A. This is true for wall sheathing in many instances, but not for roof sheathing. To understand why, we need to look at how the grain of the plies is oriented relative to the direction of the applied force. Each layer of wood in plywood is oriented either parallel or perpendicular to the long direction of the sheet. Most of the shear force is resisted by those plies whose grain runs parallel to the direction of the applied force. So for 3-ply plywood, for instance, which has two face plies running parallel with the long dimension of the sheet, and a single central ply running perpendicular, most of the wood fibers are oriented parallel to the length of the sheet, so that is the plywood’s stronger direction.

This fact is reflected in the Uniform Building Code’s nailing schedule for structural panel shear walls (1997 UBC, Table 23-II-I-1), which permits the allowable shear for 3/8-inch and 7/16-inch panels, if oriented horizontally across the wall studs, to be increased to that of corresponding 15/32-inch panels. As plywood gets thicker, this rule is...

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