What causes wood to warp, and what wood is least likely to warp?

Because they are not dry. When your construction lumber loses moisture it has to change its shape.
Look for a local lumber yard and buy kiln dried wood. If they don’t sell 2×4s for framing walls your in the right place.:)

Good Luck

-- Aj

With construction lumber from the BORG, warpage is caused by :

Looking at it wrong
Humidity
Roll of dice
Time of day
Leap year
Non-leap year
Full moon
Peace in the middle east
Direction of your bunions
Position of jupiter in the sky

Something I’ve learned about lifts of Douglas fir. The best boards are usually on top or the bottom unfortunately the ones on the bottom are scarred badly from fork lifts. And the one on top are warped from direct sun light drying them unevenly. :(
SMP is right a lot depends on your Karma. :)

Good Luck

-- Aj

I’m surprised more haven’t responded. When I buy boards I not only look at straightness, but also look at the end grain pattern on both ends.

We have five sawmills in our area. This is typically want happens to most construction lumber. Most sawmills will leave the logs brought in to them out in the yard for a while before processing. During this time they have the logs sprayed and saturated down with water to soften the bark before running through the mill. Then the logs are run through for debarking. Then go through a cutting process to cut the logs down to size. Then a planning process to size. Then separate the lumber by grade.

With some sawmills, construction lumber is stacked outside and air dried after milled, then bundled and wrapped for shipping once they get to a certain moisture % (not all of them). We have a couple sawmills out here that will kiln dry the lumber to bring down the moisture content (to speed up the drying process time), before restacking-bundling-wrapping and shipping. There’s also sawmills out here that once the lumber is cut-bundled-wrapped, they shipped directly to the distribution centers and stores without even being dried (air or kiln). The stores will say this stuff is ‘right from the sawmill’. I’ve hauled out of each of these locations for 19 yrs. The construction lumber at Menards, Lowes, Home Depot, Builders Square and many other lumber yards I feel aren’t really even air dried. Even treated lumber is treated green and shipped with no real drying period. The only air drying most construction lumber see is while being shipping and sitting in distribution centers and stores. Usually the cheapest priced lumber because they have no actual drying expense.

So with using construction lumber expect more wood movement in the wood. look at the end grain pattern on each end. Read up on some of the wood books that explain ‘shopping for lumber’ and ‘wood movement’. Read-up on Riftsawn, Quartersawn, Plainsawn-Flatsawn, Heartwood, Sapwood, Pith. Plainsawn-Flatsawn is more prone to cup. Every type of warp has a name (Bow, Crook, Cup, or Twist). Read-up on wood movement to understand more on wood movement.

Chapters in this “Wood Handbook,” pretty much explain everything you might want to know & more than any of us really need to know.

https://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

Chapter 6 explains lot about both hard & softwood processing & grading systems:
https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_06.pdf

Nice free reference might want to save & refer back to different chapters as needed.

-- Bill

You can avoid warped lumber fairly easily by careful selection. Here are the basic things I look for when selecting lumber (roughly in order of importance):

• Stick with #2 grade or better labeled kiln dried.
• Obviously ignore any boards that are already showing signs of warping or are obviously wet at the lumber yard. It can be difficult to tell so get yourself a moisture meter and take it to the yard with you.
• As you pull each board off the pile, put one end on the ground and sight down it to see just how straight it is now. If it already some sort of curve to it, it just going to continue warping.
• Start by looking at the ends of the boards on the stack and look for boards that do not have the center rings (sometimes called the pith) of the tree running through them. The first few rings are typically juvenile wood which is usually less dense and more prone to cause warping as well as splits and checks.
• Select boards that have narrower growth rings (this usually automatically eliminates the pith).
• Give preference to boards where the rings run across the narrow dimension and do so consistently from end to end. This quarter sawn or rift saw ring orientation is more stable. Plane sawn boards are okay as long as they don’t show pith or significant diagonal grain.
• Select boards where the grain runs as straight as possible and avoid ones where the grain runs diagonally from end to end.
• When possible, buy wider and longer boards than the dimensions you need. For example if you want 2×4s, buy 2×8s or 2×10s and rip them to width. This gives you a chance to cut pith or juvenile wood and knots out of the board. They generally have to use better quality logs to start with to get longer and wider lumber. With the wider width there is more likelihood of having pith but you can cut it out and by doing so wind up with a board that is more like #1 grade than #2 and you often end up with quarter or rift sawn grain.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

@WoodenDreams Thanks for that explanation. I can remember each time I passed a saw mill they had an array of sprinklers keeping all the logs wet.

The sprinklers are to mostly to keep logs from starting to split at the ends as they wait to be milled. Saw and plywood mills often have to buy logs when they can so that they have inventory when weather or other factors slow down supply and keeping the ends wet will prevent most degradation while the logs sit on the lot. Since they still have the bark on them, this won’t add much moisture to the logs. Plus, when cut, logs usually have the most moisture they are ever going to have.

During a logging and milling course I took in college, we toured a bunch of saw and plywood mills. A few of them even stored their logs in ponds. It was a long time ago so I don’t remember for sure but that may have mostly been plywood mills that were going to soak the logs in hot water 8 or more hours anyway to make them easier to peel.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.