NASCLA Accredited Commercial General ContractorWoodMedium
A contractor is preparing to install roof sheathing on a low-slope commercial roof. The structural engineer's specifications call for the use of 5/8-inch thick plywood sheathing with a span rating of 40/20. What does the '40' in the span rating '40/20' primarily indicate?
- AThe minimum thickness of the plywood in 1/16th inches.
- BThe maximum recommended span in inches for roof applications.
- CThe number of plies (layers) within the plywood panel.
- DThe maximum recommended span in inches for subfloor applications.
Show answer & explanationAnswer & explanation
Correct answer: B. The maximum recommended span in inches for roof applications.
The first number in a plywood span rating (e.g., 40/20) indicates the maximum recommended spacing of supports (in inches) when the panel is used as roof sheathing. The second number, '20' in this case, would indicate the maximum spacing for subfloor applications.
Why the other options are wrong
- A. The span rating does not directly indicate thickness in 1/16ths; thickness is usually specified separately (e.g., 5/8-inch).
- C. The span rating does not directly indicate the number of plies in the plywood panel.
- D. The second number (20) in a span rating indicates the maximum recommended support spacing in inches for subfloor applications.
Plywood Span Rating
A two-number designation on plywood panels indicating the maximum recommended spacing of supports in inches for roof sheathing (first number) and subfloor (second number) applications.
- Example: 40/20 means 40 inches for roof, 20 inches for subfloor.
- Determined by panel thickness and strength.
- Must match or exceed the framing layout.
Memory trick: Roof first, floor second: that's the span rating's simple word.