Flatsawn Lumber Is Not So Flat: How To Fix Cupped Wood

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.

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.

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.

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.

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.

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About wunderwoods

Hi! My name is Scott Wunder and I am the owner of WunderWoods Custom Woodworking. We build wine cellars, built-ins and furniture from local woods, here in St. Louis, MO. Recently, I finished a three-year term as the President of the St. Louis Woodworkers Guild, which had me writing a monthly article for our newsletter. I love to write, especially about wood, and found that I still had more to say. Every day I run into something wood related that I realize some of my customers don't know and this seems like a great forum for sharing what I have learned (instead of telling the same story to each person). The main thing to remember is that I try to keep it light and as my wife always reminds people that have just met me, "He is joking."

26 responses to “Flatsawn Lumber Is Not So Flat: How To Fix Cupped Wood”

  1. Timothy Gibbs says :

    Hi, Scott. Thank you for the education. I’ve enjoyed reading this post, and must say that if your woodworking abilities are as strong as your writing, you’re in good shape (which is more than I can say for my two, gigantic slabs of Karri wood.

    The slabs in question are roughly five meters in length x 3 meters wide, and about 10 cm thick. They were stored in Western Australia, at the timber company, but not as thoughtfully as you have prescribed. Unfortunately, they’ve cupped pretty severely, leaving “waves” which span the width of each slab about 70 cm apart.

    The good news is that at least one of these slabs is not going to become a tabletop, but rather an enormous piece of wall art, so the cupping in its case causes only a visual disturbance – nonetheless, I want to get rid of it. My question is, will the reverse drying method you’ve described above work for these slabs? Or is it time to pullout the electric handplaner?

    Thank you for your time.


    – Tim

    • wunderwoods says :

      First of all, I want to see some photos of the wood you are talking about so I can get a feel for the issue, plus I just want to see a piece of wood that big. Second, I think that yo may be able to coax the wood a bit, but it is a taller issue with rough wood. The same principles apply, but there will be some planing, no matter how you approach it. Send over some photos.

  2. Wade says :

    I’m new to woodworking. I’ve been using 1/4″ thick flatsawn cherry and walnut to make trivets and coasters. I just had a large beautiful piece of walnut cup in my garage. This post was very helpful. Thanks! My question is, once it’s cut into 4″ circles and even up to 8″ diameter circles, will it still cup then? Or is it stable once I’ve cut it that small? I then seal it with a benwood matte sealer hoping that adds to the stability too? Would love your thoughts! Thank you much!

    • wunderwoods says :

      Wood cups no matter what size it is. The amount of cup is just more visible or obvious in wider pieces. Cutting it smaller will not stop it from cupping, but it will bother you less. The wood cups because it is wetter on one side than the other. If you do anything that makes one side wetter or drier than the other, the wood will cup. Sealing the wood only slows down the moisture loss or gain in the wood, thereby slowing the rate of cupping, but it can still cup.To keep the wood flat, do your best to allow both sides of the wood to breathe. Store your wood cutting boards standing up, either in a drying rack or a cabinet, leaning against the wall. Adding feet to the bottom of the boards also helps to keep them off of the countertop, allowing the bottom of the board to breathe as well. Avoid putting a wet board without feet flat on a countertop. It will dry on the top and stay wet on the bottom, and it will cup for sure.

  3. Joe says :

    Just wanted to comment and thank you for your blog posts. I come across them frequently when I’m googling different woodworking questions.

  4. Steve Wurgler says :

    Thank you for your informative post. I was hoping you could help me. I have a 20″ wide piece of 170 year old barn wood that I want to make into a dresser top, but it is cupped rather badly. Any suggestions. It is about as dry as paper throughout.

    • wunderwoods says :

      Most of my tricks work on wood that was machined flat after sawmilling and drying and then cupped. Barn wood is usually installed wet, so the cupping is now “built-in” to the wood. On barn wood we often have to do some ripping, flattening and reassembling. I know you don’t want to do it to a 20″ wide board, but you will probably have to. Your other choice is to thoroughly soak the wood until it flattens out on its own, meaning it went back to its original state and then redry with clamps and cauls. We have had some success with this method, but still not totally flat and it takes awhile to do.

  5. Diane says :

    Great article and the first to make sense to me regarding what happened and why and isn’t saying to plane or cut. thanks! So I have an oak 1 by 6 that was cut to make a floor transition ramp of sorts so its thickness is not consistent as it is a wedge now. So I had it in my garage workshop for months laying flat on my workbench while I sanded, stained, and poly coated the top only. Now the thinnest section has cupped upward away from the table by 2 mm…which would mean it isn’t flush with the floor and forcing it down will likely crack it. It is winter here in Ohio so the sun thing is likely out. I tried the hair drier with no luck but it is 8 ft long so it difficult to heat throughout and consistently. Also the ‘dry side’ gets hot when I do this since it is so thin at the wedge part so I don’t know if it’s making the drier part even drier. Now I have it laid with the unfinished side up in the house. Do you think this will work since it’s likely been cupped for a while? Will it work due to the wedge and uneven thickness? If it does work and go flat, I will poly all sides evenly. Any other tips on prep or install so it doesn’t recup after install when one side has no airflow?

    • wunderwoods says :

      Assuming that your lumber was kiln dried when you started, the most likely scenario is that the unfinished side soaked up humidity from being outside and began to swell, causing the cup. If this is the case, simply bringing the piece inside to dry out the unfinished side will bring it back around to flat. After it is flat, finish the bottom with the same number of finish coats as the top to help keep things in place. By the way, it seems like you really paid attention and have a firm grasp of the situation.

  6. Jason Wheeler says :

    i have a 40″ w kanpher counter bar top that is starting to rainbow on me after i finished it with bar top resin only on the top and sides. what do i do ?

    • wunderwoods says :

      The top is moving because only one side was finished. The unsealed side is drying more than the sealed side now that you have brought it inside into a climate controlled and dry environment. The top will eventually level out after the entire thickness of the wood reaches the same moisture content. In the summer the cup may even reverse as the unsealed side gains moisture. As long as the bottom remains unsealed you will have wild swings in humidity and flatness. At some point when the wood looks flat, it would be a great idea to fully seal the bottom to help prevent future cupping. Also, make sure to loosely attach the top, so that it can move in width to prevent the wood from cracking.

  7. Erin says :

    Hello there! I just purchased a 38″x108″ live edge piece of kiln dried white oak. The gentleman we bought it from said just to wet it down and place 4 sets of 2x4s on the top and bottom,and secure them with clamps and it should work itself out. I found you because I didn’t ask if it should be hot water or cold water and from the things I’ve learned, his suggestions seem really optimistic to me. Is there hope? If there is and it does flatten, I plan on using tung oil to protect it. Should I make sure to do both sides to prevent it from re-cupping? Thanks in advance.

    • wunderwoods says :

      We have used a similar method in our shop and had some improvement. The slab we tried it on dried with a terrible cup and only had the weight of the stack on it. We completely rewetted it, soaking it in wet blankets for a couple of weeks until it was completely flat. We then sandwiched it between oak 3×4″ on both sides and retried in the kiln. The wood still wanted to cup, but the overall result was flatter. This will take lots of time and effort on your part for white oak because water moves slowly through that species and any changes will take a long time to take place. If the piece is so cupped that it is unusable, then it really can’t hurt to give it a go and see if you can make any progress, just don’t plan on it being quick. If you do happen to get it flat enough to use, make sure to seal both side evenly to maintain flatness.

  8. Josh Philpot says :

    I had a round top cup on me recently. I used a water-based poly with 6 coats on the top but only 3 on the bottom—I sprayed all top coats first and allowed to dry before coating the bottom. It was a thick film but glassy and beautiful. It cupped in the rainbow formation. I tried using cleats but the long grain cupped with the wood that makes up the top, unfortunately. I’m assuming this is because of uneven coats?

    Also, to remedy the situation, I was thinking of belt sanding both sides down to the bare wood to get the moisture even again before re-coating evenly, one side at a time. Would you say this is the best option? I would hate to start over. It’s Ash, by the way.

    • wunderwoods says :

      If you wait long enough, it has a good chance of going back to flat or near flat, but not until it had entirely dried out, which will take awhile since it is fully coated. I would let it go for awhile and see what happens. Sanding the finish off of both sides will make the evening out happen faster, but then you will need to refinish. Once the piece has dried and leveled out, then I would coat the bottom to get the film as even as possible, but the movement will be much less dramatic after it has fully acclimated.

  9. Bob says :

    I took some heavily-painted maple or birch knobs off an old chest of drawers I decided to refinish and dumped them in a water-based stripper for nearly a day before I got to them. The paint was lifted off tho not all the original finish, but in the process, the round knobs had become oval, and I’ve seen little evidence that they’re returning to their original shape as they dry. Should perhaps have watched them more carefully, but am still astounded. They’re not end-grain turned incidentally. Have any ideas?

  10. John James says :

    Make that six-plank construction (hard to see with dark stain), with a maximum 5/16 deviation on one end of the finished side (3-4/16 on the other).

    • wunderwoods says :

      Having finish on one side makes things trickier. The side with finish will change slowly and the unfinished side will change quickly. All of my tricks will still work, but you need to compensate for the finish, which means drying out the finished side will take much longer. If the top of the rainbow is on the finished side (which is what I think you were describing), you will need to put that side in the sun to dry it out, but wait hours instead of just minutes.

      • John James says :

        Thanks for the reply! Got a little more info from the maker… Silver maple, stained and coated with acrylic poly on top and sides. Glued butt-joints.

        The finished top is the dry (concave) side. The slab was kiln dried to 5% moisture–but they never coated the bottom, so here I am.

        Basically, I need to dry out the UNfinished (convex) side of the slab.
        Does the bottom of the cup being on the UNFINISHED side change things?

        Are you saying this puppy will bow back in minutes? (I was thinking it would take all day…) Upper 90s – 100+ here, maybe 40-50-% humidity. Los Angeles area.

        Will wood and acrylic poly stretch that fast without cracking?

        Can I coat it hot, in the sun, when it flattens out?

        Sorry for all the Qs; this is new to me–thanks again!

  11. Vince says :

    Just wanted to compliment you on a most informative site. It is so helpful to understand the reason why to visualize what is happening (and hopefully correct). Keep up the great work!

    Vince King

  12. Linda Keres Carter says :

    Thanks for explaining this. I”m midway in a project to improve the R-value of a steel door by applying a 1/4″ veneer of oak to it with contact cement. Will the cement seal the wood while not adding moisture to the same degree as paint I apply to the exposed side? Should I seal the wood first before cementing? Should I screw in and countersink some screws to help the cement out?

    • wunderwoods says :

      The cement and the door it is glued too will seal the back side appropriately, and you don’t need to seal the wood first. I don’t think adding screws will help anything.

  13. Alec says :

    I’m doing some flat sawn balusters and while priming noticed the boards were starting to cup. The wood has been testing at under 10 percent. Is there a remedy for this?

    • wunderwoods says :

      Usually the boards will cup if one side is treated differently from the other, meaning one side gets wet (primed) and the other does not. If you treat both sides the same, they may cup in the beginning, but will typically flatten back out once dry and stabilized, if they started out dry.

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