Lumber Cupping

[Keith Strong]

I am currently at the finishing stages of a project. While sanding the piece I noticed two boards starting to cup. I surfaced the lumber myself by removing a little material in three stages. Obviously the lumber was not properly dried. Is there anything I can do to get the lumber to cup the other way. Unfortunately there is snow on the ground so I can not do the grass/sun trick. Can I finish the concave side which will allow the convex side to dry and hopefully shrink? What about putting a fan or light on one side? Thanks for your help. You guys have bailed me out before.

[fRED mCnEILL]

I have straightened warped plywood by standing it upright in a small bathroom and using a fan/heater. Aim it at the convex side. The wood dries causing it to move.

[glenn bradley]

Certainly not what you want to hear but, I have pretty much given up on "salvaging" errant parts of a piece. I just make new ones and the ill-behaved parts become a smaller part of something else down the road. That's just me.

[Joe Shinall]

Keith I feel your pain. We have no lumber mills around here and I had a small keepsake box I just made for a friend's wedding present that I have to give them tomorrow. Had to buy red oak from the big orange borg.

The box is done and finished and tonight I just noticed the lid started to cup just a hair. Not really noticeable to anyone except a woodworker of course. But enough that if I had another week I would make a new lid. That's what I get for last minute project and making a lid of a solid board.

[Richard M. Wolfe]

The guy I work with in the shop just made a couple big (about 20"x24" and about an inch and a half thick) cutting boards and used alternating walnut and ash for contrast. He finished them and leaned them up against each other for a few days and when he laid them on a table each had a cup of a little over an eighth inch. He could afford the time and just set them aside three days ago with the convex side up. They are just about flat now.

I had the same thing happen with a wide but short ash board several years ago except it cupped about a quarter inch. It must really have been wet because it took flipping it about three times when the cup reversed itself. Works OK as long as you have the time to wait on it.

[David Thompson 27577]

{you told your story, then asked........}

Is there anything I can do to get the lumber to cup the other way.

{Then you said some more stuff.......}



Sorry Keith, but no, there is probably nothing you can do, short of re-milling the parts.

[george wilson]

We would WET THE CONVEX side of cupped wood. It seems wrong,I know,but when the wood dried,it would often make the board flat again. I even saw the furniture conservators do this to the tops of 18th.C. tables. They even placed a piece of plexiglass over the convex table top to make sure the water sank in deep before letting it dry out and pull the top back flat.

[Myk Rian]

We would WET THE CONVEX side of cupped wood. It seems wrong,I know,but when the wood dried,it would often make the board flat again.
Wetting the CONCAVE side >) has worked for me.



The box is done and finished and tonight I just noticed the lid started to cup just a hair. Not really noticeable to anyone except a woodworker of course. But enough that if I had another week I would make a new lid. That's what I get for last minute project and making a lid of a solid board.
That happened to me after finishing the dominos box I made for my Aunt.
After I finished a new top, the warped one straightened out on its own.

[Keith Strong]

I am going to start by putting a fan on one side of the wood. If by some chance I am able to get the wood to flatten. Once I apply a finish to all the sides, will this stabilize the wood or will the wood eventually go back to being cupped again?

[Howard Acheson]

Are these individual boards or boards in a panel or assembly? How wide are the boards? Are they in a position that air can get easily to both sides?

[Keith Strong]

Its a knock down bookshelf. The boards are 8" x40". I have them placed on the floor concave side down with the fan on top. Once assembled air will be able to circulate on all sides.

[Randy Dutkiewicz]

Keith I feel your pain. We have no lumber mills around here...

Ain't that the truth!!! I have to make special trips up to Atlanta to get some lumber. Sure wish someone would open a good lumber yard down here someday:(

[Randy Dutkiewicz]

[QUOTE=Joe Shinall;1574826]Keith I feel your pain. We have no lumber mills around here QUOTE]

Ain't that the truth!!! I have to make special trips up to Atlanta to get some lumber. Sure wish someone would open a good lumber yard down here someday:(

[Lee Schierer]

I'm a big fan of applying finish on all sides of pieces. I think it solves a number of movement issues. When wood dries it shrinks and if one side dries more or faster than the other you can get cupping. By wetting the short side (concave) or drying the long side (convex) you can sometimes straighten out a piece. Once it is flat, finish both sides equally as quickly as possible and most of the time it will stay put, but not always. Boxes are particularly problematic since one side of the lid is open to the room and the other side is isolated from the room. Again finishing both sides will help.

Cupping can also be caused if you glue the lid to side pieces that run across the grain. I've seen box tops curl up the side pieces because the lid couldn't move relative to the edge pieces.

[Rob Holcomb]

I've never had much luck with the wetting the wood method. It works good when you do it but after the wet side dries out, the boards just cup again. Wood will do what it wants. Now I just make a new piece or if that's not feasible, I live with it and call it "Character"

[John TenEyck]

If the wood was not uniform in moisture content from one side to the other when you milled it flat, no amount of fussing is going to make it permanently flat now. If the wood was uniform in MC when you milled it then it must have absorbed more moisture on one side since, which could easily happen if you left it lying flat on a bench for example. In this case, just set the board on edge or stand it up, so that air can get to all sides, and it will flatten out again on its own in a day or two.

[Scott T Smith]

Whether or not you can save the board often depends upon the amount of moisture gradient between the faces, and the species / thickness of the wood.

Keep in mind that hardwood is usually dried down between 6% - 8%, yet it will equalize with the environment outside of the kiln at 12% - 14% (sometimes less - it depends upon your temp and RH %). If I take an oak board out of the kiln, within a day or so the shell of the board will equalize with the environment, but the core of the board is still down around 6% or so. Therefore, if I resaw it cupping will occur.

If I let i.e. equalize for several weeks, and then resaw, I see little movement because the core has had time to equalize along with the shell.

If the board is eastern red cedar, then the core acclimates much more quickly and within a few days I can safely resaw. The denser the species, the longer it takes to acclimate. Also, whether the wood is open or close pored (white oak versus red oak) makes a difference in how long it takes to equalize.

This has nothing to do with whether or not the lumber was dried correctly, and everything to do with the recent environment in which it was stored.

So, if there is only a percentage or so difference between the core and the shell, it is usually correctable by some of the methods listed by others. If there is a significant delta (6% or greater), then it is more difficult to correct.

[Larry Edgerton]

If they are cupped it is probably in the way they are sawn, and you can screw around with all kinds of ways of temporarily making them go the other way, but in the end, they will do what they want to do. If you can't live with the cup, replace them would be my suggestion.

[Carl Beckett]

I agree 100% with Scotts comment:

This has nothing to do with whether or not the lumber was dried correctly, and everything to do with the recent environment in which it was stored.

I work with a lot of home milled wood (from various sources otherwise destined for firewood). Then air dry for a while (years), but inevitably I use wood that isnt quite properly dried.

I notice movement after moving it from the outside stack to my inside overhead stack (unconditioned building)

Then more movement when moving from the inside unconditioned building to the inside, psuedo conditioned shop (unheated attached garage)

Then more movement when moving from the shop to the living space (usually in finished form).

But here is the big one recently - I made a medicine cabinet for my bathroom that had reasonbly large flat front doors. They are about 12" wide each, and about 3ft high. I have small kids and the bathroom gets VERY humid at bath time (almost like a steam room), but I put the fan on and never really thought about it.

I left on vacation for a week a while back and came home and the doors on this cabinet were bowed out a good 2 inches across the face!!! Note that this piece was happily hanging for about 5 months or so with no signs.

My thoughts - the inside of the cabinet had the high humidity air. After letting the room dry out for a few days, there was a moisture difference between the back of the doors and the front of the doors. The wood took this in on one side and shrunk on the other (outside) surface. Thus, cupping outward.

Also note that this piece was finished with spray laquer. Not adequate to slow moisture movement. I should have used a more durable finish, but the point is to reinforce the suggestions here to finish ALL sides of a piece to slow moisture migration to a uniform rate

And heres the other fun part - we started using the bath/shower again daily, and the boards WENT BACK FLAT!

The next time I left for days I OPENED the doors first, so they could acclimate evenly from both sides and they didnt warp!

Im pretty comfortable this can be repeated - So I reinforce the recommendations here to either dry the convex side, or moisten the concave side. In the summer time this can be done by laying on the grass at night and letting the temp difference cause condensation (good if you have a lot of boards to do)

Once you get it flat - make sure its exposed to the same conditions from all sides at all times.