Expert: Jamie Yocono - 1/10/2007

Question
I've glued oak planks together and then to 3/4 inch oak plywood and now it
has a bow in it.  How do I get it straight?

Answer
Hi Frank,

I hate to be the bearer of bad news, but unfortunately, you've done one of the big "no-no's" of woodworking. I will explain what you did wrong, and then try to talk you through a few solutions. But I just can't guarantee any positive results. I wish I had better news for you.

First off, your problem is a somewhat common one, so don't feel too bad about it. And it's a little hard to describe by writing about it. It would be a heck of a lot easier to explain it in person, than have to type it here, but I will try.

OK, the big mistake was gluing the solid wood to the plywood. The solid wood planks by themselves would have probably been fine to a table top, provided you did a good job with the edges of the planks. They needed to be cleaned up on a jointer, with a nice, square edge so that they all clamped together with no voids. Sometimes, if you have a very nice (sharp) blade in your tablesaw, you can glue the boards straight from the saw, and not run them over a jointer first. That is to say, a good saw blade cut creates an adequate surface for glue.

But really.... that's not the problem here. What you did was create a moisture imbalance of the whole table top. Wood is an organic material, and reacts to humidity changes in our environment. It swells up a bit when it's humid out, and shrinks a little when it's dry, like the dead of winter. So when you glued the solid wood planks to the plywood, you created an unbalanced top.

That's why it bowed.

Because the two sides of your table top are different, ambient moisture is being absorbed (or lost) at a different rate, from the top to the bottom.

I hope that makes sense!

Here's another example... let's say you're making a table top out of plywood. And when you're done, you only varnish the top of it, but not the underside. By doing that, you've once again created an imbalance from the top to the bottom. And it will bow.

The proper way to make a table top is to treat both the top and the bottom equally, whether it's with a laminate, or with finish, or whatever. So if you had put plywood on the top of your table, thus making a "sandwich" of plywood/wood/plywood, your top probably wouldn't have bowed.

That said, I'm wondering why you attached the solid wood to the plywood. If you did it for thickness, there might have been better ways to accomplish that. You could have glued up a double layer of solid wood around the perimeter, keeping the grain all running in the same direction. For the long edges, that's pretty simple to do, but for the ends, it takes a little bit of work.  But that would have been the correct way to make a top appear thicker.

OK- how do we correct it? I'm assuming that it's not possible to remove the plywood. If it's glued down, you'll probably waste too much wood in cutting it all apart. But if you had to, you could rip the table top apart at the plank glue lines, and then turn the planks upright and re-saw them off of the plywood. But like I said, doing that is probably going to waste a lot of wood, and involve a good deal of labor. So I'm not sure you want to attempt that.

The only way I can think of to "possibly" flatten out your table top would be to attach some solid wood stretchers from the underside, going across the grain. These would have to be fairly beefy, and would have to be attached vertically. What to I mean by that? Think of a floor joist. You know how they rest on their edge, so that they don't bow? That's what I mean. For example... with a 2 x 4, if you had one resting flat between two sawhorses and you sat on it, it would bow. But if you had it resting on it's edge and sat on it, it wouldn't bow. That's because wood it stronger in one direction than the other.

So it might be possible to flatten your table top by attaching some stretchers to the underside. But you're probably going to have to screw through the top, going through the solid wood, then the plywood, into the planks. You can plug the screw holes with solid wood plugs.

Now that creates some other problems, mainly that when the wood moves, it could possible crack where you have sunk the screws. But there's a possibility that the plywood might keep it from moving as much as it normally would. So you might be OK there, but I can't guarantee it. Normally you wouldn't do anything like this!

I sure hope what I write here makes sense. You've created an imbalance between the top and the bottom, and that's why you've got the bow. Correcting it is difficult, sort of cutting it all apart and redoing it correctly. The solution I've suggested (the stretchers) is a stab in the dark, there's just no telling if the top will stay flat or not. Sorry I can't help you out with a better solution, but that's just the way it is.

OK, good luck, I hope this helps. Please feel free to write back if you have any further questions after reading this. And if this answer was helpful... please take a minute and rate my service. Thanks!

Jamie Yocono
Wood It Is! Custom Cabinetry
Las Vegas, NV
www.wooditis.com