Flatten lumber

Nail it down.

The real answer is probably put it in a container large enough to pump steam around it for a couple of hours, take it out and bend the hot board and clamp to let it dry and set. Seen it done for making furniture but who in their right mind does it for a 2X4? Easier to buy a straight piece.

I think he meant a 2 foot x 4 foot piece, not a 2x4

As someone else said, nail (or screw) it down. Or are you not going to use it for underlayment?

"underlayment lumber" What's that?

When I hear "lumber", I think of a board. (2x4, 2x8, 4x4, etc) When I hear "underlayment" I think of sheetgoods, e.g. plywood (luan, perhaps), particle board, maybe even MDF depending on the application.

What type of product are you asking about? What are you going to do with it/why do you need to flatten it?

A piece of luan underlayment could be "flattened" by attaching it to a frame made of "lumber". ;-)

Assumed plywood. Probably right, but definitely assumed.

OK. Italian builder was building a house and sent his brother to the lumber yard to buy 2X4's. The yards man asked the brother how long he wanted them. Brother said he'd ask the builder. He comes back and says, "We want them a long time. We building a house."

You must not buy much lumber.

Straight flat pieces are the exception.

Andy

I want to make a small shelf with it.

Underlayment is a plywood that is 1/8 inch thick.

Perhaps if you gave us some more details, we could offer something useful.

For now, I'll go back to my original thought. Make a frame and attach the p lywood to that. A 1/8" thick shelf is going to need a lot of support anyway.

Not so.

While it is true that 1/8" plywood is often used as underlayment, "underlayment" is not any specific size or type of material.

That would depend on where you buy your lumber.

Allow me to quote from an article I ran across just last night:

"Beginners often start by perusing the limited options available at home centers but are eventually drawn to the greater variety and quality available at lumberyards and specialty hardwood stores."

Then there's mail order, Craigslist, reclaimed wood dealers, etc.

If you are serious about woodworking, don't limit yourself by shopping where the crap is sold. I've built projects from red oak that I've bought on-line, reclaimed Douglas Fir that I found on Craigslist, etc. All perfectly straight.

I still have a bunch of the reclaimed Doug fir that was used to build this bed. Not a warped or twisted board in the lot. Some pieces were over

You need to tell that to Home Depot as that is what it is labelled as.

Andy

Most wood is air dried instead of kiln dried.

Kiln dried wood has around 6% moisture as opposed to 15% moisture for air dried wood.

Higher moisture content = more warpage.

Andy

o hold much of anything.

I'll go with a frame made of 2x2 inch wood.

Shelf will support a DVR which weighs less than 5 lbs.

If wood was flat I could use simple shelf brackets.

Andy

to hold much of anything.

he plywood to

Or you could just go get a flat board or a flat piece of anything.

If you are using that higher math to support your claim that "Straight flat pieces are the exception" I still say you are shopping for your wood in all the wrong places.

Not my job.

Besides, I don't use Home Depot's labeling as my go-to source for definitions. They call lots of things "underlayment" and you know what? If you use the Material drop down on this page, they don't even list "wood" as a choice for an underlayment material, yet they label one of their wood sheet goods as such.