Commercial Building Inspector B2Floor ConstructionMedium
A commercial building inspector is observing the installation of a new wood floor system in a Type III-B construction. The plans specify wood I-joists. According to the IBC, when a floor assembly is required to be fire-resistance rated, what is the minimum nominal thickness of gypsum wallboard required on the underside of wood I-joists to achieve a 1-hour rating?
- A1 inch (25.4 mm)
- B3/4 inch (19.1 mm)
- C5/8 inch (15.9 mm)
- D1/2 inch (12.7 mm)
Show answer & explanationAnswer & explanation
Correct answer: C. 5/8 inch (15.9 mm)
For a 1-hour fire-resistance rating for wood floor assemblies, the IBC typically requires the use of 5/8 inch (15.9 mm) Type X gypsum wallboard on the underside of the joists, as specified in approved fire-resistance rated designs (e.g., from UL or design manuals referenced by IBC).
Why the other options are wrong
- A. 1 inch gypsum is usually reserved for 2-hour or higher fire-rated assemblies, often as multiple layers or specialized fire-rated panels.
- B. 3/4 inch gypsum is not a standard thickness for fire-rated assemblies in this context; typically, multiple layers of 5/8 inch or thicker specialized products are used for higher ratings.
- D. 1/2 inch gypsum wallboard is generally not sufficient to achieve a 1-hour fire-resistance rating when applied directly to the underside of wood joists.
Wood Floor Fire Rating
Achieving a fire-resistance rating for wood floor assemblies often involves protecting the underside with gypsum wallboard, with specific thicknesses and types required for different ratings.
- Type X gypsum wallboard is required for fire-rated assemblies.
- 5/8 inch thickness commonly provides a 1-hour rating.
- Part of a tested and approved fire-resistance rated assembly.
- Protection is for the combustible wood elements.
Memory trick: Wood's fire shield, gypsum's yield, Type X's field.