Flattening a Cupped Board

I love wide boards. My most cherished timbers are some 18″-wide cherry boards from a huge, standing dead cherry tree that I found back when I lived in South Mississippi. They have some beetle holes from the decade or more that the tree stood forlornly in the forest after it died. But they’re wide. Also, some 17″-wide white pine boards from a tree that died in front of my house when I lived in North Georgia. It was a yard tree, full of knots and pitch, but those boards…so wide. And of course, a stack of 18″ to 24″-wide poplar planks from a discarded butt from a logging job in Mississippi. The loggers left the butt because it splintered as it fell, but there was still plenty of good wood for me to salvage. And it was worth it, because, well, wide. Reeaaally wide…You know?

It traumatizes me greatly to have to glue up boards to make a wider panel. Sometimes it’s unavoidable, like when I made my 48″-wide sassafras table. Alas, there were no 48″-wide sassafras boards to be had. At least not in my wood stash. But usually I just end up designing my work to suit the stock that I have on hand.

Such was the case when I began work on a small tavern table over the weekend. I began with the intention of reproducing a small table from the MESDA collection. I’ve had my eye on that piece for years, ever since Chris Schwarz built a version for Popular woodworking a few years back. The original is from Charleston, South Carolina around the 1720’s. Not bad-looking for a 300-year-old.

Photo credit: popularwoodworking.com

Actually, to say that I’m building a reproduction would probably ruffle the feathers of an ardent antiquarian. The original was made of cypress. I don’t have any cypress. The original utilizes some unconventional drawer construction. I don’t mind the unconventional, but I don’t hesitate to build things a bit differently if it suits my fancy. The original turnings exhibit quite a bit of variance from one leg to another. This was presumably not the intention of the maker, so I definitely will not be trying to replicate that variance. Mine will be different, too, but in their own unique way. And the original has a 21″-wide top. I have some poplar that wide, but it’s 8/4 stock that is destined for Windsor chair seats. My widest 4/4 poplar is 19″. Therefore, my table will be 19″ wide.

See how easy that was? No need to glue a 2″ strip to the edge of a gorgeously wide plank. I suspect the original table would have been 19″ wide also, if the maker’s widest cypress board had been 19″ wide.

There is only one problem with my 19″ wide poplar boards. They are cupped. Pretty badly. If I tried to plane this sucker flat, I’d be left with a 1/4″-thick top. That ain’t good.

taverntable 072

So what’s the solution? Well obviously, I just need to cut the sucker apart. Flatten out three narrow boards individually. That’s obviously the right decision, because then I can feed the boards through my 12″ planer! Then I can glue them back together, and no one will be the wiser!

Screw that. All of it. The day you see me cutting apart my wide boards so that I can straighten out a minor cup/fit them through my planer/orient the growth rings differently/or any other stupid reason is the day that you will know the I’ve gone to the Dark Side, Anakin Skywalker-style.

A wide, mildly cupped board can easily be flattened with a little TLC. Here’s how I do it:

1) Wet down the concave side with a healthy amount of water. Make sure to really let it soak in. Sometimes I even leave a wet cloth on top of the board for good measure. I prop it up on some stickers to get good airflow to the convex side.

taverntable 074

2) Come back a few hours later. Hopefully, the board has straightened out a bit. In this case, the board was perfectly flat after leaving it overnight. If not, add more water and wait longer.

taverntable 075

3) Now the board is flat (or maybe even cupped a little to the opposite side). You want it to stay flat. So, clamp it tight to a couple of cauls. Leave it in the clamps for a couple of days. Better yet, until you’re ready to use it. More than likely, the board will stay flat when it is completely dry.

taverntable 076

Now you’re ready to sharpen your hand planes and get to work.

This technique does have its limitations. The narrower or thicker your board is (basically, the closer it is in cross-section to a square rather than a long rectangle), the less likely you are to have success with this method. I’ve had great luck with stock 12″ and wider, and a little over an inch thick. I wouldn’t expect this to work with 8/4 stock (but maybe with more time and water? I dunno, I don’t do tabletops with 8/4 stock. I use it mostly for legs so the boards can be as cupped as they want to be).

Good luck, and go give those wide boards the respect they deserve.




4 thoughts on “Flattening a Cupped Board

  1. That’s a clever technique! I’ve found that sawing the board apart and regluing it is a pretty short term fix. If the board wants to cup, it’ll keep on cupping, even after the surgery, unless I constrain it in some way.

    So how do you restrain the top over time? Breadboards? Battens? Something else?


    1. Good question. In my experience, screwing a board to the aprons is sufficient to keep a top from warping. Even a board that started out fairly cupped. I’ve used breadboard ends in the past – but only on chest lids, not tabletops. I tend to use pretty agreeable woods, though. Cherry, walnut, birch, poplar, white pine. I select clear wood without funky grain. They all behave pretty well once planed and finished. You have to make those allowances for expansion and contraction, because that’s always going to happen no matter what, but cupping is pretty easily restrained. Think about how easy it is to take a board 19″ wide and 7/8″ thick board and put a 1/4″ cup in it. It really doesn’t take too much pressure. That’s all the pressure that the screws need to exert to keep that top tight to the aprons (I use pocket screws for attaching tops, by the way). I’ll admit that it doesn’t always work out so well. I have a friend who had a helluva time trying to keep some flatsawn sycamore shelves flat. Sycamore is a screwy wood though. Like elm, lots of interlocking grain. Anyway, I attribute most of my success to good stock selection. Hope that helps!


    1. It’s highly unlikely that you would run into problems with compression set from the method described above. The classic example of compression set would be soaking an axe head in water to tighten it up. The wood swells, but not as much as it wants to, since it’s restrained by the axe head – therefore, the wood is compressed, and when it dries out, it shrinks smaller (and thus the axehead is looser) than before. In this example, one episode of soaking and drying can irreparably damage the axe handle.

      In the examples that Flexner describes in the article that you linked to, the thing that is restraining the wood from swelling is the opposite (dry) face of the board. Obviously, wood itself has more flexibility than than the steel walls of an axehead, so the effect is far more modest. As he states, “The compressions are cumulative. So, after many wettings and drying outs, the result is shrinkage.”

      A single episode of wetting and drying should not be sufficient to damage your board. For more modest cases of cupping, though, you can definitely try gentler methods: I’ve had luck leaving the convex side of a slightly cupped board in a sunny window, and the gentler warming/drying of that side was sufficient to straighten the cup. There are many ways to skin this cat, and my hope with this post is to get a few people thinking of ways to work with the boards rather than default to cut-up/re-glue. Thanks for your comment, though. I feel this topic might deserve some more discussion in the future!


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s