Question: 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.

Answer: 
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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.
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.