Building BC - Spring 2007: Page 9 of 11

ACQ Treated Wood and Use of Metal Fasteners and Connectors

Since the withdrawal of the chromated copper arsenate (CCA) treated wood for residential construction in 2004, there has been some concern among builders and others in the industry with the durability or possible premature corrosion of metal fasteners and connectors when used with wood treated using some preservatives,such as alkaline copper quaternary (ACQ).

In response to this concern, the HPO funded a field research study to examine the condition of fasteners and connectors in contact with ACQ treated lumber at four building complexes throughout the Lower Mainland. Additional samples from Vancouver Island and Lower Mainland builders were also reviewed.

This research was carried out with the support and assistance of the Technical Advisory Committee of the Canadian Home Builders' Association of B.C. The research study was completed in the fall of 2006, and is available on the HPO website.

The research study found evidence of premature corrosion. In some cases the corrosion was related to the fact that incorrect metal connectors and fasteners were used in conjunction with the treated wood, in other words, not meeting the recommended G185 level of galvanizing, or use of aluminum. In other cases, however, differing amounts of premature corrosion were observed on galvanized metal components depending on:

• the amount of exposure to weather and rainfall
• contact with incised or unincised treated wood, and
• damage to zinc protective coating to fasteners during nailing.

The study's author, Professor John Ruddick with the University of British Columbia and president of Mychem Wood Protection Consultants Ltd., recommended that more research needs to be carried out to clearly isolate the nature of the problem. This would involve recovering connectors and fasteners to identify the degree of corrosion and the damage to zinc protective coating. It would also examine different wood species that have been treated to assess the impact of preservative retention, mobile copper and residual water, among other factors, on the rate of corrosion.

Prompted by the HPO research report findings, the Canadian Wood Council organized a forum in January 2007 in Ottawa with the participation of all industry participants: residential builders, wood treaters, chemical suppliers, fasteners and connectors manufacturers, regulators, and scientific community representatives. At this national forum current issues, ongoing research, methods of working together and next steps were discussed. From this forum the HPO, in partnership with other industry organizations, is considering further work involving a more in-depth field survey and controlled laboratory research.

The scope and methodology of the research proposal are now being reviewed by interested parties and the project is expected to begin later this year.

Additional information on the use of ACQ pressure treated wood and the compatibility with fasteners and connectors in residential construction can be found in the HPO's Builder Insight #1 ACQ: Alkaline Copper Quaternary Treated Wood, and the online resource guide.