FR Assemblies

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Keep the list of Western treaters that make and supply fire retardant treated lumber and plywood handy by downloading the Sources - Fire Retardant Treated Wood.

Fire-Resistance Rated Assemblies

To assist designers in meeting a structure's fire requirements, building codes allow the use of pre-designed fire-reisstance rated assemblies. These assemblies combine the fire resistance of all of the materials used to achieve a one-hour or two-hour rating, as determined by testing the specific assembly design. For less common assemblies, there is an alternative code-accepted process to calculate the fire resistance based on the materials used.


The most common fire-rated assemblies are floor/ceiling systems and wall assemblies. These are rated for fire exposure to one side or on both sides, depending on the application.

Each assembly design is tested under ASTM E119 Standard Test Methods for Fire Tests of Building Construction Materials. Testing is done by third-party laboratories, such as Underwriters Laboratories (UL), which certify the assemblies meet the prescribed standard.

Assembly performance under fire

The one- and two-hour ratings are intended to describe how long the assembly will maintain its structural capabilities when exposed to fire. During testing, each assembly is loaded to 100 percent of its design load and then exposed to flames to determine its endurance performance.

Fire-rated assemblies define a prescriptive list of materials used and the configuration of such materials. These include fire-retardant-treated lumber and/or plywood, gypsum board, insulation batts and blankets, and in some designs, fasteners. For assemblies rated for exterior exposure to fire, there are required exterior facings such as fiber cement siding, stucco or brick.

Design specifications

Each assembly design offers a diagram for the confirguation of the materials needed to achieve the specified fire-resistance rating. This includes spacing of framing members and minimum dimensions for insulation, gypsum board or boards and exterior facings.

Fire retardant makers each have developed and tested assemblies utilizing lumber and plywood treated with their formulations. The most common assemblies they offer are one- and two-hour rated bearing walls.

Use the links below to review certified assemblies for the following fire retardants:

Alternatives to tested assemblies

In addition to tested assemblies, the building codes recognize an alternative process called Component Additive Method, or CAM, for calculating fire resistance. The CAM process was developed in the 1960x and can be used for calculating the fire resistance of load bearning and non-load bearing floor, wall, ceiling and roof assemblies.

For wood assemblies, the CAM process uses the provisions of Section 722.6 in the International Buiilding Code. The full process, including sample calculations, are detailed in the publication DCA 4 - CAM for Calculating and Demonstrating Assembly Fire Resistance published by the American Wood Council.