The fire resistance rating of a wood frame assembly is determined by adding the time contributions of its key protective and structural elements.
For wood frame assemblies, the calculation of the fire resistance rating is based on summing the time assigned to different parts of the construction that contribute to its ability to resist fire penetration and structural integrity under fire conditions.
Based on common methods used in building codes and standards, the fire-resistance rating of a wood frame assembly is equal to the sum of the time assigned to the membrane on the fire-exposed side, the time assigned to the framing members and the time assigned for additional contribution by other protective measures such as insulation.
This means that the total fire resistance time is the cumulative result of how each component performs when subjected to fire. The calculation involves adding the individual time values attributed to:
- The Membrane on the Fire-Exposed Side: This protective layer, often gypsum board, is the first line of defense. Its thickness and type significantly contribute to delaying the fire's progress.
- The Framing Members: The structural wood studs or joists themselves have an inherent ability to withstand fire for a period as they char, maintaining some structural capacity.
- Additional Protective Measures: Other materials integrated into the assembly, such as insulation, can provide further fire resistance by reducing heat transfer and protecting the framing for a longer duration.
By summing the time contributions from these three elements, the overall fire resistance rating for the wood frame assembly is established. This rating represents the duration, typically in minutes or hours, that the assembly is expected to maintain its fire resistance performance criteria in a standard fire test.
