Quartersawn lumber stays flat, but flatsawn lumber does not (ironic, I know). Flatsawn lumber cups during the drying process and it even cups after it’s dry if not cared for properly. Wide boards are especially fussy and panel glue-ups can be a giant pain in the tuchus.

I deal with cupped lumber all of the time, and I was reminded of this common problem when a friend of mine was trying to figure out why his wide panel glue-ups had cupped. Whenever I am asked about this, my first question is always, “How did you store your panels after they were assembled and surfaced?” The answer is usually that they laid the panels flat on a table. A quick bit of logic says that a flat panel on a flat table should stay flat, but that isn’t how it works, at least not with solid wood.

Solid wood needs to expand and contract evenly, on both sides, to stay flat. If the panels are placed flat on a table, they can breathe on one side but not on the other. The bottom side will remain as dry or wet as it started, but the top side will shrink or swell depending on the ambient humidity in the room. Usually, this  problem arises when lumber is moved from a non climate-controlled environment (like a garage or barn) into a dry, climate-controlled shop, so the top of the panels will shrink and the lumber will cup up and away from the table as it dries.

This glued up panel couldn’t breathe on the bottom since it was flat on a table. The top dried out a touch after processing and the panel cupped.

In a perfect world, rough lumber would be stored for months in the exact same, hermetically sealed environment where the processing is going to happen, but since we don’t live in a bubble, that’s not really possible. Even if you store the lumber in your climate-controlled shop and build in your climate-controlled shop, the climate still changes – in small increments from day to day and more dramatically from season to season. And, since you know that these changes will make your wood expand or contract, it is even more imperative to store surfaced lumber and panels properly to make sure your flat work stays flat.

Again, storage is the key, and there are two approaches to keep things flat. The most common way is to store the wood so that it can breathe on all sides. This is done by keeping it stacked flat on sticks or by storing it upright at an angle, perhaps leaning against a wall. The other approach is to not let the wood breathe at all and keep it wrapped or covered in plastic. I commonly use both tactics, leaning panels against the wall for short-term storage, usually during a day of processing and then covering them with a sheet of plastic for longer storage. Note that dramatic changes in flatness can happen in just hours if the conditions are right (or wrong, in this case).

From fresh sawn lumber (in this photo) to finished product, storing wood on sticks is the best practice.

For short-term storage (hours to days) standing wood upright is a great choice. Make sure air is able to get to all sides.

Now, let’s say you didn’t follow this advice and your panels developed a cup in them. They were planed and sanded flat and ready to be put into the door frame before you left the shop, but when you returned the next morning they had a noticeable rock. Since everything was already to final thickness, what options do you have? There is no meat left to machine flat and the wood can’t really be bent back into shape… or can it?

No, it can’t really be bent back, but it can be coerced back by doing the reverse of what caused the cup in the first place. The key is understanding the cause of the problem.

First, you need to identify the wet side and the dry side. If you are looking at a cupped panel from the end and it is shaped like a rainbow with the legs down, then the bottom side is the drier side. It is drier, tighter and smaller, and the outside edges are pulling together. The top side is wetter, looser and bigger, and its outside edges are pushing apart. These two forces, one pushing and one pulling, are working together to make a cupped panel.

After you have identified the problem, the solution is to treat the panel to the opposite conditions. This can be done by drying the wet side or wetting the dry side, but since almost all problems in woodworking are from wood that is too wet (at least around here), you should choose to dry the wet side.

I recommend to use a hairdryer for convenience, but on nice sunny days you can put the sun to work for you too. Both work fine, but the sun can fix a lot of panels at a time, quickly and quietly. The sun works great because it focuses all of the drying energy on just one side, and it focuses it on the entire side, not on just one spot like a hairdryer. (Be aware that some woods, like cherry, change color quickly in the sun and may be a better choice for inside drying).

The process is simple. Put the dry side down on a flat surface, one that restricts air movement across the bottom of the wood. The wide board or panel will be sitting like a rainbow, with the two legs down and the center up. Then just proceed to dry the top side, either with the sun or a hair dryer. If you are not in a hurry, you can simply move the wood to a drier environment, like the inside of your house on a cold winter day and let it dry out on the top side overnight. Any way to dry the top side while the bottom remains as it is should do the trick.

Use a hair dryer (like in this photo) or put the panels out in the sun with the wetter side of the wood up to reverse the cup.

Keep an eye on the panels and check them regularly. With a hair dryer you will probably end up propping it up in a position to blow on the panel and check it every thirty minutes. In the sun, check the progress every hour. If you just move them to a drier environment, check them once or twice a day. Even with regular checks it is not uncommon to go too far and overcorrect. If you let the wood bake too long on one side and it starts to cup the other way, just flip it and dry the other side. Eventually, you will get a feel for how long it takes and end up with a flat panel, and now a drier panel (both good things).

Follow these guidelines for flat wood:

1. Build with quartersawn lumber. Quartersawn wood doesn’t cup.
2. Store lumber in the rough. If the lumber goes wonky you will still have extra thickness to machine flat.
3. Store lumber and build in an environment similar to where the piece will end up.
4. Quickly build with lumber after it is machined. Don’t give it a chance to move on you.
5. If you can’t build immediately, store wide boards and panel glue-ups properly. Give them air on all sides or no air at all.
6. Make sure assembled furniture stays flat by finishing both sides of solid wood panels the same. This is especially important on wide glue-ups like tabletops.

Remember, wood moves and changes size all of the time. It is your job as a woodworker to understand how these changes happen, how to prepare for them and how to control them. And, luckily, in the case of wide wood, you may even have the chance to correct them.