Is there a way to fix a cupped board?

[Erik Lindseth]

Hey all,

I have a newbie question. I just bought some 1x12 inch pine board to build a locker cabinet. I went somewhat cheap on the lumber and spent a lot of time looking for straight boards. I finally got what I needed, but when I got home I noticed one of boards had a subtle cup in it.

My question, is there a procedure to flatten a board, i.e. clamping to hard flat surface. I don't have a planer. Thanks for any help.

[Alan Schwabacher]

Sometimes the joinery will hold a cupped board flat, so the problem is to get the board to flatten temporarily, so you can attach everything. To temporarily flatten a board, drying the outside of the curve and moistening the inside, may be enough. Sometimes putting the board concave side down on the grass in the sun will do it. Or just clamping it as you assemble might be enough.

If the board actually needs to stay flat, there's not much else you can do than cut it. You do have some choice as to how. The usual approach is to rip it down the middle, joint the edges square, and glue it back together. But if only one face needs to look good, sometimes ripping down the middle only partway through will allow you to force the board flat, and the other face will still look OK.

[Chip Lindley]

Welcome to SMC, Erik. Most will tell you, NO Way to actually "un-cup" a board without jointing/planing it flat. (and thinner)

But in the case of your lumberyard pine, you may be able to clamp the corners tightly enough to flatten the cup, then glue and nail or screw the joint together. Before you glue the joints, dry-fit the corners and clamp to see if you like what you see.

You can always return the board and pick out a flatter one...if one exists! Good luck!

[Erik Lindseth]

Thanks guys. You just confirmed all the thoughts I had. I think the cup is subtle enough that clamping/joining just might be enough to straighten it out, may try the moistening tip to uncup just prior to joining.

Thanks again, I love this forum.

[Lex Boegen]

Here's another alternative for you to consider:

Work around defects by cutting the small parts that you need from this board. A cup or twist that is noticeable over an eight foot length, may not be as severe when the part is cut down to eighteen inches (or whatever.)

I don't have a jointer, but I have used both a tablesaw and a router table as a jointer substitute. Since you're probably using a tablesaw anyway to rip and crosscut the boards, you can use it as a jointer too:

Ensure that the blade on your tablesaw is set to exactly ninety degrees and rip the board down the middle on a tablesaw. Adjust the fence so that the board now just barely touches the blade and make another pass on each board. You only want to remove 1/32-inch or so. This pass joints the board. You now have two slightly cupped boards instead of one deeply cupped board. Glue and clamp your freshly jointed edges together. Of course, if you have a jointer then use it instead of the tablesaw.