Where can I find load ratings for two-by lumber laid flat?

How much load can a 2x4, 2x6, and 2x8 each hold is laying horizontally, yet turned on it's side? We have found numbers for wood turned upright, but it wasn't what we were looking for.

• The critical missing piece to your question is span. Without that, the number be almost anything. Commented Nov 22, 2016 at 16:16
• Would the load rating not be the same as the thinner dimension? ie, would the load rating of a sideways 2x4 would be the same load rating as a 2 2x2s?
– DA01
Commented Nov 22, 2016 at 17:27

The reason you can't find tabulated information on dimension lumbar laid flat (usually called "weak axis bending") is because the strength is so small that there is virtually no structural application. The only time you would use 2x lumber like that is something like flooring where the boards are almost continuously supported and are carrying very little weight.

However if you want to get a sense of the strength you can compare the moment of inertia of the various cross-sections and orientations. Bending strength is proportional to that value.

I have done that math for you:

• a flat 2x4 has about 20% the strength as a standing one
• a flat 2x6 has about 7% the strength as a standing one
• a flat 2x8 has about 4% the strength as a standing one

You can see that especially for the deeper joists you lose practically all of the strength.

• I'm designing an application where there will be load carried on a horizontal member kept under tension, so the load will essentially be transferred horizontally to a vertical beam. That means an upright, eight-foot 2x12 has a 1/4 inch metal rod (with a threaded end) mounted through its narrow side, and 6,000 pounds are siting on those metal rods (6 from bottom to top). So about 3,000 pounds of spaced lateral load. Commented Aug 6, 2019 at 23:55