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, that’s an increase of only 8%. What’s more, you only get the full effect of this extra thickness if there are no splices in the plywood near the middle of the span, or better yet, no splices at all. For headers at openings wider than 8 feet, that’s not often the case. But it’s these longer headers that will most likely need some extra strength.

Combine these drawbacks with size limitations and the plywood almost never makes a critical difference in safety. What I mean by size limitations is that when I design a header, the numbers may tell me I need two 2x9s. Since two 2x9s are about 30% stronger than two 2x8s, the 2x8s plus 8% from 1/2 inch of plywood wouldn’t be strong enough. And I wouldn’t ask the framer to rip some 2x9s, I’d simply call for 2x10s. What’s more, he’ll probably use double 2x10s for all his headers, big and small. Because headers only come in certain depths, there’s usually extra strength in the 2x10s to begin with. And that extra strength in the 2x10s means that the small extra strength from the plywood is rarely important. But the thickness is helpful.

—C.D.