Commercial Building Inspector B2Floor ConstructionMedium
A commercial building inspector is reviewing the structural plans for a new three-story office building. The plans show a floor system utilizing 2x10 wood joists spaced at 16 inches on center. The designer has specified a required fire-resistance rating of 1 hour for the floor assembly. Which of the following methods is typically used to achieve this fire-resistance rating for a wood-framed floor assembly?
- AApplying a 5/8-inch Type X gypsum wallboard ceiling directly to the underside of the joists.
- BReducing the joist spacing to 12 inches on center.
- CUsing fire-retardant treated wood (FRTW) for all joists.
- DIncreasing the joist size to 2x12 members.
Show answer & explanationAnswer & explanation
Correct answer: A. Applying a 5/8-inch Type X gypsum wallboard ceiling directly to the underside of the joists.
Applying specific thicknesses and types of gypsum wallboard to the underside of wood joists is a common and effective method to achieve required fire-resistance ratings for wood-framed floor/ceiling assemblies, as documented in UL or other listing services.
Why the other options are wrong
- B. Reducing joist spacing increases structural capacity and stiffness but does not inherently provide a fire-resistance rating to the assembly.
- C. FRTW primarily reduces flame spread and smoke development, but typically does not contribute to a fire-resistance rating of a wood-framed floor assembly without additional protective layers like gypsum board, unless specifically tested as a complete assembly.
- D. Increasing joist size primarily affects structural capacity and deflection, not fire resistance unless specifically tested as part of an assembly.
Wood Floor Fire Rating Methods
Techniques used to achieve specified fire-resistance ratings for wood-framed floor assemblies.
- Gypsum wallboard is the most common method.
- Specific thickness and type of gypsum are critical.
- Assemblies must be tested and listed (e.g., UL).
Memory trick: Wood needs a 'Gyp'sum shield to resist the fire.