Troubleshooting a Cupped Table Top

How to deal with badly cupped boards in a table top — and how to prevent the problem next time. March 14, 2006

Question
I build props for event decorating/rental and a while back I built 4 cedar harvest tables, on the fly, measuring 43" x 104". The top construction is 9 board biscuit joinery (alternating grain) - 1" thickness. Because of the urgency I didn't attach my aprons to the table top until the day of the event and the next day they were disassembled, transported and stored - separate tops, legs and aprons. Within 2 weeks I caught sight of the tops belly dancing into s-curves and cupped edges (as in the image below) at the tables ends, while the long outside edges remained pretty much true.

I know that had I properly joined the top to the apron immediately, and maybe added breadboard ends, I might have avoided this result. Similarly, gluing, clamping and screwing doesn't feel like it's going to be enough persuasion at this time, especially on the tops with the cupping on the corner edges (like the image below).

I've experimented with cedar steam bending - it's nice and I like it, but I'm apprehensive to try to build something big enough to house one of these tops if my chances of success are limited. What do you steam savvy carpenters think? Or how best to approach repair without steam and without widdling my big tables into consoles?

Forum Responses
(Furniture Making Forum)
From contributor H:
Was the bottom side of this top finished equally to the top? To me it doesn’t look like it because it absorbed more moisture than the top and expanded causing the curve upward. Screwing it down will only serve to make the top crack.

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From contributor L:
The last thing you want to do is introduce more moisture into the scene by steaming. You need to go the opposite way and try to dry the warped side. Take your tabletop and put the convex side towards the sun so that it will dry out the moisture and let the top flatten out. Check on the top every 15 minutes, you don't want it to got the other way. I suppose you could use heat lamps too, but the sun is free and much more even. Don't do all the tops at once, do one first and see the results, if they are what you are looking for then proceed with the rest.

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From contributor G:
I did similar with a blanket chest top. I left it in my car for 14 daylight hours for delivery later. Well, even with breadboard ends I had a minipogada. I brought it home, took the top off and placed in my bathroom for a couple of days, and every once in a while I turned on the hot water, closed the door and basically steamed it. Thing calmed down nice enough and it dried in a couple of days to equilibrium. Then it was refinished and delivered.

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From Gene Wengert, forum technical advisor:
Indeed, steaming is one way to go. But it is not 100% perfect. What happened was that the top of the table was exposed to a dry environment and it dried quickly and shrinkage forces were large and the piece cupped. The back side, which dried eventually, did not shrink as much because it dried slowly and could not overcome the rigidity of the drier and stronger top. (Wood gets stronger when dry.) The same effect can occur if you have a dry table top and expose it to humid conditions and there is an excellent coating on the top, so only the back side shrinks or swells.

You need to:
1. Assure in the future that the wood is at equilibrium with its environment, as it takes a MC change for wood to warp. Then use a vapor resistant coating so that any RH changes will not rapidly affect the wood moisture content.

2. At this point, you need to restore the piece to flatness. Adding water slowly will not do a great job. Rather, add moisture quickly to the cupped face (the top). This will flatten the table. Then slowly re-dry it to the correct MC, holding it flat. It will likely not be perfectly flat, so you may need a large drum sander.

3. Alternate plan. Rip it into several pieces, joint the new edges that are perpendicular to the face and re-glue. Then sand it smooth.

Note: There is so much cup that even if you had secured the edges to the skirt, I doubt that you could have held it flat.