On Thursday, July 2, 2020 at 12:59:53 PM UTC-4, pyotr filipivich wrote:
Sonny on Thu, 2 Jul 2020 05:05:26 -0700 (PDT)
typed in rec.woodworking the following:
On Thursday, July 2, 2020 at 12:11:00 AM UTC-5, DerbyDad03 wrote:

For 2 x 4 walls, you should seriously consider
adding a piece of 1/2" ply or OSB between the 2 x X's to build it out to the full width of the 2 x 4 top plate.

+1. Do you understand what Derby is saying, here?

Yes. Adding a plywood "stiffener". expansion.

Just to be clear, "stiffener" has (next to) nothing to do with it.

If your walls are 2x4, they are 3.5 inches "thick". A double 2x(4,6,8) header
is only 3". The 1/2 ply/OSB is to make the header as "thick" as the walls so
everything is flush when (if) you want to finish the wall, inside or out, as well as fully filling the space for maximum support.

The addition of the 1/2 ply doesn't add much strength when you consider that
probably half of the laminations are running the wrong way and not helping
at all.