Curling of a Cutting Board Set in a Concrete Countertop
Woodworkers attempt to cure a problem created by placing wood in a moist situation on one side. No easy answers here. June 18, 2009
Question
I am looking for some help with a problem reported to me by a customer. I made a fairly large and thin cutting board out of hard maple and Bloodwood. It measures about 32 x 18 and is 7/8 thick. It is inset into a custom concrete counter about 1/2 inch with 3/8 inch above the surface. I glued with Titebond 3 and I put on two heavy coats of butcher block oil (from woodcraft - mineral oil) and wiped off the excess. The board was looking perfect and laying nice and flat when I installed it. Now, about two weeks later, the customer said the cutting board is curling up about 1/4 inch and it now rocks in the countertop recess instead of laying flat. Any ideas what might have caused it and how I may be able to fix the problem?
I am wondering if it is because the board is sitting on the counter surface and the top surface is maybe drying quicker than the bottom? But the wood was nice and dry before I glued it up so I am not sure what to do.
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Forum Responses
(WOODnetWORK Forum)
From contributor A:
You sealed the top and not the bottom. Moisture got in the bottom and swelled the wood, but the top didn't get moist and swell at the same rate, due to being sealed.
From the original questioner
Actually I put 2 coats of mineral oil on both the top and bottom. Any ideas on how to correct the problem?
From contributor B:
Mineral oil does very little to prevent moisture from getting into wood. This is not a problem of uneven drying, it's a problem of water getting in between concrete and wood and being absorbed. A cutting board this large, this thin, and loose around sinks is really pushing your luck no matter how you make it.
It might straighten out if you removed it and let it dry, but the wait to see if that worked would inconvenience your customer so I'd just remake the thing. I'd then coat the bottom and edges with a tough film finish, perhaps 3 coats of polyurethane, and cross my fingers.
From contributor C:
I have to agree with contributor B on this one. The finish you are using won't do much in the way of preventing moisture movement. In addition, you could have extra moisture hitting the bottom if the concrete top is fairly new. Even if it isn't, the top is going to exchange moisture while the bottom isn't.
The thing you need to remember is that no matter what the moisture content of the wood is when you install it, it's going to change. Unless your in a building with very rigid climate/humidity control you'll have fluctuations with the weather. I'm not sure how you resolve this problem, since I'm guessing your client almost certainly won't want a heavy plastic build finish on their cutting board.
From the original questioner:
Thanks for the help. To contributor B: If I coated the bottom and edges with poly, wouldn't that be bad since the top can still breathe and the bottom can't?
From contributor B:
I know we're always saying that you should treat both sides of a panel the same way to keep it "balanced," but this is a case where such balance is simply impossible. It hardly matters that both sides were coated with the same finish if one side is exposed to air and the other is sitting in a puddle. I don't think there's a completely safe, "right" way to handle this, so I'm picking the least of the evils I can think of.
From contributor D:
This is never going to be flat. You need the same air to both sides and you have concrete under. Sealing the board will be of little value, with no air circulation to the underside. It is a bad design and failure is waiting. There is a process where the wood in put into a tank of plastic and extreme pressure is applied. The plasticized wood is much more stable. Even solid surface would warp under this condition.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
I certainly agree that moisture is the problem here, as everyone seems to already appreciate and the moisture issue is one of a gradient from back to front. THis gradient,if it is set up quickly, will actually cause some permanent size change, so even when the MC becomes uniform, it will not flatten the piece. I suggest coating the back and edges with beeswax (or perhaps Bri wax), several coats, and then use an acceptable (food safe, etc.) wax coating on the top and have that coating applied frequently.
From contributor F:
I'm assuming the cutting board is removable for cleaning. If so, replace the cutting board, coat both sides with oil or wax as Dr. Gene suggests. Then advise the customer that they need to keep the area dry when not in use. It will be tedious and won't necessarly prevent warpage, but will help minimize it. By the way, I would also let the customer know that wood and water never mix well.
From contributor G:
You might try sawing off the first 3/4 inch. Flatten it back, planer, jointer or with a plane. Then put some threaded rods and tighten them real good with some countersunk bolts and washers. Then glue back up the 3/4 inch strip you took off. It's going to be more or less a mini workbench top.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Contributor G has a good idea. Using a metal rod or angle iron (or two; does not have to be a bolt) that cannot bend is often done for tall doors and windows to keep them from warping due to the moisture gradient from inside to outside.
From contributor H:
Regarding the threaded rods, I would resist this urge, all due respect to Dr. Gene and others. This creates the example I have used for many years to teach why some wood assemblies crack. If the expansion is limited by two iron beams, or nuts and washers on a threaded rod, or any two immovable objects, then as moisture is added thru RH increase, puddles or whatever, the wood will expand - or try to expand. However, since it cannot move due to the immovable objects, all the fibers (think soda straws) will crush just a little bit to accommodate the unstoppable expansion. Then, when the MC decreases, the wood will come back to a narrower dimension than the original, and cracks can develop.
When you seek to immobilize solid wood, there will be trouble.
If you are committed to the top, I would suggest two large sliding dovetails across the grain on the underside. Peg the forward, exposed end of the dovetail pins to the top for flush, and let the back of the top come and go. The dovetails will allow expansion and contraction, and also work to keep it all flat. Size accordingly.
From contributor I:
I think some of this problem could be alleviated by the way the wood top is secured to the cabinet. If there is a solid plywood top on this cabinet, it may be preventing some air circulation to the bottom of the cutting board. On a cabinet like this, I would use a stretcher front and back that had tongued ends floating in a groove in the sides of the cabinet- this will allow the top to be screwed down and still move, while providing some support to the top. The concrete counter looks like it is supported by the sides and back of the cabinet(s). I would also use a good epoxy sealer on the edges of the concrete in the opening- concrete gives off a lot of moisture that is probably being absorbed into the edges of the wood top. I also just noticed that the cutting board is thinner that the concrete top, and is sitting on another board. Is it screwed or laminated to the board below? This could be causing an imbalance problem.
From contributor J:
Why not just remove it, drill through the countertop, and put it back down with screws from the underside? Make the holes in the top oblong to allow for movement, and be done with it.
That way your customer will be happy, you won't get any more callbacks, and it will take only one service call to fix it.
From contributor C:
I hadn't seen the pic when I wrote my first response. Now that I have I think that this is just an example of a bad idea. To have a solid top like that and then put an oil finished wood cutting board with a sink in it is just not practical. You can try the steel rods and extra wax coatings for a short term fix, but my feeling is your probably going to end up with wood movement in more than one direction over time. And regardless of the flatness issue the lifespan of this cutting board will be relatively short. The only advice I can offer is when this one does start to come apart, you could try making the next one out of Ipe or Teak, something very hard, stable, and most importantly good around wet areas.
From contributor K:
I agree with contributor C. Bad idea from the get go. Make them a new one out of Corian with different colors like you have and make a nice wooden one for the counter.
I hadn't seen the photo either and I think I would have passed on this one. Of all suggestions given, Don has the best one. Make it out of corian and be done with it. Put the photo in a book titled: "Things I will never do again."
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
To contributor H: The metal rod does inded work and the restricted expansion is what we are trying to obtain. I have seen bolts used rarely, while a strong metal rod or angle iron to resist bending is more common. I do agree that the picture does indeed show a very tough situation for wood. Wax is a perfect vapor barrier and liquid barrier, so it will work, but I do not know about food safety. If you use wax, alwatys heat it slightly to achieve better penetration. With some species (for example, white oak, coastal Douglas-fir heartwood), there may be very little penetration no matter what is done.
From contributor O:
Bad design. It will never stay flat under those conditions.
From contributor N:
Take a masonry bit and drill several small diameter holes in the concrete under the insert. It wont cure the problem but it will slow it down.
From contributor M:
Make a new board and then use salad bowl oil, using several coats and it will dry like varnish only shinier. I use it all the time on my boards and never get cuppage.
From contributor L:
I am no expert but I have an idea. Everyone here seems to agree that the design is the problem. Why not fix the design instead of changing the finish or attempting to force the wood to stay still when the laws of physics say wood expands and contracts? Here is an idea: if required, replace the board with a new flat one, then place it on spacers so that there will be air flow under as well as on top of the board. I am sure a creative wood worker can make a 1/2" space "seem" like it is not there, like a hidden drawer at the back of a book case.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor P:
After re-acquiring the shape, you may be able to reduce the probability of the problem occurring again. Oils are great, but there are two types (hardening and non-hardening) and they have specific characteristics that determine their value on a project. You used non-hardening oil, one of the best treatments for things like bread boards and butcher blocks. However, non-hardening oils don't challenge losses and gains of moisture. For initial treatment, it would have been fine if followed up with a hardening oil, then ideally a surface coat (long oil polyurethane).
