HELP! Warped Desk Top

[Daniel Hartmann]

I recently built myself a 42" tall standing desk. It's been in my house for over a month now. We just had a turn in weather that caused me to turn on the heat and after going away for the weekend I found the top warped 1/8" (The gap measured from the apron top to the bottom of the desk top). The desk was situated over a heat vent, which I suspect is the culprit. Ive moved the desk. Is there any chance of it flattening out or am I going to have to flatten it again? The top of the desk is 1 1/8" thick african mahogany. The top is attached to the desk aprons with metal S clips. The base is painted so I used Poplar and Pine. When the top warped it enlarged the groove that captures the S clip. The african Mahogany top is made from one 12" wide board cross cut and glued up. I cross cut the board probably 6 months ago, the lumber was bought from a hardwood dealer and I was told it was kiln dried. Any help is greatly appreciated.

Thanks.

[Stuart Tierney]

1/8" in a weekend, over a heater vent?

I wish things were so tame here...

Try 1/2" bow in 4 hours...

And back out again in 6 hours...

(Even wondered why Japanese joinery is the way it is, and why everything pretty much interlocks? It's because if you didn't do that, everything would twist itself to destruction in a week.)

It should come back to flat again, once the top surface and bottom surface have had a chance to equalise again. Just another point, are both sides of the top 'finished', as in have a finish on them? Having the upper surface finished and the bottom not will cause the top to bow/warp rapidly with humidity changes, made worse by anything that will dry/wet the two sides differently.

But in short, it should come back. Give it some time and it'll be fine.

Stu.

[Daniel Hartmann]

thanks for the reply. Both surfaces are not finished. I guess I'll be shellacking the bottom!

[Brian Holcombe]

Does it have breadboard ends or anything that would help prevent the top from cupping?

[Daniel Hartmann]

No, no bread boards

[Daniel Hartmann]

Assuming I can get the top to flatten out I might add some hard maple battens

[Brian Holcombe]

Clips alone are not enough to keep a solid wood top from cupping. If you did not want to see breadboards on this piece you could get nearly the same function from the base if you built supports that spanned the width and attached them with screws through slotted holes to allow for expansion of the top.

[Tom McMahon]

Whether or not the top will straighten out by it self depends on whether it has taken on a compression set or not. Here is part of a post I wrote on straightening a warp.

"When a piece of wood is confined and it absorbs moisture it tries to expand but can’t, when it can’t expand the force crushes some of the cells, with the cells crushed it can no longer expand and contract fully, keeping it warped or compressed. Compression set takes place whenever the wood tries to expand and is restricted. It also takes place when one side of a board tries to expand and the other side does not. This is normally caused by uneven moisture exposure. Compression set also explains why joints loosen over time. It is the explanation of most cracks, loose screws, loose nails any thing that restricts the movement of the wood can cause it."

The whole post is here.
http://millcrek.wordpress.com/2012/08/16/straightening-a-warp-compression-set/

[george wilson]

If the top is unfinished,or can be stripped,wet the CONVEX side of the top. Get it GOOD and soaking wet. It will warp more than ever while wet,but when it dries,it will shrink and pull the top down flat. THIS is how the furniture conservators in Williamsburg flatten warped antique tops.

It seems counter intuitive,but it works,believe me.

[Chris Griggs]

If the top is unfinished,or can be stripped,wet the CONVEX side of the top. Get it GOOD and soaking wet. It will warp more than ever while wet,but when it dries,it will shrink and pull the top down flat. THIS is how the furniture conservators in Williamsburg flatten warped antique tops.

It seems counter intuitive,but it works,believe me.

George. I just want to take this moment to say how constantly amazed I am by your wealth of knowledge. My word...who would think to do this on there own? You're the man George!

[Mel Fulks]

George, Tom , Thanks for good efforts on this, but what a tough sell! I worked with a guy who in his free time repaired furniture. One day he said,"I wish I could figure out a way to unwarp old table tops ". When I described the correct method as you have done today ....he acted like I was crazy. I took a sample,warped it ,then straightened it ,showing him every step. He said nothing .

[Daniel Hartmann]

The top is finished with shellac. can I just scrap it off or plane it off?

[phil harold]

George beat me to it
the underside was dried out by the heat vent
so moisten the bottom side...

[Tom McMahon]

You could scrap or plane it off, or you could dissolve it with alcohol and wipe it off with a rag. Do not wet the concave side it will just make the warp worse when it dries. Wet the convex side, I know this is counterintutive but it can work. It helps to restrick the board from moving while wet, I use aluminum angle iron clamped to the piece until totally dry. What you want to do is cause the cells on the convex side of the board to crush to counteract the crushed cells on the concave side. The concave side of the top is already crushed, if you crush the cells on the concave side again it just makes the warp more pronounced.

[george wilson]

Chris,I was in a position for many years,with a World class furniture conservation shop next door to me in the museum,to learn many things. So,I can't take personal credit for knowing this.

[Chris Griggs]

Chris,I was in a position for many years,with a World class furniture conservation shop next door to me in the museum,to learn many things. So,I can't take personal credit for knowing this.

Either way, I'm just glad you're here/willing to share the knowledge. I'd never think to wet the convex side...completely counter intuitive.

[Daniel Hartmann]

Thanks for all the great advice. Looks like I'll be stripping the shellac off the top!

[Brian Ashton]

Assuming I can get the top to flatten out I might add some hard maple battens

If it doesn't flatten out on it's own. Take the top off and lay it on concrete cup side down for a day or so or till it's flat again. On really bad warped tops I've thrown them out side on the grass cup side down to flatten them but 1/8" isn't much. Once it's flat finish both sides fairly soon after. If it keeps warping you have a very reactive board and will need to think about battens or cuts in the back side to remove the ability of the board to warp.

[Brian Ashton]

George beat me to it
the underside was dried out by the heat vent
so moisten the bottom side...


I think what you're saying is the opposite of what George was saying. He said wet the convex side but you're saying wet the concave side.

[Brian Holcombe]

I'm a little baffled by this. The finish is going to be stripped so that this water method can be applied, and if it straightens the board then what? Will you no longer need to change the structure to eliminate this type of movement? Likely it will be just as cupped in a few months.

Why not apply the structural changes to the base first and see if it pulls the top down flat before stripping the shellac?

[Mel Fulks]

Brian, your point on structural changes makes sense ,but the top in question is so thick and made from a wood that moves more than some animals. Since the water on convex side method is used to balance the water applied to the tops over time it is possible that since the damage in this case was caused by drying ,the top could be fixed by stripping the top and exposing TOP side to same heat source. Then finished on both sides. If the desk is going to be used over that heat vent I might even add CUSTOM HEAT DEFLECTOR (CHD). A piece of cardboard with some air space between . When warp is caused by water in repeated cleanings then water on bottom is the lab proved fix.

[Tom McMahon]

The key to repairing a warped top is understanding that there is more going on than just expansion and contraction due to moisture or heat. I know I am not a great communicator and you may not believe me any way, so here is a link to a paper on correcting a warp using compression set, written by a conservator at Williamsburg and published by WAG. I've been restoring wooden objects for around 40 years and for the first 25 I had little success correcting warps, but once I discovered this information I've had much more.
http://cool.conservation-us.org/coolaic/sg/wag/1995/WAG_95_howlett.pdf
There is also a lot of information if you Google "compression set in wood".

[Pat Barry]

I tend to think that the method outlined of wetting the convex side of the piece is in appropriate for the newly manufactured desktop that the OP is describing. I think that is a completely different proposition than trying to do restoration work on historical pieces. To me, the water trick would only provide a very temporary fix and that fix would soon result in the same issue. I think the issue seems to be lack of uniform dryness of the desktop itself, possibly due to lack of acclimation time in the shop before the glueup and final flattening occurred. If the wood were dry it would not have warped to begin with. Now of course, this is all assuming that the attachment method of the top actually did not in itself induce the warping condition by restraining the natural movement of the top.