Three Piece Slab Top - cupped

[andrew whicker]

Hi,

I have glued together 3 walnut slabs together to get a 40" x 8ft table top. When I glued it together, there wasn't a cup. Now I have a huge cup. (like 1/2" down from the center on both sides).

I have no idea how this happened or why.

It is changing seasons here, so Relative Humidity is bouncing around some. I had the table outside with a heavy moving blanket on top and draped over the sides for one night. It had a slight freeze that night. The bottom of the table is concave, the top is convex.

I live in Northern Utah. It's dry here year round. I think I'm going o get a MC tool to see if I can find a MC difference in the top of the table vs bottom.

Should I just wait this out? Let it re-balance itself? The table top is 2.25" thick.

thanks and cheers,

[andrew whicker]

The top is now sitting in my shop, uncovered.

My only thought is to let it sit until the MC throughout the wood equalizes.

[andrew whicker]

It is noticeably better today than yesterday. Going to keep waiting

[Andrew Seemann]

The top is now sitting in my shop, uncovered.

My only thought is to let it sit until the MC throughout the wood equalizes.

The bigger problem is that the moisture content of the air will never equalize and be stable for all time. As long as the humidity changes, so will the wood. That movement will need to be taken into account in the design, because you can't stop it. The wider the cross grain piece, the bigger the problem. Table tops are particularly sensitive to moisture, since they usually have wide crossgrain sections. And people want them to be flat, which the wood does not want to be due to the curved growth rings.

[Steve Jenkins]

I’d set it on a couple sawhorses on edge with a 4’ clamp on each end clamped to the sawhorses so it is less likely to tip over. This will allow plenty of air to get to both faces. Hopefully it will flatten out. Be sure to finish both faces equally

[Matt Day]

If you had it outside and covered one side, then one side had air flow and the other didn’t and cupping is the wood’s reaction. Trying the reverse, covering the other side, might bring it back to flat.

In general, If the top is destined to be inside, I’d keep it inside.

[Mikail Khan]

Did you apply finish to the top and the underside?

MK

[John TenEyck]

Stand it on end or sticker it. Chances are it will flatten out again on it's own.

John

[andrew whicker]

It's sitting flat on sawhorses inside the shop (no blankets). I'm filling the cracks with epoxy. When that is done, I'll take into the house.

Thanks for all advice.

[andrew whicker]

I also wouldn't mind having it run through a sander, so I'm waiting on it getting back to the way it originally glued up a few months ago.

1) Epoxy
2) Wait for best case flatness
3) finish inside to get it stable

That's my plan so far atleast.

[Joe Jensen]

Please get a copy of "Understanding Wood". It does a great job of explaining how changes in humidity cause wood to move. Flat sawn wood moves around 3X as much as quarter sawn wood with any given change in relative humidity in the room. I have a 37" wide red oak kitchen table that is all flat sawn. I live in Phoenix and the humidity indoors here goes from a low of around 20% to a high of 44%. That table will change in width 1/4" to 3/8" over than change in humidity. If it was quarter sawn it would move about 1/3rd as much. If you had a perfectly quarter sawn board. If the humidity increased it would get wider and thicker. If the humidity dropped it would get thinner and narrower. Same for a perfectly flat sawn board. Most boards are not perfectly quarter sawn or perfectly flat sawn. They have grain that curves when you look at the end grain on the board. When humidity changes the space between the growth rings will change and with the growth rings 3X as much. This will make the board curve. If you flatten it when the humidity is say on the low end of the range for your home, when the humidity changes it will warp. Every time. The book above has tables for nearly every wood telling you how much it moves. When gluing boards up you should alternate which way the grain curves so with a change in humidity they will warp in alternating directions. If you want to eliminate warp you need to have either perfectly flat sawn or perfectly quarter sawn wood. To minimize warp of a panel you glue up narrower panels. Truely fine antique furniture has a solid lumber core of narrow pieces glues up with alternating grain so it stays really stable and a nice thick figured grain veneer on top. I have an old industrial wood text book and in that they say to be stable never have parts wider than 6X the thickness. That means a 3/4" board should be about 4" wide or narrower.

[Lee Schierer]

When you get to the finishing stage, be sure to apply equal amounts of finish to all sides. This will greatly reduce the chance of it cupping again.

[Lyndon Klassen]

What if the OP had ripped his slabs to narrow strips say 2 inches or so. Would this have helped?

[Andrew Seemann]

What if the OP had ripped his slabs to narrow strips say 2 inches or so. Would this have helped?

It might have helped with the cupping, and if it was possible to make it more vertical grain, with some of the expansion. No matter what, there will still be a small amount of cup or wave in the top, and it will still expand and contract with humidity changes. The bigger problem is that it makes your table look like a bowling alley with all those narrow boards:)

But that shows the problem of dealing with wood movement. Designs that look good sometimes don't deal well with wood movement, and designs that do well with wood movement, sometimes don't look so good. Ideally you can find things that look good and deal well with movement, like frame and panel construction, tongue & grove, etc. Things like tables can be hard to design for wood movement because of how we want the top to look. Something like a big huge table made from a single slab of thick wood looks great, unfortunately its nearly impossible to keep it from cupping and uncupping with the seasons.

[Joe Jensen]

What if the OP had ripped his slabs to narrow strips say 2 inches or so. Would this have helped?

If he ripped and flipped ever other one over it would be stable. This is because the warp would alternate for each board.

https://www.woodworkingnetwork.com/wood-products-red-book/red-book-woodworking-tools-best-practices/panel-processing-guide/The-Secret-to-Flat-Panels-269418501.html

[andrew whicker]

The table cupped the OPPOSITE of what the wood grain would suggest that it would do.

I think this is caused by lack of airflow on the top of the table (blanket) and airflow on the bottom (no blanket).

[Mel Fulks]

Is it possible it's in the joint itself? Best to joint face in, face out, face in,rather than depending on fence being 90 degrees. But it could be something else.

[andrew whicker]

It was flat a few months ago.

[Frank Pratt]

If he ripped and flipped ever other one over it would be stable. This is because the warp would alternate for each board.

https://www.woodworkingnetwork.com/wood-products-red-book/red-book-woodworking-tools-best-practices/panel-processing-guide/The-Secret-to-Flat-Panels-269418501.html

Logic suggests that it would work that way, but I've read many studies that say it just doesn't make much of a real world difference. My first priority is arranging the grain in the most pleasing manner. If I can get the grain alternating without sacrificing the look, I'll do that.

[Andrew Seemann]

The table cupped the OPPOSITE of what the wood grain would suggest that it would do.

I think this is caused by lack of airflow on the top of the table (blanket) and airflow on the bottom (no blanket).

Increasing humidity will cause wood to cup one direction. Decreasing it will cause it to cup the other direction. This is caused by the curve in the growth rings. Depending on what the moisture content of the wood was when it was planed, a change in humidity can either cup the board, return the board to flatness, or cup the board the opposite direction.

The wood is going to move with changes in humidity no matter what you do. The design of the table will need to restrain the cupping. Messing around with blankets or wet towels or anything like that will only help with getting it flat enough to mount the top to the framing. They will not cause the wood to become permanently flat, nothing will.

[Carl Beckett]

The table cupped the OPPOSITE of what the wood grain would suggest that it would do.

I think this is caused by lack of airflow on the top of the table (blanket) and airflow on the bottom (no blanket).

I have experienced this and it was dramatic. I wondered if it was more than just airflow (heat on one side that was covered, especially if that faced the sun, changed relative humidity on that surface, or even condensation type effect since you mentioned it was out overnight and temperatures cycled).

[peter gagliardi]

It is simple, really. You put, what is the equivalent of a big sponge- the blanket, on top of the table, and set it outside. During the night, the condensation and frost saturated and soaked through the blanket into the other big sponge below it- the wood top.
You now are seeing the extreme effects of moisture differential on a wide top. It will probably take many days, to a couple weeks for it to equalize and come back hopefully to flat or very close.

[Joe Jensen]

Increasing humidity will cause wood to cup one direction. Decreasing it will cause it to cup the other direction. This is caused by the curve in the growth rings. Depending on what the moisture content of the wood was when it was planed, a change in humidity can either cup the board, return the board to flatness, or cup the board the opposite direction.

The wood is going to move with changes in humidity no matter what you do. The design of the table will need to restrain the cupping. Messing around with blankets or wet towels or anything like that will only help with getting it flat enough to mount the top to the framing. They will not cause the wood to become permanently flat, nothing will.

Exactly correct. Everyone should read the book "Understanding Wood". After you understand that finish will not prevent changes in humidity from changing the shape of wood.

[Joe Jensen]

I have experienced this and it was dramatic. I wondered if it was more than just airflow (heat on one side that was covered, especially if that faced the sun, changed relative humidity on that surface, or even condensation type effect since you mentioned it was out overnight and temperatures cycled).

It is due to changes in the moisture content in the wood. If you put a piece in the sun it dries one side faster than the other and it warps. If you put a piece on a damp basement floor it will absorb some moisture and warp. This warpage is not a mystery. The book "Understanding Wood" has extensive tables of any wood you are likely to encounter with data on how much the wood will move in each direction with changes in humidity.