Residential Building Inspector B1Roof-Ceiling ConstructionHard
A residential building inspector is performing a rough-in inspection for a new roof system. The roof has a pitch of 6:12 and uses 1/2-inch plywood sheathing. What is the maximum allowable span for this sheathing if the rafters are spaced 24 inches on center and the sheathing is applied with the long dimension perpendicular to the rafters?
- A24 inches
- B32 inches
- C20 inches
- D16 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 24 inches
According to IRC Table R803.1, for 1/2-inch plywood sheathing applied with the long dimension perpendicular to supports, the maximum allowable span when rafters are spaced 24 inches on center is 24 inches. The table provides span ratings for various sheathing thicknesses and support spacings.
Why the other options are wrong
- B. 32 inches would typically require thicker sheathing, such as 5/8-inch, for 24-inch rafter spacing.
- C. 20 inches is not a standard maximum span for 1/2-inch plywood with 24-inch rafter spacing.
- D. 16 inches is the maximum span for 3/8-inch plywood with 16-inch framing or 1/2-inch plywood with 12-inch framing.
Roof Sheathing Span Rating
The maximum distance between supports (rafters/trusses) that a given thickness and type of roof sheathing can span without excessive deflection.
- Determined by sheathing thickness and composition.
- Influenced by orientation (long dimension perpendicular/parallel).
- Governed by IRC Table R803.1 for wood structural panels.
Memory trick: Sheathing Span: 'S' for 'Spacing', 'S' for 'Strength', 'S' for 'Sheathing'.