Summary

New preservatives may cause failure of structures dependant on inadequately protected: 

• Steel hangers and hardware for wood framing
• Aluminum flashings and cladding
• Nails and fastenings
• Anchor bolts

Background

For many years, the most common preservative used in pressure treated wood for building construction applications was Chromated Copper Arsenate (CCA). Commencing in early 2004, the pressure treated wood industry ceased producing CCA treated wood for many residential uses. A number of replacements have been developed including Copper Azole (CA), Alkaline Copper Quaternary (ACQ) and Sodium Borate (SBX). CCA treated wood is still available in Canada but it is limited to a number of specific applications including utility poles, plywood, shingles/shakes and permanent wood foundations. 

With the change, ACQ and CA will almost certainly be the preservatives used in generically designated “pressure treated” sawn lumber in use in building construction. SBX treated lumber is available but would need to be specified as such.

Some of these “next generation” wood preservatives are significantly more corrosive to metal than was CCA treated wood. Current information suggests that this increase in corrosion particularly applies to CA (and ACQ but not to SBX) 

The increase in corrosiveness results from the higher proportions of copper used in the formulation, and affects metals commonly used in construction including aluminum sheet, steel fasteners and connectors that come into contact with the wood. 

Practices and customs that were adequate with CCA treated lumber may not be appropriate for use with the new products.

The use of hot-dipped galvanized or stainless steel fasteners with the pressure treated wood, and avoidance of contact of aluminum with pressure treated wood have been long standing recommendations; although, in practice, these have frequently been overlooked with no serious consequences (other than pitted aluminum flashings). With the new preservatives these recommendations should no longer be ignored. Thus it is important to specify and obtain appropriate corrosion resistant materials to ensure adequate service life for fasteners and connectors. 

Architects and their consultants are advised to review their designs, specifications and site review activities in recognition that this increased corrosion can lead to loss of service life or early failure of components that are not sufficiently corrosion resistant including steel hangers and connectors, screws, nails, anchor bolts, and sheet metal flashings and cladding which are in contact with the wood.

Most of the available recommendations focus on: 

• ensuring that the protective coating on the fasteners and connectors is more durable and long lasting than may have been provided by prior practices,
• eliminating incompatible contact between the wood preservative’s chemicals and metals.

The following recommendations have been derived from several industry websites. Most of these are USA based with ASTM references. CSA equivalents have been provided for reference but care should be taken to confirm the appropriateness of these. It may be anticipated that further information and recommendations will become available as awareness of the issues evolves. 

• Aluminum should not be used in contact with pressure treated wood. Spacer materials or other physical barriers are recommended to prevent direct contact of pressure treated wood with aluminum products. 
• Reliance on a “generic” galvanizing requirement may not be sufficient. Currently available information recommends use of hot-dipped galvanized fasteners meeting ASTMA153 Class A or B1 G185 (CAN/CSA-G164 minimum zinc coating of 600 g/m2) and connectors meeting ASTM A653 Class G-185 sheet (CAN/CSA-G164 minimum zinc coating of 600 g/m2) or better. These recommendations would substantially increase the thickness of the protective zinc coating compared to the minimum typically available as being “hot-dipped galvanized”. 
• Electroplated galvanized fasteners are not recommended as the thin coating is likely to erode or chip leaving the underlying steel exposed. Coated products such as “deck screws” are not recognized as providing appropriate protection against these chemicals unless specifically recommended by the preservative or fastener manufacturer. 
• Stainless steel fasteners and connectors are recommended for use with treated wood for “optimum performance and longevity” and for “severe” applications. “Severe” can be taken to mean any location that may be exposed to moisture accumulation. Where stainless steel is used, types 304 or 316 are recommended. 
• Avoid mixing stainless steel with galvanized steel. The dissimilar metals can cause loss of galvanization and protection. 
• Specific types of pneumatic nail-guns may be required for hot-dipped galvanized fasteners. Stainless steel fasteners may be an alternative (where available) for conventional nail-guns. 
• SBX is not recommended where it will be exposed to liquid water, as the preservative will leach out. Typically it is limited to interior uses such as sill plates and other framing components and exterior above ground uses under a well maintained three-coat paint finish.

The National Building Code and the Ontario Building Code require the use of Pressure Treated Wood in a limited number of specific applications including:

• Wood in direct contact with the ground or framed into concrete below ground level. 
• “Structural wood elements” within 150mm of ground. 
• “Wood framing members” without a dampproof membrane separating the wood framing member from concrete in contact with the ground. 
• In termite areas, pressure treated wood is required for “structural wood elements” unless the clearance to ground exceeds 450mm. 
• In high rainfall areas, building components where moisture accumulation may be anticipated. 
• Retaining walls over 1.2m high or stabilizing earth supporting building foundations.

Procedure

1. Consider carefully whether the use of pressure treated wood is appropriate, advantageous, or required for the specific application and circumstances. Consider whether there are alternatives that will provide adequate performance. Be aware that corrosion resistant fasteners may also be recommended for some “naturally decay resistant” woods.
2. Familiarize yourself with the range of preservatives and the recommendations respecting the appropriate use of each.
3. Specify a specific preservative chemical formulation suitable for the intended use. Although CA and ACQ will be most often encountered in ordinary use, there are other products that are substitutes for CCA, including SBX, There are variations of formulation within each general classification. It can be anticipated that new formulations will evolve. CCA is still available for limited, appropriate building construction applications.
4. Specify the standard of protection required on any fasteners and connectors that will be in contact with pressure treated wood.
5. Review the various fastener and connector manufacturers’ specific recommendations as to which products they endorse for use with each type of wood preservative chemical formulation. Much of the available information concerning recommended fasteners and connectors for use with these new products originates with the fastener manufacturers.
6. Learn how to identify the correct fasteners and protective coatings and check for general conformity to your specifications at the construction site. The recommended increases in protection represent an increase in cost over the prior “generic” or “standard” practices, which may provide a disincentive to strict adherence.
7. Review the issues with other consultants engaged on the work to ensure that their specifications and review services reflect these concerns.

Resources

This bulletin has been prepared with assistance of Dr. Paul Morris of Forintek Canada Corp., Canada’s national wood products research institute. 

The Canada Wood Council and Forintek Canada Corp.’s shared website, www.durable-wood.com contains a number of articles concerning pressure treated wood, including a link to information on related CSA 080 standards. It is anticipated that more detail on the new preservatives and appropriate protection of fasteners will be forthcoming, and accessible from:

www.durable-wood.com/treated/index.php 

Other website references that address the issues include:

Canadian:

www.citw.org/using_specifying/residential/ptwfacts.htm 

Canadian Institute of Treated Wood

American:

www.csinet.org/s_csi/doc_specifier_article.asp?TRACKID=&CID=851&DID=8269

Construction Specification Institute

www.strongtie.com/productuse/corrosion.html?source=hppromo

Simpson Strong-Tie Co.

www.osmose.com/acrobat/NaturewoodFastenerInfoSheet.pdf 

Osmose Holdings Inc.

www.treatedwood.com/fastener.pdf

Chemical Specialties Inc.

www.ufpi.com/product/ptlumber/fasteners.htm 

Universal Forest Products, Inc.

Disclaimer

The above is neither exhaustive nor complete, and is intended solely for the purpose of introducing the issues. It is specifically not intended as a substitute for your own research, education and conclusions on the subject. It is your responsibility to adequately inform yourself and to exercise your own professional judgment respecting the matters addressed.