Photos courtesy of ProWood

The steps in the process of creating FRTW are 1.) pressure treatment 2.) kiln drying after treatment and 3.) protective (wrapped) storage and handling.

Photos courtesy of ProWood

The two primary FRTW products are dimensioned lumber and plywood.

Structural Considerations

It must be noted that the treatment process can affect the physical and structural properties of the wood. Depending on the wood species (spruce, pine, fir, etc.), the type of product (stud, joist, plywood, beam), and its application (wall, floor, roof), the structural strength originally associated with the untreated wood is reduced somewhat when subjected to the fire-retardant process described above. Therefore, the FRTW manufacturer is required to provide strength adjustments based on the intended use of the wood which must be factored into the structural design of the building.

The structural durability of FRT lumber and plywood is best verified according to the latest and most stringent versions of ASTM strength durability standards. FRTW is routinely tested by independent accredited laboratories, following two directly relevant industry standards: 1.) ASTM D5664 Standard Test Method for Evaluating the Effects of Fire-Retardant Treatments and Elevated Temperatures on Strength Properties of Fire-Retardant Treated Lumber and 2.) ASTM D5516 Standard Test Method for Evaluating the Flexural Properties of Fire-Retardant Treated Softwood Plywood Exposed to Elevated Temperatures. These tests are used to develop strength design factors of fire-retardant treated lumber and plywood, respectively, for various use conditions in buildings and elsewhere.

The National Design Specifications (NDS), Wood Handbook, and other publications have cautioned against the use of any wood product in environments exceeding 150˚F. Based on the strength data generated when tested per industry protocol at an accredited third-party laboratory, professional engineers have calculated design values and span adjustments to modify the untreated design values for lumber and span ratings for plywood. These design values are applicable at temperatures up to 150˚F for lumber and 170˚F for plywood.

Because of these considerations, it is appropriate to request the relevant structural data from manufacturers for their particular products. Commonly, FRT plywood needs to be slightly thicker (approximately 1/8”) than untreated plywood for the same structural application. Dimensional lumber may also have some minor span and strength limitations, although in most cases, it is not a significant design issue.

DESIGNING WITH FRTW IN BUILDINGS

As already noted, building codes allow FRTW to be used in multiple locations in buildings to satisfy structural and fire resistance needs. In particular, both FRT dimensional lumber and plywood can be considered for use by design professionals in some specific locations in buildings.

• Dimensional FRT lumber can be used for all stud framing in partitions and walls where permitted including studs, sill plates, sole plates, jack studs, and headers. It is also suitable for blocking and backers in wood or metal stud framed walls in all building types. It can also be used for roof systems including roof trusses, rafters and related framing and blocking. Floor systems can benefit with floor trusses, floor joists and framing all potential uses of treated lumber. Dimensional FRT lumber can also be used to create various other carpentry items, such as built-up beams, stairs, and millwork.
• FRT plywood can similarly be used for sheathing and enclosure of framed systems including exterior wall sheathing, roof sheathing over trusses or rafters, and floor sheathing/ subflooring. It can also be used on interior or exterior walls for non-structural sheathing or backer boards such as for mounting electrical or other equipment.

In short, FRT products can be used virtually anywhere that untreated wood products can be used, within the parameters of the applicable code requirements.

Typical Wall Assemblies Using FRTW

Like most construction assemblies that are tested for fire resistance, those using FRTW are specifically assembled per typical construction techniques and tested according to the relevant testing standards. Typical exterior wall assemblies are based on FRT 2x4 framing at 16” on center or FRT 2x6 framing at 24” on center plus FRT plywood used as exterior sheathing. Standard batt insulation is assumed between the framing members. The exterior may or may not include house wrap or other air and water membranes plus any common exterior facing. The interior is presumed to be finished with gypsum board (drywall) – 1 layer for a 1-hour fire-resistance rating and 2 layers for a 2-hour fire-resistance rating. Wall assemblies such as these have commonly been tested by UL labs and others and include the details of the parameters of the testing including fasteners, gypsum finish, etc. that need to be included. Copies of these tested assemblies are routinely available from manufacturers.

Sometimes, a question may arise regarding the intersection of a floor assembly with an exterior wall assembly. As we have seen, it is not uncommon for the code to require a 2-hour exterior wall, acceptably built using FRTW, but only requires a 1-hour rated floor or roof. The question then pertains to the intersection of the wall and the floor or roof and how to ensure the proper fire rating is maintained. Flooring and roof members need to rest on or be connected to the wall so it is that area of overlapping or intersecting construction that may be a design concern. If, for example, a 1-hour rated floor can be constructed without using FRTW, but a 2-hour rated exterior wall does need to be FRTW, does the intersecting portion of the floor also need to be FRTW? Those who have looked at this question closely rely on the intent of the code which, in this case, is to contain or compartmentalize fire within a space. The concern is with the interior spaces defined by either the wall sheathing or ceiling sheathing for the floor or roof. The conclusion is that as long as the wall and floor or roof assemblies each meet their independently required fire-resistance ratings, they comply with the code since the continuity of the interior surfaces is maintained.

images courtesy of ProWood

FRTW can be used as the basis for structural walls with either a 1-hour or 2-hour rated assembly as documented by accepted testing standards.

Specifying FRTW

Specifying FRTW is directly linked to its manufacturing process, codes, and testing standards. There are several considerations to address when writing specifications that include it.

• Quality Control: Documentation of a manufacturer’s quality control program for specified FRTW products should be available from manufacturers. Similarly, it is common for a 50-year Limited Warranty to be available against structural failure. Typically, such a warranty protects against a reduction in strength caused by the fire-retardant chemicals.
• Testing: A full range of product testing is recommended, beyond the fire rating tests already mentioned. In particular, hygroscopicity testing conducted by a third-party independent laboratory is the best way to confirm that, compared to untreated wood, fire-retardant-treated wood does not pick up excessive moisture under test conditions. A more complete list of testing includes the following:
• ASTM E84/ UL 723 Surface Burning Characteristics
• ASTM 2768 Extended 30-minute Test
• ASTM E119 Fire Tests of Building Construction
• ASTM D3201 Hygroscopic Properties
• ASTM D5516 Flexural Properties Plywood
• ASTMM D5664 Flexural Properties Lumber
• ASTM D6305 Strength Design
• ASTM D6841 Treatment Adjustment Factors
• AWPA E12 Determining Corrosion of Metal
• AWPA M4 Care of Treated Wood
• AWPA P50 Standard for Fire-retardants
• AWPA T1 Treatment Standard

• On-Site Storage and Handling: Exposure to precipitation or moisture during storage and installation must be avoided since FRTW is not intended for exterior exposure or applications. If any FRTW product becomes wet, it must be allowed to dry before enclosing it in construction. Finished products should be wrapped or bagged and stored 6 inches off the ground and under cover whenever possible. It is best practice to store all FRTW in a dry, covered space on a job site.
• Products: FRTW is available in a wide range of softwood species including Spruce-Pine-Fir, White Spruce, Red Spruce, Engelmann Spruce, Black Spruce; Southern Pine, Lodgepole Pine, Red Pine, Ponderosa Pine, Jack Pine; Douglas Fir, Alpine Fir, White Fir, Balsam Fir, Hem Fir, and Western Hemlock. Only fire-retardant-treated wood that is visibly clean and free of surface residue should be used.
• Labeling: FRT lumber and plywood products need to be clearly identified. Each piece shall be stamped with the critical information required by building code authorities including the UL FR-S designation and any ICC-ES report number. Additionally, all FRTW products should be color-tinted for easy identification on the job site.
• Installation: Structural systems, which include FRT treated lumber or plywood, should be designed and installed in accordance with the adopted building code using the appropriate lumber design adjustment factors and plywood spans from manufacturers’ structural testing information.