlier formulation, ACA, which is no longer available in the United States. ACZA contains copper oxide (50%), zinc oxide (25%), and arsenic pentoxide (25%). The color of the treated wood varies from olive to bluish-green. The wood may have a slight ammonia odor until it is thoroughly dried after treatment. ACZA is listed in AWPA standards for treatment of a range of softwood and hardwood species and wood products. The minimum ACZA retentions are 4.0 kg/m3 (0.25 lb./ft.3) for wood used above ground or 6.4 kg/m3 (0.4 lb./ft3) for wood used in ground contact. A slightly higher retention, 9.6 kg/m3 (0.6 lb./ft.3), is required for wood used in critical structural components. As with other preservatives containing ammonia, ACZA has an increased ability to penetrate into difficult to treatwood species. Treating facilities using ACZA are currently located in western states, where many of the native tree species are difficult to treat with CCA.

Copper Azole

Copper azole is another recently developed preservative formulation that relies primarily on amine copper, but it also includes a co-biocide to further protect wood from decay and insect attack. The first copper azole formulation developed was copper azole – Type A (CBA-A), which contains 49 percent copper, 49 percent boric acid, and 2 percent Tebuconazole. Minimum retentions of CBA-A are 3.3 kg/m3 (0.20 lb./ft.3) for wood used above ground and 6.5 kg/m3 (0.41 lb./ft.3), for wood used in ground contact. A retention of 9.8 kg/m3 (0.61 lb./ft.3) is specified for critical structural members placed in ground contact. More recently, the copper azole – Type B (CA-B) formulation was standardized. CA-B does not contain boric acid and is comprised of 96 percent copper and 4 percent Tebuconazole. Minimum retentions of CA-B are 1.7 kg/m3 (0.10 lb./ft.3)for wood used above ground or 3.3 kg/m3 (0.21 lb./ft.3) for wood used in ground contact. A retention of 5.0 kg/m3 (0.31 lb./ft.3) is specified for critical structural members placed in ground contact.

Wood treated with either copper azole formulation has a brownish-green color and little or no odor. The formulations are listed in AWPA standards for treatment of a range of softwood species. Although listed as an amine formulation, copper azole may also be formulated with an amine-ammonia formulation. The ammonia may be included when the copper azole formulations are intended for treatment of refractory species. Recent studies have demonstrated the ability of such a formulation to adequately treat Douglas-fir. The inclusion of the ammonia, however, is likely to have slight effects on the surface appearance and initial odor of the treated wood. Wood treated with copper azole formulations has become increasingly available in recent months.

Copper Dimethyldithiocarbamate

Copper dimethyldithiocarbamate (CDDC) is a reaction product formed within the wood after treatment with two different treating solutions. It contains copper and sulfur compounds. CDDC is standardized for treatment of southern pine and some other pine species at copper retentions of 1.6 kg/m3 (0.1 lb./ft.3) for wood used above ground or 3.2 kg/m3 (0.2 lb./ft.3) for wood used in ground contact. CDDC-treated wood has a light brown color and little or no odor. CDDC was introduced several years ago, but because plant conversion may be more expensive with CDDC, it is not currently commercially available.

Copper HDO

Copper HDO (CX-A) is an amine copper-based preservative that has been used in Europe and was recently listed in AWPA standards. The active ingredients are copper oxide, boric acid, and copper-HDO (Bis-(Ncyclohexyldiazeniumdioxy) copper). The appearance and handling characteristics of wood treated with copper HDO are similar to the other copper-based treatments. CX-A has been evaluated in a range of exposures, but at this time has only been standardized for uses above ground. At the time of this writing (August 2004) EPA registration of CX-A was pending.

Borates

Borate preservatives are sodium salts such as sodium octaborate, sodium tetraborate, and sodium pentaborate that are dissolved in water. Borate preservatives have received considerable attention in recent years because they are inexpensive and have low mammalian toxicity. Borate-treated wood is also odorless, colorless, and may be painted or stained. Borates are effective preservatives against decay fungi and insects. Borate preservatives are diffusible, and with appropriate treating practices, they can achieve excellent penetration in species that are difficult to treat with other preservatives. However, the borate in the wood remains water soluble and readily leaches out in soil or rainwater. Borate preservatives are standardized by the AWPA, but only for applications that are not directly exposed to liquid water. Borate-treated wood should be used only in applications where the wood is kept free from rainwater, standing water, and ground contact. An example of such a use is framing lumber in areas of high termite hazard.

Sources of Supply

Information on companies that manufacture or distribute preservatives listed in the AWPA standards can be found on the AWPA website (www.awpa.com/faq/faq4.htm), or at the websites of other associations such as the Southern Pine Association (www.southernpine.com/producers.htm) or Western Wood Preservers Institute (www.wwpinstitute.org).

Will I Notice a Difference with the CCA Alternatives?

From a practical, end-use basis there is little difference between CCA and the recently developed alternatives. The appearance, strength properties, and handling characteristics are very similar to CCA. The alternatives are slightly more expensive however, and the cost difference increases at higher preservative retentions. With the possible exception of ACC, the alternative treatments may be somewhat more corrosive to metal fasteners than is CCA. In-service corrosion with the CCA alternatives is difficult to evaluate because of the absence of long-term service data, but major fastener and preservative manufacturers are working to minimize corrosion concerns. The use of stainless steel fasteners or fasteners treated with a high quality hot-dip galvanization process is generally recommended, but the formulations continue to evolve and the preservative suppliers are the best source of up-to-date information on suitable fasteners. Because the treatments contain copper, direct contact of the treated wood with building components that contain aluminum should be avoided.

Specifying Preservative Treatments

To guide selection of the types of preservatives and loadings appropriate to a specific end-use, the AWPA recently developed Use Category System standards. The UCS standards simplify the process of finding appropriate preservatives and preservative retentions for specific end uses. They categorize all treated wood applications by the severity of the deterioration hazard.

The lowest category, Use Category 1 is for wood that is used in interior construction and kept dry, while UC2 is for interior wood that is occasionally damp but is completely protected from the weather. UC3 is for exterior wood used above ground, while UC4 is for wood used in ground contact in exterior applications. At the other end of the spectrum is UC5, is for treated wood in contact with seawater and marine borers. To use the UCS standards, one needs only to know the intended end-use of the treated wood. A table in the UCS standards lists most types of applications for treated wood, and lists the appropriate Use Category and Commodity Specification. The Commodity Specification lists all the preservatives that are standardized for that Use Category, as well as the appropriate preservative retention and penetration requirements. The user needs only specify that the product be treated according to the appropriate Use Category.

Take Care in Selecting Retentions and Types of Treated Wood

CCA was often only sold at one retention (0.4 lb/ft3) that was used for either above-ground or ground contact applications. Because the new preservatives are more expensive, the wood is now treated to a lower retention for above-ground use. Wood treated for above-ground use should not be placed in contact with the ground. LOOK CAREFULLY AT THE END TAG to determine the appropriate end-use for each piece treated wood. Employees in retail lumber yards may inadvertently mix lumber treated to above-ground and in-ground retentions.

Quality Assurance

With the rapid changes taking place in the treating industry it is more important than ever to ensure that wood is being treated to standard specifications. The U.S. Department of Commerce American Lumber Standard Committee (ALSC) accredits third-party inspection agencies for treated wood products. Quality control overview by ALSC-accredited agencies is preferable to simple treating plant certificates or other claims of conformance made by the producer without inspection by an independent agency. The ALSC Treated Wood Program currently has eight accredited independent third-party agencies headquartered throughout the United States and Canada. Updated lists of accredited agencies can be obtained from the ALSC website at http://www.alsc.org. Look for a quality mark or symbol of an American Lumber Standards Committee (ALSC) accredited agency on the back of the end-tag.

Once a bundle is opened, personnel at a retail lumber yard may inadvertently mix wood treated for different end-uses. This picture shows an example of this problem. Look carefully at the end-tag to determine whether or not that wood can be used in contact with the ground.

This document has been distilled from material authored by: Stan Lebow, Research Scientist and Jerrold Winandy, Project Leader, USDA Forest Service, Forest Products Laboratory, Madison, WI and Donald Bender, Professor/Director, Wood Materials and Engineering Lab, Washington State University, Pullman, WA.