315. The plywood open secret (they WON'T tell you)

[Make Things]

[nova_robotics]

2x4s aren't 1.5x3.5 because they shrink when they dry.

A 2x4 is 1.5x3.5 for historical reasons. You used to buy rough cut 2x4s and then mill them yourself to finished dimensions at the worksite. Sawmills eventually caught on to the fact that shipping all that extra wood made no sense, and just started selling finished cut boards which were SMALLER than 2x4, but it meant that you didn't have to do any finishing at the job site. It has nothing to do with moisture.

[Make Things]

2x4s aren't 1.5x3.5 because they shrink when they dry.

A 2x4 is 1.5x3.5 for historical reasons. You used to buy rough cut 2x4s and then mill them yourself to finished dimensions at the worksite. Sawmills eventually caught on to the fact that shipping all that extra wood made no sense, and just started selling finished cut boards which were SMALLER than 2x4, but it meant that you didn't have to do any finishing at the job site. It has nothing to do with moisture.

You're absolutely right that modern 2x4s are planed to 1.5x3.5 for historical and standardization reasons. But drying still plays a role in how those sizes are chosen...green lumber shrinks as it dries, and mills account for that before final surfacing. So yes, it's both history and moisture.
That said, my video was actually about plywood, where moisture has nothing to do with the undersizing. In contrast to solid wood, plywood is engineered...and modern 23/32" panels are deliberately shaved thinner than their 3/4" labels.

Thanks for your comment!

[TrickieDickie]

I learned very little from that, so the manufacture decided to make the plywood thinner to save money...so that is it. How much more expensive would it be, it is damn expensive now. It appears all plywood is thinner than the posted retail thickness, I remember a time it was not. I thought it has something to do with going metric.

[Make Things]

I learned very little from that, so the manufacture decided to make the plywood thinner to save money...so that is it. How much more expensive would it be, it is damn expensive now. It appears all plywood is thinner than the posted retail thickness, I remember a time it was not. I thought it has something to do with going metric.

Yeah, I hear you...it does feel like a ripoff when the prices go up and the material gets thinner. I’ve definitely noticed that shift too.
The video was just a quick heads-up for beginners about why dados might be loose if they assume plywood is exactly 3/4”. But you’re right...it goes deeper than just "saving money." Some plywood is made to metric sizes (like Baltic birch), but a lot of U.S. plywood is just shaved down to reduce cost and still technically qualify as “3/4” by retail standards. One sheet times millions of cuts? It adds up.
It’s frustrating...but knowing the trick helps avoid sloppy joints.