In any given year, Simpson Strong-Tie fields several questions about the use of our connectors and fasteners with pressure-treated fire-retardant wood products. Most often asked is whether this application meets the building code requirements for Type III construction, and whether there is a legitimate concern about corrosion. While there haven’t been any specific discussions on this topic in the SE Blog, there have been related discussions surrounding sources of corrosion, such as: Corrosion: The Issues, Code Requirements, Research and Solutions, Corrosion in Coastal Environments, Deck Fasteners – Deck Board to Framing Attachments. This post will explore several resources that we hope will enable you to make an informed decision about which type of pressure-treated Fire-Retardant-Treated Wood (FRTW) to choose for use with steel fasteners and connectors.

One factor contributing to the frequency of these questions is the increased height of buildings now being constructed. With increased height, there is a requirement for increased fire rating. To meet the minimum fire rating for taller buildings, the building code requires noncombustible construction for the exterior walls. As an exception to using noncombustible construction, the 2015 International Building Code (IBC®) section 602.3 allows the use of fire-retardant wood framing complying with IBC section 2303.2. This allows the use of wood-framed construction where noncombustible materials would otherwise be required.

In the 2009 IBC, Section 2304.9.5, “Fasteners in preservative-treated and fire-retardant-treated wood,” was revised to include many subsections (2304.9.5.1 through 2304.9.5.4) dealing with these wood treatments in various types of environmental applications. Section 2304.9.5.3 addressed the use of FRTW in exterior applications or wet or damp locations, and 2304.9.5.4 addressed FRTW in interior applications. These sections carried over to the 2012 IBC, and were moved to Section 2304.10.5 in the 2015 IBC. FRTW is listed in various other sections within the code. For more information about FRTW within the code (e.g., strength adjustments, testing, wood structural panels, moisture content), the Western Wood Preservers Institute has a couple of documents to consult: 2009 IBC Document and 2013 CBC Document. They also have a number of different links to various wood associations.

As shown in Figure 1 below, fasteners (including nuts and washers) used with FRTW in exterior conditions or where the wood’s service condition may include wet or damp locations need to be hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper. This section does permit other fasteners (excluding nails, wood screws, timber rivets and lag screws) to be mechanically galvanized in accordance with ASTM B 695, Class 55 at a minimum. As shown in Figure 2, fasteners (including nuts and washers) used with FRTW in interior conditions need to be in accordance with the manufacturer’s recommendations, or, if no recommendations are present, to comply with 2304.9.5.3.

In Type III construction where the exterior walls may be FRTW in accordance with 2012 IBC Section 602.3, one question that often comes up is whether the defined “exterior wall” should comply with Section 2304.9.5.3 or 2304.9.5.4. While there are many different views on this point, it is our opinion at Simpson Strong-Tie that Section 2304.9.5.4 would apply to the exterior walls. Since the exterior finishes of the building envelope are intended to protect the wood and components within its cavity from exterior elements such as rain or moisture, the inside of the wall would be dry.

There are many FRTW product choices on the market; take a look at the American Wood Council’s list of treaters. Unlike the preservative-treated wood industry, however, the FRTW industry involves proprietary formulations and retentions. As a result, Simpson Strong-Tie has not evaluated the FRTW products. In our current connector and fastener catalogs, C-C-2015 Wood Connector Construction and C-F-14 Fastening Systems, you will find a newly revised Corrosion Resistance Classifications chart, shown in Figure 3 below, which can be found on page 15 in each catalog. The FRTW classification has been added to the chart in the last column. The corrosion protection recommendations for FRTW in various environmental applications is set to medium or high, corresponding to a number of options for connectors and fasteners as shown in the Corrosion Resistance Recommendations chart, shown in Figure 4. These general guideline recommendations are set to these levels for two reasons: (1) there are unknown variations of chemicals commercially available on the market, and (2) Simpson Strong-Tie has not conducted testing of these treated wood components.

The information above is not the only information readily available. There are many different tests that can be done on FRTW, as noted in the Western Wood Preservers Institute’s document. One such test for corrosion is Military Specification MIL-1914E, which deals with lumber and plywood. Another is AWPA E12-08, Standard Method of Determining Corrosion of Metals in Contact with Treated Wood. Manufacturers of FRTW products who applied for and received an ICC-ES Evaluation Report must submit the results of testing for their specific chemicals in contact with various types of steel. ICC-ES Acceptance Criteria 66 (AC66), the Acceptance Criteria for Fire-Retardant-Treated Wood, requires applicants to submit information regarding the FRTW product in contact with metal. The result is a section published in each manufacturer’s evaluation report (typically Section 3.4) addressing the product use in contact with metal. Many published reports contain similar language, such as “The corrosion rate of aluminum, carbon steel, galvanized steel, copper or red brass in contact with wood is not increased by (name of manufacturer) fire-retardant treatment when the product is used as recommended by the manufacturer.” Structural engineers should check the architect’s specification on this type of material. Product evaluation reports should also be checked to ensure proper specification of hardware and fastener coatings to protect against corrosion. Each evaluation report also contains the applicable strength adjustment factors, which vary from one product to another.

Selecting the proper FRTW product for use in your building is crucial. There are many different options available. Be sure to select a product based on the published information and to communicate that information to the entire design team. Evaluation reports are a great source of information because the independently witnessed testing of manufacturers has been reviewed by the agency reviewing the report. Finally, understanding FRTW chemicals and their behavior when in contact with other building products will ensure expected performance of your structures.

What has been your experience with FRTW? What minimum recommendations do you provide in your construction documents?