AWC's Design for Code Acceptance No. 4 (DCA 4) Component Additive Method (CAM) for Calculating and Demonstrating Assembly Fire Endurance describes a procedure to calculate the fire endurance rating of a wood-frame wall, roof, or floor/ceiling assembly. The procedure is based on combining fire resistance times assigned to each separate component of the assembly without the need for additional fire testing. Section 722.6 of the 2012 International Building Code (IBC) provides the basis for this approach and is limited to determining a maximum fire resistance rating of one hour. A simple example of a one hour interior wall is shown in Figure 1 and the accompanying table. Note that a single layer of 5/8-inch Type X gypsum board is assigned a time of 40 minutes and 2x4 wood studs are assigned 20 minutes for a total of 60 minutes.
One question that is often asked is whether this methodology can be applied to individually exposed wood members. For a large structural member with protective membranes directly applied to all of the exposed surfaces of the structural member, there is no code-referenced methodology in the United States to determine the fire-resistance rating of the member. However, research conducted at the USDA Forest Products Lab (FPL) concluded that the fire resistance of a structural wood member with a protective membrane directly applied to all of the fire exposed surfaces can be obtained by adding a fixed time for the protective membrane to the fire resistance of the unprotected element. The tests indicated that times of 30 minutes for a single layer of 5/8-inch Type X gypsum board and at least 60 minutes for a double layer of 5/8-inch Type X gypsum board will result in estimates for the fire resistance of protected structural wood members consistent with the failure times observed in tensile fire-resistance tests of protected structural wood members.