Residential Building Inspector B1Roof-Ceiling ConstructionEasy
A residential building inspector is reviewing the installation of roof sheathing on a new construction project. The roof has a rafter spacing of 24 inches on center. What is the minimum nominal thickness required for plywood roof sheathing with a span rating of 24/16?
- A1/2 inch
- B3/8 inch
- C5/8 inch
- D3/4 inch
Show answer & explanationAnswer & explanation
Correct answer: A. 1/2 inch
For plywood roof sheathing with a 24/16 span rating, the first number (24) indicates the maximum allowable span when used as roof sheathing. Since the rafter spacing is 24 inches, a 1/2-inch nominal thickness is required. This aligns with IRC R803.2.1.1.
Why the other options are wrong
- B. 3/8 inch plywood typically has a lower span rating and is not suitable for 24-inch on-center rafters.
- C. 5/8 inch plywood is thicker than required for this span rating and rafter spacing, though it would be acceptable.
- D. 3/4 inch plywood is significantly thicker than required for this span rating and rafter spacing, though it would be acceptable.
Plywood Roof Sheathing Span Rating
A two-number designation on plywood panels indicating the maximum allowable spacing for roof supports (first number) and floor supports (second number) in inches.
- The first number is for roof applications.
- The second number is for floor applications.
- The rating must meet or exceed the actual support spacing.
Memory trick: Roof first, floor second, always check the span.