Commercial Building Inspector B2Roof Ceiling ConstructionMedium
A client is planning to construct a new commercial building with a roof assembly utilizing steel open-web joists. The roof is required to have a 1-hour fire-resistance rating. What is the typical minimum thickness of sprayed fire-resistant material (SFRM) required for steel joists to achieve a 1-hour rating, assuming a standard density SFRM and typical joist sizes?
- A1 1/4 inches
- B1/2 inch
- C1 inch
- D3/4 inch
Show answer & explanationAnswer & explanation
Correct answer: D. 3/4 inch
For steel open-web joists requiring a 1-hour fire-resistance rating, a typical minimum thickness of 3/4 inch of sprayed fire-resistant material (SFRM) is often specified. Actual requirements can vary based on specific joist dimensions, SFRM density, and tested fire-rated assemblies.
Why the other options are wrong
- A. 1 1/4 inches is likely for higher fire-resistance ratings (e.g., 2 hours) or larger/heavier steel members.
- B. 1/2 inch is generally insufficient for a 1-hour rating on steel joists.
- C. 1 inch would provide a higher rating or margin of safety, but is not the typical minimum for 1 hour.
SFRM Thickness for 1-Hour Rating
The minimum thickness of sprayed fire-resistant material (SFRM) applied to steel structural members to achieve a 1-hour fire-resistance rating, typically 3/4 inch.
- Protects steel from high temperatures.
- Thickness varies by rating and steel member size.
- Must be applied per tested assembly designs.
Memory trick: Steel needs a shield, fire's heat it must yield. 3/4 inch is the minimum for a 1-hour field.