Residential Building Inspector B1Floor ConstructionMedium
A builder is installing a wood structural panel subfloor over floor joists spaced 16 inches (406 mm) on center. The subfloor will receive a resilient flooring finish directly. According to the IRC, what is the minimum nominal thickness required for the wood structural panel subfloor?
- A7/8 inch (22.2 mm)
- B5/8 inch (15.9 mm)
- C3/4 inch (19.1 mm)
- D1/2 inch (12.7 mm)
Show answer & explanationAnswer & explanation
Correct answer: B. 5/8 inch (15.9 mm)
For joists spaced 16 inches on center, the IRC requires a minimum nominal thickness of 5/8 inch for wood structural panel subflooring when a resilient flooring finish is applied directly over it. This ensures adequate stiffness and prevents deflection.
Why the other options are wrong
- A. 7/8 inch is thicker than necessary for this application and exceeds the minimum requirement.
- C. 3/4 inch would be acceptable but is not the minimum requirement in this scenario.
- D. 1/2 inch is typically allowed only if a separate underlayment is installed over it, or for joists at 12 inches on center.
Subfloor Thickness (Wood Structural Panel)
The minimum thickness of wood structural panel subflooring is determined by joist spacing and the type of finish flooring, to ensure adequate support and prevent deflection.
- Depends on joist spacing and finish type.
- 5/8 inch is common for 16" o.c. joists with direct resilient flooring.
- Refer to IRC Table R503.2.1.1(1) or similar.
Memory trick: Sixteen on center needs five-eighths strength.