Residential Building Inspector B1Floor ConstructionEasy
An inspector is evaluating a floor assembly in a residential basement. The plans indicate the use of a concrete slab-on-ground over a vapor retarder. What is the minimum thickness required for the concrete slab for habitable spaces?
- A4 inches (102 mm)
- B5 inches (127 mm)
- C2.5 inches (64 mm)
- D3.5 inches (89 mm)
Show answer & explanationAnswer & explanation
Correct answer: D. 3.5 inches (89 mm)
The 2021 IRC specifies a minimum thickness for concrete slabs-on-ground. For habitable spaces, the slab must be at least 3.5 inches thick to ensure adequate structural integrity and load-bearing capacity.
Why the other options are wrong
- A. While 4 inches is common, 3.5 inches is the minimum required by the IRC.
- B. This thickness is greater than the minimum required and would be acceptable, but not the minimum.
- C. This thickness is typically insufficient for habitable space slabs and does not meet IRC requirements.
Concrete Slab-on-Ground Minimum Thickness
The IRC specifies a minimum thickness for concrete slabs-on-ground, particularly for habitable spaces, to ensure structural integrity.
- Minimum 3.5 inches (89 mm) for habitable spaces.
- Minimum 4 inches (102 mm) for garage slabs.
- Must be placed over a vapor retarder when required.
Memory trick: Habitable's half-inch more than three.