You're installing roof sheathing on a hip roof with a 6:12 slope. The sheathing panels are 4x8 OSB sheets installed with the long dimension perpendicular to rafters spaced 24 inches on center. What is the minimum thickness required?
Correct Answer
A) 5/8 inch
With 24-inch rafter spacing and panels installed perpendicular to supports, Michigan Building Code requires minimum 5/8-inch OSB sheathing for adequate structural performance.
Why This Is the Correct Answer
Michigan Building Code requires a minimum of 5/8-inch OSB sheathing when rafters are spaced 24 inches on center and panels are installed with the long dimension perpendicular to the supports. At 24-inch spacing, the sheathing must span a greater unsupported distance, requiring the thicker 5/8-inch panel for adequate structural performance and deflection control.
Why the Other Options Are Wrong
Option B: 23/32 inch
23/32 inch (approximately 0.719 inches) is slightly thicker than 5/8 inch (0.625 inches) and would exceed the minimum requirement — it is a valid structural choice but the question asks for the minimum, which is 5/8 inch.
Option C: 7/16 inch
7/16 inch is typically the minimum for wall sheathing or for roof sheathing with rafters at 16 inches on center — it is insufficient for 24-inch rafter spacing, as the larger span creates greater deflection and load demands.
Option D: 15/32 inch
15/32 inch is the minimum thickness for roof sheathing with rafters at 16 inches on center with panels perpendicular. At 24-inch spacing, this thickness does not provide adequate stiffness or load resistance.
Memory Technique
Rafter spacing drives sheathing thickness: 16-inch OC → 15/32 inch minimum; 24-inch OC → 5/8 inch minimum. Remember: wider spacing = thicker sheathing. Think 'spacing up, thickness up.'
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