Residential Building Inspector B1Roof-Ceiling ConstructionMedium
A residential building inspector is reviewing the installation of roof sheathing. The roof rafters are spaced 24 inches on center. What is the minimum nominal thickness of plywood sheathing required for a roof with this rafter spacing, assuming the sheathing is applied with the long dimension perpendicular to the rafters?
- A3/4 inch (19.1 mm)
- B5/8 inch (15.9 mm)
- C1/2 inch (12.7 mm)
- D3/8 inch (9.5 mm)
Show answer & explanationAnswer & explanation
Correct answer: C. 1/2 inch (12.7 mm)
For rafters spaced 24 inches on center, the minimum nominal thickness for plywood roof sheathing, when applied with the long dimension perpendicular to the rafters, is 1/2 inch (12.7 mm) according to IRC tables for wood structural panel sheathing.
Why the other options are wrong
- A. 3/4 inch is thicker than required for this spacing and would be over-specification.
- B. 5/8 inch would be acceptable but is not the minimum required.
- D. 3/8 inch is typically for 16-inch on center spacing or less.
Roof Sheathing Thickness
The minimum required thickness of wood structural panel roof sheathing is determined by the spacing of the supporting rafters or trusses.
- Plywood and OSB panels have specific span ratings.
- Long dimension perpendicular to supports is the typical installation method.
- Thicker sheathing is required for wider rafter spacing.
Memory trick: Sheathing thickness depends on how far the rafters spread.