Need help with table top movement issue

[Patrick Hannon]

Hey there everyone I'm new to this site and not sure if this is the appropriate place for this post so I apologize if its not. Anyway my question is I made a coffee table about a year ago. I have recently had some trouble with cupping its about an 1/8" on one side and 1/16" on the other the top is two 5/4 book matched maple boards 50" in length and 18" wide. When I attached the top I didn't have a good understanding of wood movement and I attached the top with with figure eight fasteners instead of buttons. I was told that this would not allow for movement if they were used length wise along the grain. I also only put poly on the top and sides of the top. Are either of these things responsible for my cupping issue and if so is there anything I can do to fix it? Also if you notice because of the design I can only attach the top length wise so screwing it to the base doesn't really pull it flat.

[Steve Baumgartner]

Did you put anything on the underside of the top? If not, that is a possible (though not definite) cause as it will make the boards gain and lose moisture at different rates through the top and bottom surfaces. To flatten it, you need to try to get the relative moisture content the same as when you originally flattened the top boards. A lamp left on beneath the top will provide gentle heat and gradually dry it. Light misting with water repeated every so often will moisten it. But don't overdo it or you may overshoot and have to wait for it to come back! If you can get it flat again, apply the same poly on the bottom surface and cross your fingers.

[Kevin Jenness]

That is not much cupping considering the width of the boards, the apparent lack of crosswise stretchers to pull them down on and the fact that you didn't finish the bottom of the slab. In fact, you should consider it a miracle. You might gain a little by running some stretchers between the long rails and using buttons to fasten the top to them and the long rails, although the evident overhang will leave a considerable part of the top unrestrained. The figure eight fasteners may be making the long rails bow in at this time of year- have you checked that with a straightedge? If so that is putting the top in tension and at risk of splitting. Finishing the bottom equally with the top will help to minimize further cupping,, though unlikely to remove it..

[Patrick Hannon]

I'll count my blessings that it wasn't worse, I did not check to see if the stretchers were bowing but when I used the figure eights I was hoping that the fact that they do have a little give to them and that the side aprons are long and more flexible then the top might allow for the top to expand enough. I removed the fasteners on one apron earlier today and a was hoping that it might flatten out after a few days. Is this unlikely? And if it is would you go ahead and apply poly now to the bottom even with its current cupping?

[John Coloccia]

Did it bend down or up? In other words, is the top concave, or convex? I would have to think it's cupped down (convex), probably because the back dried out and is pulling the rest of the top down. Forget about any thoughts of "pulling" it straight. You'll only risk splitting it somewhere else. Assuming it cupped the way it really should have cupped, a possible solution is to rehumidify it for a while, and then when it's back straight, finish the back as you did the top. You may want to consider removing the top and sticking it in your shop, with the finished part sitting towards a wall. At some point, the weather will turn normal again (sometime in the spring), and everything will hopefully straighten out.

Anyhow, that's just my opinion. If it's cupped the other way, you may just have to live with whatever the wood wants to do.

[Patrick Hannon]

It cupped so that the two edges curve down towards the bottom and the middle bulges up slightly on the top of the table. Is this consistent with excessive moisture loss on the untreated bottom?

[John TenEyck]

If it's cupped down, as John suggested, just finish the bottom now. You don't have to wait until the top is flat. After you finish it, the moisture in it will redistribute over a week or two and it should flatten out again as the moisture equalizes.

John

[John Coloccia]

It cupped so that the two edges curve down towards the bottom and the middle bulges up slightly on the top of the table. Is this consistent with excessive moisture loss on the untreated bottom?

Yup. Moisture leaves the bottom, the bottom shrinks, and it yanks the wood down. You're in good shape. Humidify that thing however you wish (spritz with water, or just detach it so it doesn't split and WAIT), finish the bottom, and you'll go a long way towards mitigating the issue. Be aware that if you spritz with water, you'll raise the grain and you'll want to sand before finishing, if you care about a smooth finish on the bottom.

[John Coloccia]

If it's cupped down, as John suggested, just finish the bottom now. You don't have to wait until the top is flat. After you finish it, the moisture in it will redistribute over a week or two and it should flatten out again as the moisture equalizes.

John

Does that really work? I've never had the guts to try it!

[John TenEyck]

Does that really work? I've never had the guts to try it!

If the top is warped due to non-uniform moisture across the thickness it will work as well as adding water to the dry side to get it flatten out and then finishing it. In both cases the initial moisture content will be non-uniform across the piece, but equilizes after finishing. If the top is warped for some other reason, then neither approach will work.

John

[Ethan Melad]

While you wait for the top to flatten, you may want to add a couple stretchers across the width of the table. Drill with oversize or elongated holes, that way when you reattach the top you can account for movement rather than leaving the figure 8 fasteners.

[Sam Murdoch]

While you wait for the top to flatten, you may want to add a couple stretchers across the width of the table. Drill with oversize or elongated holes, that way when you reattach the top you can account for movement rather than leaving the figure 8 fasteners.

If you have the courage to change the design Ethan is making a good suggestion. The most inconspicuous, but likely not very effective - too short, is so set a couple of cross stretchers between your rails.

A more radical esthetically, but better functionally, idea might be to let the stretchers into the rails and bring them out closer to the outer edge of the top. The stretchers could be fastened directly into the rails and then attached to the completely finished top using the elongated holes.

The table as designed will, I think, always result in a dynamic top, regardless of the finish. The finish will mitigate the problem but I think only the stretchers will correct it.

Having said that - I like where you were going with this design .