Slab Flattening Gone Awry

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Forum topic by JJDWoodworking posted 07-05-2020 01:15 PM 709 views 0 times favorited 11 replies Add to Favorites Watch
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5 posts in 759 days

07-05-2020 01:15 PM

Topic tags/keywords: slab flatten cupped oak

My 16year old has suddenly taken an active interest in woodworking. He and I are building a desk. He chose an oak slab I have and he hand planed it flat. Two days later he went into the shop and it had cupped again. I am guessing it wasn’t completely dry although it had sat in my shop for 2 years. Maybe some built up tension was released – would welcome suggestions on what went wrong there. So the slab is now about 3/4 of an inch thick with about a half inch of cup in it. What are my options for Flattening it? Breadboard ends? Steel bars with wide screw holes? Battens? Cut shelves lengthwise on the underside? Rip it and glue it back together? Thanks for the help.

11 replies so far

View bondogaposis's profile


6025 posts in 3565 days

#1 posted 07-05-2020 01:30 PM

A picture will help you get better answers. Particularly a shot of the end grain.

-- Bondo Gaposis

View JackDuren's profile


1677 posts in 2173 days

#2 posted 07-05-2020 01:45 PM

How wide is the slab?

View Axis39's profile


536 posts in 810 days

#3 posted 07-05-2020 01:56 PM

My vote for safest future for the slab would be to rip, joint and glue it back together, unfortunately.

That’s real bummer, especially for your son! Don’t let him get discouraged!

-- John F. SoCal transplant, chewer uppper of good wood

View Lazyman's profile (online now)


7802 posts in 2601 days

#4 posted 07-05-2020 02:54 PM

If the center of the tree runs through the slab, it may be tough to avoid that unless you wait until the moisture content is down around 10%.

Did you leave it laying flat on a workbench, table or floor for example? Especially right after planing it and you expose new wood, uneven moisture loss on the side exposed to the air can cause it to cup. Try laying with the concave side down on your lawn with a little sun shining on the convex side for a little while and see if it will even out. The sun will dry the convex side while the ground will give up just a little moisture into the concave and it may flatten back out. Keep an eye on it as it might not take long if it is pretty warm outside. When you bring it back in, side lean it against the wall or bench so that air can circulate around it or it will just cup again. If you do lay it flat for any length of time, make sure you have some boards under it so air can circulate. I would wait a couple of weeks before you proceed with your build to see if it will settle down.

One other thought: if you didn’t flatten the bottom, it is a good idea to plane a little off the bottom as well so you get fresh wood on both sides.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View JJDWoodworking's profile


5 posts in 759 days

#5 posted 07-05-2020 03:57 PM

View JJDWoodworking's profile


5 posts in 759 days

#6 posted 07-05-2020 03:58 PM

Thanks all for the help so far. I appreciate it The slab is 20×53.

View LesB's profile


3061 posts in 4657 days

#7 posted 07-05-2020 04:14 PM

This is a complicated problem and a 20” wide slab is hard to stabilize. First your climate in VA tends to be humid and you probably have air conditioning in the house which dries the air so that will add to the problem of wood movement on the completed project. Oak is one of the more “active” woods when it comes to movement. I also noticed the while the top of the slab may be flat the thickness viewed on the end in the picture does not appear to be uniform. That may just be the picture.
One solution was already given. Rip the board into say 4 pieces and glue back together alternating the end grain by flipping every other board. This will reduce the cupping effect of the growth ring curve. Look at pict. #2.
If you complete the project with this slab be sure to seal both sides with what ever finish you use so the gain or loss of moisture is equal on both sides. I would use dowels or biscuits on the glue joints to help align the pieces and add a bit of stability and strength to the joint.

Hopefully this will only give your son a challenge to learn more about wood and wood working. Like other skills it takes time to learn.

-- Les B, Oregon

View shampeon's profile


2167 posts in 3397 days

#8 posted 07-05-2020 04:21 PM

First off, good on you and your son. Hand planing a slab flat is tough work, but the knowledge he’s gained is going to be so great going forward.

The previous posts have good advice for trying to deal with the cup, so the only other thing I’ll add is that 20” wide 3/4” thick slabs are going to be hard to keep flat for very long. Moving it from outside to inside will cause movement, as it adjusts to the relative humidity of the shop. So yeah, let it sit for a while with air movement on both sides, and if it’s still cupped, forcing it flat with e.g. steel bars will probably result in cracks.

-- ian | "You can't stop what's coming. It ain't all waiting on you. That's vanity."

View SMP's profile (online now)


4723 posts in 1119 days

#9 posted 07-05-2020 04:42 PM

Personally I consider 3/4” thick more of a board than a slab. Hard to tell from the pics on my cell phone but looking at growth rings it looks like its cupping opposite of the normal way? Also realize after planing you have to let it acclimate again, check moisture. At this point I would probably just let it dry a few days, the. screw some battens underneath, and/or utilize an apron to keep it flat.

View RCCinNC's profile


512 posts in 1540 days

#10 posted 07-06-2020 02:22 PM

If you screw battens underneath be sure to slot the screw holes in the battens to allow for expansion/ contraction, and if making end caps, use keyed bread board style ends, glued or physically fastened only to the center few inches of the wide board. Years ago, I knew someone who glued and screwed a cap end to a four foot wide glue up top for his dining table. One night, he heard was jerked awake by a “gunshot” from his dining room. Because the table top couldn’t freely shrink, it had “very loudly” split right down the middle.
I’m in North Carolina just south of you guys, so dealing with humidity changes is always at the forefront of my design considerations.
Lastly, I’d try to acclimatize the wood to the hopefully “conditioned” area that it will ultimately be placed. Bottom line, if you can’t get it fairly flat to start with on its own, I’m afraid shampeon is probably right. Forcing it flat will likely split your board. You can rip and reglue it, alternating the grain, but I suspect that each individual board will bow or twist so much that by the time you get them all flat and square you’re not going to have much left in the way of final thickness. I’m afraid wide boards, especially plain sawn like yours, can be very frustrating to work with!

-- Live to putter...putter to live!

View Aj2's profile


4030 posts in 3011 days

#11 posted 07-06-2020 04:11 PM

I see a board too thin for the width. My guess is over time it will cup in the other direction. Looking at the end grain the growth rings will try to lay flat over time.
I recommend your son start planing the high spot on side without epoxy and watch the board carefully. There’s a good chance you will get it moving in a favorable direction.

Good Luck Nice looking Hickory

-- Aj

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