A. Scott McVicker, a structural engineer in Palo Alto, Calif., responds: There are two properties of OSB panels at play here: its strength in shear and its bending strength. When using OSB for corner bracing, shear strength — the ability of the panel to resist lateral racking movement — is the issue, and the panel’s shear strength is not affected by its orientation. But place that same sheet up on the roof across rafters 24 inches on-center and stand in the middle of the span between two rafters: Now the bending strength of the panel is at play. That’s what the arrows refer to.

If you look carefully at the sheet of OSB in question, you’ll see that most of the fibers are oriented parallel with the length of the sheet, making that the strongest axis in bending.

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