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MHRestorations

11'x4' Oak Slab Problems

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I have a kiln-dried oak slab sitting at 11 feet long, 4 feet wide (at it's widest point) and an original thickness of 2 3/4".  I built the popular router sled flattening system, and it worked like a dream. I had it dead flat on Friday all across the top.  It is now the following Thursday, and there is just over a quarter inch of cup in the middle of the board.  This slab was kiln dried, sat in the customers barn (out of the elements, but not temp controlled) for 4 months, then in my temperature controlled shop for a month before I started flattening it.  The other thing that is confusing to me, is that when I started working on the slab, the side opposite the one I am working on is what had the cup in it. Anyone ever deal with an issue similar to this in the past? 

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Thick slabs take time to acclimate to the humidity in their surroundings. I can be kiln dried but that doesn't mean it won't ever move. This is what happens when you remove wood unevenly and one side reveals more moisture than another. Something that large may take a long time to reach EMC, also sudden humidity swings can cause problems as well. 1/4" over 4' isn't much. This is all assuming it was dried completely and properly. If it was dried too fast and is still wet in the middle you'll have issues.

just an FYI EMC when it's 30 degrees outside and 85% humidity (typical for winter) is 14%. A shop heated to 65 when it's 30 degrees outside is goign to have RH of around 25%-30% depending has an EMC of 6%. So the slab in a non temperature controled building is going to have to do a LOT of drying once moved into a shop.

I acclimate my 8/4 wood for at least a month before i used it. It entrees my shop around 10% and i usually have to dry it down to 7 or 8%. thicker wood takes more time. More moisture difference takes more time.

Take the guesswork out, if your doing this professionally get a moisture meter.

These are top notch, don't let their website fool you. http://www.electrophysics.on.ca/

Wagner are also good but cost more and do the same thing.

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That's good to know.  I have a general understanding of wood drying principles, but the specifics are good to see.  So let me ask another question: what would you do now with this slab? The customer and I decided to flatten only one side to save as much thickness on the piece as possible, and a table base being made for this would account for any height differences on the underneath side.  

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How much thickness did you remove? 

Agree with all that Chesnut said. Also I know that oak that thick is difficult to dry correctly. Oak is notorious for being a tough wood to dry evenly and the thicker the slab the more issues. There is a chance is was not completely dry internally. Need a moisture meter to determine that. 

 

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Well flattening 1 side is part of the problem. You exposed internal moisture unevenly. You are asking the wrong person 1/4" over 48" isn't enough to be a problem imo I'd put it on a base that could pull it somewhat flat and call it done. I also fully understand that it might cup back the other way at some point and that slabs can be unruly. You or the client has to make that decision.

So first thing I'd do is get that sucker secured. Use thick cauls or steel bars something that can resist a lot of force and clamp that thing flat. Until it's dry it's just goign to continue to move.

5 minutes ago, MHRestorations said:

If I flip it now (with the help of 3 other guys) and remove some material from the other side, will it improve or hurt the situation?

 

probably just get cup the other direction. I'd restrain it let it dry and then remove the cup if you need to. You aren't really goign to know when it's dry unless you have a way to measure the moisture.

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6 minutes ago, MHRestorations said:

If I flip it now (with the help of 3 other guys) and remove some material from the other side, will it improve or hurt the situation?

 

It may cup back, wood pretty much has a mind of it's own.  Moisture content is what's causing your problem. You need to allow the moisture escape in all directions, not just one.

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@Bmac I've taken off right around 3/8 off the top. The side has such a large ridge in the middle (in addition to being slightly cupped) that there was a lot of material to be removed.  I haven't had the help to flip it at all yet, so I may do that, take the belt sander to the rough side, which should allow some moisture to escape that direction as well. Then flip it back over, clamp the sucker down and let it sit for a bit?

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1 hour ago, MHRestorations said:

@Bmac I've taken off right around 3/8 off the top. The side has such a large ridge in the middle (in addition to being slightly cupped) that there was a lot of material to be removed.  I haven't had the help to flip it at all yet, so I may do that, take the belt sander to the rough side, which should allow some moisture to escape that direction as well. Then flip it back over, clamp the sucker down and let it sit for a bit?

Ok, that's a good amt and just doing that to one side likely is a big factor like mentioned above by numerous people. As Spanky said, thick boards are hard to dry, and oak is the harder than most to dry correctly. I'm betting it wasn't all the way dry and taking 3/8th off one side means the newly exposed wood is wetter that the previously exposed wood on the opposite side.. Now what is happening is the newly exposed wood is drying more now and contracting, thus causing it to cup to the newly exposed side.

You really need to figure out how wet it is.

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You mentioned flipping it over a couple of times. If this piece is resting on a solid surface, I would suggest placing small strips of wood under it to elevate it and allow for air circulation.

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We don't have a moisture meter in the shop, but sounds like we will need to get one. Any recommendations on what brand/model to go with?  K Cooper it is currently sitting on two separate carts, so it is elevated with plenty of space for air circulation. I wish I could say that was the reason I put it on the carts, but I'm not that smart yet!

 

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25 minutes ago, MHRestorations said:

We don't have a moisture meter in the shop, but sounds like we will need to get one. Any recommendations on what brand/model to go with?  K Cooper it is currently sitting on two separate carts, so it is elevated with plenty of space for air circulation. I wish I could say that was the reason I put it on the carts, but I'm not that smart yet!

 

Chesnut has some good recommendations above, with a thick slab you really should go pinless. He mention Wagner, which is a quality one, and another model I haven't heard about but I'm interested in finding more about.

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At that thickness, I'd take the reading from even a good-quality pinless meter with a grain of salt.

Do you know the environment that this slab will eventually live in? I made an oak slab table with wood that was air dried 12 months, solar kilned, and acclimated to my unconditioned shop for 4 months. 6 months after going to the client's home, his dehumidifier had sucked the MC to half it's value at delivery, and the top had cupped about 3/4".

Oak holds a LOT of water.

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Sounds to me like oak slabs are more work than they are worth.... Thanks all for moisture test recommendations.  I think my best plan of action at this point is to live with the 1/4" cup in the middle, and let the customer know that it could continue to move (potentially in either direction, correct?) once it is in his home.  

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50 minutes ago, wtnhighlander said:

At that thickness, I'd take the reading from even a good-quality pinless meter with a grain of salt.

I'd take a reading at that thickness from any meter with a grain of salt. There are arguments to be made that the pin meters are even less accurate when thickness increases lot of factors there wood species pin depth ect. What it will tell you is the moisture in the top 3/4" on both sides.

My pin less does read fairly deep. I know that if i have my hand behind a 4/4 board my hand throws off the reading.

1 hour ago, Bmac said:

Chesnut has some good recommendations above, with a thick slab you really should go pinless. He mention Wagner, which is a quality one, and another model I haven't heard about but I'm interested in finding more about.

I can't remember where i heard about Electrophysics. Think on here somewhere. They make a stand up meter that just works for a heck of a deal. They were covered by one of the woodmags in 2010, i think FWW. I have the CT808 been really happy with it. I know wagner does a lot more marketing and is a lot flashier but it's all the same tech.

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+1 on the Electrophysics meter.  The model I have is both pin and pinless, so best of both worlds.  Documentation was good and easy to use.  I paid $203 a year ago.  

Neither pin nor pinless mode is going to get that deep, but it will give you an idea of what's going on inside and gives you something objective to follow as the wood acclimates (or as you remove some surface material).  

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I think I would true up the slab and start from there. If you build a base not level to accommodate an uneven top it could get complicated. After truing the slab it likely will move so it may need some drying time. If there is no time for drying  there is risks to the finished project. You may get away with it. Tough call. Depends on how risk averse you are. I would save the slab for another day and get a dry piece. But I have become cautious in my old age...

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The walnut slabs are out of the kiln now. At around 7% moisture. They was 23.5” wide going in, they are now 22.75” wide.

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