Residential Building Inspector B1Floor ConstructionEasy
A builder is constructing a floor system where the subfloor will be 3/4-inch oriented strand board (OSB) with tongue-and-groove edges. What is the maximum allowable spacing for floor joists when using this subfloor material, according to the IRC?
- A32 inches on center
- B19.2 inches on center
- C16 inches on center
- D24 inches on center
Show answer & explanationAnswer & explanation
Correct answer: D. 24 inches on center
IRC Table R503.2.1.1(1) specifies that 3/4-inch oriented strand board (OSB) subfloor with tongue-and-groove edges can be installed over floor joists spaced a maximum of 24 inches on center.
Why the other options are wrong
- A. This spacing is generally too wide for typical residential subfloor materials without supplemental blocking or thicker panels.
- B. This spacing is often used for 19/32-inch OSB or plywood.
- C. This is a common spacing but not the maximum for this subfloor.
Subfloor Joist Spacing
The maximum on-center distance between supporting floor joists, dictated by the thickness and type of the subfloor material to prevent excessive deflection.
- Thicker subfloor allows wider joist spacing.
- Tongue-and-groove edges increase spanning capability.
- IRC tables specify maximum spacing based on subfloor type and thickness.
Memory trick: Subfloor Thickness Dictates Joist Spacing.