A. The most important thing the plywood adds is thickness. Of course, the plywood does add some strength, but for several reasons engineers almost never count on this strength in their designs.
Only the layers of plywood with the grain oriented horizontally (parallel with the direction of the header) are really adding any strength. A quick look at the thicknesses involved shows that the additional strength is small. If half the layers in 1/2-inch plywood are horizontal, that’s 1/4 inch of extra material. Compared with 3 inches of 2x10,...
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