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I am in the process of building a dining table out of solid walnut. I don't have the skills or equipment to make the tabletop myself, so I got my wood supplier to do it for me. The tabletop is 6/4 walnut, 7' long by 40" wide, and is made up of 6 smaller boards glued together. The shop is reputable and seems to know what they are doing. I received it about a month ago, and since then, it's been standing behind my living room couch leaning against the wall (long side on the floor). It was leaning at maybe a 10 degree angle, with the underside of the piece facing away from the wall.

Anyways, now that I've completed the base, I'm paying attention to the tabletop, and I notice that there is some significant cupping. The concave side (the underside) goes in about 3/16" in the middle down from the edges.

Unfortunately, I didn't check for cupping when I initially received the piece, so I'm not sure if it came like this or if I caused it myself somehow. Regardless, the damage is done, so what are my options now?

I've seen some articles/videos where they dampen the concave side of a cupped board and let it sit overnight to straighten it out, but will that work on my 6/4 tabletop? I tried applying this technique yesterday evening, but this morning I don't measure any difference in the amount of cupping.

Any advice would be greatly appreciated.

Note: I don't have a table saw, thickness planer, or jointer.

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  • Hi, welcome to StackExchange. First off, we can see you did some research online on possible fixes but did you look up the subject here? We have a number of related Q&As, and the Comments and Answers give a decent overview of the issues, possible fixes and their likelihood of being permanent without other measures being taken (like the installation of battens on the underside). 3/16" is a fair cup, not so bad over a 40" span tho, so you're not in too bad shape here BUT you should probably get used to the idea that you'll never get it dead flat without more intervention [contd]
    – Graphus
    Sep 18 at 17:36
  • ...up to and including getting it as flat as you can using one or more techniques, and then reflattening it (which it sounds like you don't have the tools to do yourself). And on a table this large it's probably a job best left to professional equipment anyway; I don't think I'd want to try to do it myself and I have a full selection of hand planes and lots of experience flattening warped material.
    – Graphus
    Sep 18 at 17:41
  • "...not sure if it came like this or if I caused it myself somehow." Obviously it should have been dead flat or as close as makes no odds when you bought it. If the cupping is noticeable to the naked eye (which I'd guess it is) then you can judge for yourself whether you might have missed it when you first got it home. And it is actually quite common for large boards, and glued-up panels, to warp during storage; you could even call it normal. Only really careful storage methods can prevent it entirely, or make it so minimal it doesn't matter (e.g. a 32nd over a 30" width).
    – Graphus
    Sep 18 at 17:47
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    Can you confirm that the growth rings on the 6 individual boards alternate direction? At a minimum, they should have done that for you because it significantly reduces the amount of cupping you'll get in a big glue-up like this. If the growth rings all face the same direction (or aren't alternated board-over-board), I'd take it back to them and ask them to fix it.
    – FreeMan
    Sep 19 at 23:52
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    "they recommended making a cut on the underside (the concave side) with a circular saw to relieve the tension" ! I hope you haven't done this yet. Please don't consider going ahead with it until you read the Answers here. Because it should absolutely not be necessary. This is one of the potential fixes, solo or in combination with other tricks, but it's the least satisfying (because it permanently mars the table, and leaves a visible notch in the end grain on both ends). This really should only be done in the most extreme of cases, which this isn't....
    – Graphus
    Sep 20 at 2:30
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Any advice would be greatly appreciated.

3/16” over the length and width of the table isn’t much; if you put the top on the base and leave it unattached, it might flatten out on its own after a week or two. You could encourage it by putting something heavy, like a box of books (or two) in the middle. Whether that helps or not, you can probably also pull it flat with a screw or two through the middle of the base into the bottom of the tabletop. Depending on your design, that might mean that you have to add a piece to the base across the middle.

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There are a couple reasons for cupping like this. The first is assembly if you don't pay attention to the glue up job or don't actually make your joints square, you can get this, sometimes with rather exaggerated bends. It doesn't really sound like that is the case here, and I would certainly hope that a professional shop with any reputation wouldn't screw up like that.

The next is some boards have a lot of 'spring' in them and will 'bend' when pressure is released, some times even after cutting and squaring it, I generally see this the length of the board, and I've had Hickory give me some major headaches (but that comes with milling your own logs...) but once again, a professional shop generally uses fairly high quality lumber and it's usually the wood with sap wood and/or flat sawn that does a lot of cupping.

So the other much more likely culprit. Moisture if you had it against the wall near an air vent or under an open window or where it got a lot of sun throughout the day, there is a good chance that one side dried out faster than the other, with 6/4 lumber it's pretty easy to have uneven moisture content to happen.

'Fixing' this might be as simple as Caleb stated, putting it on your base and waiting a little bit for the moisture to restablize, it might take near the same amount of time as it did to cup in the first place. Good luck.

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