According to code, the minimum thickness for a concrete slab that is supported directly on the ground shall be _____ Inches.

• A. 4.5 inches
• B. 3 Inches
• C. 3.5 Inches
• D. 4 Inches

Answer: (C)

The main idea here is the minimum slab-on-grade thickness specified by code to ensure enough strength and durability for concrete that sits directly on the ground. A thickness of 3.5 inches provides sufficient flexural capacity and crack control for slabs under typical residential loads when the subgrade is well-prepared and compacted, and joints or reinforcement are used as required. Heavier uses or poorer soils may justify thicker slabs or additional reinforcement, but 3.5 inches is the minimum the code sets for slabs on grade. While thicker options like 4 inches or 4.5 inches are acceptable in practice for specific conditions, they exceed the minimum and aren’t necessary to meet the standard requirement.