Editor’s note: This column originally published in 2020 was produced before Gene Wengert died in 2025. It is printed here in memoriam.

Q. We are a medium-sized shop and find ourselves with a little bit of extra KD lumber that we want to keep safely without a change in moisture. How can we do this?

A. There are several ways to accomplish the storage of lumber. All involve keeping the lumber stored at an average relative humidity that is in equilibrium with the moisture content (MC) that you want.

As a rough rule of thumb, 30 percent RH is 6 percent MC; 37 percent RH is 7 percent MC; 44 percent RH is 8 percent MC; and 50 percent RH is 9 percent MC.

You can control the humidity with a complex system using electric dehumidifiers, or you can make it fairly simple.

Simplicity is achieved when you realize that whenever we have a closed room, if we heat the air in the room, the RH will drop. 

For example, foggy air at 55 degrees F will drop to around 30 percent RH if the air is heated 35 degrees F or up to 90 degrees F.

So, if we have a small room for storage, we put a humidistat in the room that is wired to turn on the heat whenever the room’s RH is too high. 

The key to success of this simple system is that the lumber must be already dry. 

If it is a little high in MC, then the little room will behave more like a kiln as the low humidity will dry the lumber. In this case, we would need some vents to exhaust this excess moisture.

Another simple system is to have lumber at the correct final MC put into a tight shipping container. 

Tight means no leaks and tight-fitting doors. 

Consider that if this container is loaded with 3,000 board feet of oak, that would weigh about 7,500 pounds, and it will take a 1 percent weight increase of 75 pounds of water to increase the moisture by 1 percent MC.

When we load this container, even if it is humid outside, the air in the container will have only a pound or two of water vapor. 

So, when we close the door, the moisture in the air will not be enough to change the moisture of the lumber by even 0.1 percent MC.

Of course, the more often we open the door when it is humid outside, the more chance that the tiny moisture changes will accumulate; so, open the door only in the afternoon when the RH is much lower than in the early morning. Note that if the roof of the container is painted a dark color, the little extra solar heat will drop the humidity inside for a few hours and can offset short periods of an open door.

Q. We made a table out of 8-inch-wide pieces of lumber, and I can guarantee that the top was flat when we shipped it, but now the customer has said that the individual pieces are cupped. Indeed, when I inspected it, he was correct. We were careful to make sure that all the pieces had the grain running the same direction (heart side down) to give a nice look. We did screw the top to the metal frame but the screws pulled out. What advice can you give?

A. First, it is important that you recognize that wood does not cup unless its moisture content changes. We might argue that the customer’s office is too dry or that the wood we used was a bit high in moisture. In either case, or a little of both, the wood pieces will cup slightly. So, you need to pay more attention to matching the MC when you make a top with the MC that it will have in service.

On an expensive table, measure the MC of every piece. After manufacturing, you should probably use a large plastic bag to keep the moisture from changing during storage and shipping.

Bottom line: With no or very little MC change, there will be no appreciable cup of the individual pieces. Note that a good finish will also slow any moisture changes which also reduces warping; slow and small moisture changes are “absorbed” by the wood without warp.

Another factor: By aligning the grain, you do achieve a striking grain pattern, but you also accumulate cup across the entire width. Therefore, with wide pieces, we do suggest alternating the pieces up and down.

Incidentally, it is worth noting that the amount of cup in an 8-inch-wide piece is reduced to 1/4 (not just 1/2) if you were to rip the piece into two 4 inch wide pieces. This is why we see so many panels made with 2 inch or 3 inch wide strips.

Cupping forces are fairly large, so it is very difficult to restrain cupping with frames and screws.