Special Report: Use of Treated Wood in Roof Assemblies

Treated wood commonly is used in the U.S. construction industry as a component in roof assemblies. In The NRCA Roofing and Waterproofing Manual, Fifth Edition, the National Roofing Contractors Association (NRCA) specifically recommends the use of decay-resistant, treated wood for blocking and nailers at roof perimeters and penetrations for fastening membrane and sheet metal flashings. Many roof product and system manufacturers also make similar recommendations for the use of treated wood.

Recent changes in the chemical treatments used in treated wood have resulted in reports and concerns about corrosion of fasteners and metals that come in contact with treated wood that use specific, current generation chemical treatments. Here is a brief background of this issue provided by NRCA.

Background

Since the early 1930s, the most widely used chemical treatment for treated wood has been chromated copper arsenate (CCA) compounds. CCA-treated wood has proven to perform successfully in many applications, including as components of roof assemblies where non-treated wood''s resistance to insects, microorganisms and fungal decay may be a concern.

As of January 2004, wood preservers voluntarily removed CCA-treated wood from U.S. and Canadian consumer markets as a result of a voluntary agreement with the U.S. Environmental Protection Agency (EPA). EPA cited the arsenic and chromium contained in the CCA treatment as being possible environmental concerns in certain exposed-to-the-weather applications, such as with outdoor furniture and playground equipment.

Wood preservers have introduced a number of CCA-treatment substitutes, including alkaline copper quat (ACQ-C, ACQ-D, ACQ-D Carbonate), copper azole (CBA-A, CA-B), sodium borates (SBX/DOT) and ammoniacal copper zinc arsenate (ACZA). These new-generation treatments contain biocides that do not include arsenic and chromium and are currently acceptable to EPA.

The long-term performance under various conditions of the current, new generation of treated wood products still is largely undetermined. Variations in chemical treatments and other variations that can affect long-term performance including product combinations, installation techniques and environmental conditions make it nearly impossible to predict the new products'' long-term performances. This problem is compounded by the fact there are few standards that adequately define the expected performance of treated wood. For example, no recognized standards exist to measure or define the corrosion resistance of metal fasteners and connectors used in contact with treated wood.

Concern of corrosion

NRCA is concerned with the increasing number of reports and bulletins from the treated wood and fastener industries regarding the increased potential for corrosion when using the current, new generation of treated wood.

Published reports of testing indicate most new, current generation treatment alternatives to CCA are more corrosive than CCA. ACQ compounds and ACZA exhibit more than twice the corrosiveness of CCA, and copper azoles exhibit slightly less than twice the corrosiveness. The high copper content in ACQ and ACZA treatments fosters a galvanic reaction. SBX/DOT treatments may be less corrosive than CCA treatments, but SBX/DOT cannot tolerate exposure to the elements.

A complicating factor is the specific chemical treatment used in treated wood is not always readily identifiable to users by the wood''s appearance, markings or product labeling.

For the roofing industry, the potential for there to be corrosion-related problems with the use of the current, new generation of treated wood is a particular concern. In roof assemblies, treated wood is oftentimes used in as a component that interfaces with or is in direct contact with metal—metal fasteners, metal flashings and other metal accessories, or metal roof decks.

To download a list of NRCA interim recommendations to address the concern of corrosion relating to the use of treated wood, go to www.nrca.net and click on the “Special Reports”.


About NRCA

Established in 1886, NRCA is one of the construction industry''s oldest trade associations and the voice of professional roofing contractors worldwide. It is an association of roofing, roof deck, and waterproofing contractors; industry-related associate members, including manufacturers, distributors, architects, consultants, engineers, and city, state, and government agencies; and international members. NRCA has more than 4,600 members from all 50 states and 54 countries and is affiliated with 105 local, state, regional and international roofing contractor associations.

NRCA is an active and progressive organization of members who share a common purpose and interest. Numerous NRCA committees and specialist teams are responsible for an ongoing development of new ideas, programs and services. More information about NRCA can be obtained at www.nrca.net.