Relieving cupping?

[forrestM]

I sawed have some white ash slabs that are between 8-9 percent that I had planed down to 1-7/8" for some countertops for my house. They are 25-1/2" wide and they look pretty good except one 37" long section is cupping pretty noticeably. I still haven't finished sanding and oiling them. Is there a way to relieve cupping in something so wide and hard? I was thinking I could make some lengthwise kerfs on the underside that would allow me to flex it back into flat. But I'm not sure how deep or how many cuts would be needed. Any tips out there?

Thanks,
Forrest

[WDH]

First off, wood does not move unless it changes moisture content. So, even if your ash was at 8 - 9% and stable, while working on it the moisture content has likely changed, probably from the change in the environment from where it was stored to the environment of where you are working on it.  One issue that many times occurs is working on a dry stable piece in a non-climate controlled garage or workshop where the piece is picking up moisture from the more humid air.  To compound that issue, people many times take a significant amount of time to work on their project leaving the piece in a more humid environment for an extended period of time since life and good intentions gets in the way.  One no-no is to leave a piece of wood in an environment where it is gaining or losing moisture laying flat on a work surface.  One side is exposed to the air and is gaining or losing moisture while the bottom that is flat on the surface does not.  So the top starts shrinking or swelling more so than the bottom and the wood moves by cupping or warping.

One way to help with this issue, if it occurs, is to flip the piece over 180 degrees for a bit and let the bottom that is now the top be exposed to the air.  I have seen cupping reverse somewhat by doing this.  The best solution is to avoid the problem in the first place by making sure the piece is exposed to the air on both sides or that both sides are fully covered so neither side is exposed to the air.  I would never leave a thick piece or a glued up panel laying flat on a work surface overnight or for several days, weeks, months especially if a kiln dried piece at say 8% moisture content is brought into a non-climate controlled space (like my workshop in my barn). 

Another issue that occurs that leads to wood movement is planing off a good bit of wood from only one side of a piece that is stable with the growth stress balanced thereby unbalancing the piece and releasing unbalanced growth stress that leads to wood movement.  This results in bow many times. 

So your issue could be from growth stress being unbalance by planing unevenly or could be from unequal gain or loss of moisture from the two faces of the piece. You might try first to position the piece in the opposite way from the initial cup and see if it flattens back out.  Anecdotally, I had a customer buy some 18" wide pecan that was 30" long and 1.5" thick to make charcuterie boards.  About 10 days later he called and told me that the pieces had cupped very badly.  The wood was kiln dried to 8% when he bought it so I knew that it was dry, so I asked him what he did with it when he got home.  He said that he laid the pieces on his porch after unloading them from his car, forgot about them, then went on a 4 day trip in December leaving them laying flat on the porch during a week where it rained every day.  Hmmm.  He brought them back, and he was all stressed out since the wood was for Christmas gifts with Christmas only a few days away.  I replaced them, and I laid the original pieces on the floor of my climate controlled lumber storage room opposite the cup, and in a few weeks they laid back down perfectly flat like the day I sold them. 

[Don P]

First off, wood does not move unless it changes moisture content.

Another issue that occurs that leads to wood movement is planing off a good bit of wood from only one side of a piece that is stable with the growth stress balanced thereby unbalancing the piece and releasing unbalanced growth stress that leads to wood movement.  This results in bow many times.  

That's two possible reasons  

On a jobsite in a hurry I've laid a board out for the dew. Unspeakably wrong, it will often lay the cup down, but now you are wide and over emc.

[WDH]

Managing and dealing with moisture content is the most significant issue that I deal with sawing, drying, and using solid wood.

My Daughter and SIL are building a new house.  All the baseboard and window trim is MDF.  Painted white.  Floors are engineered composites that snap together.  There is little real solid wood in new homes anymore.  Everything is engineered or plied or pressed together.  Depressing, but much easier to deal with than real wood movement issues.  But, I still want the real stuff even with the challenges .

My house is full of solid wood. Paneling, doors, cabinets, trim, moulding, etc.  The style today is faux wood, white farm house style.  Everything is painted, mostly white, even cabinets.  The Woke generation wants paint.

[K-Guy]

The style today is faux wood, white farm house style.  Everything is painted, mostly white, even cabinets.  The Woke generation wants paint.

I know, the same crowd that talks about wanting things natural, GMO free, gluten free and organic!!?? I guess I'm too cynical. ;D

[Don P]

Last time the painted house was popular grandad's generation painted every stick they could. By the time we came around and wanted natural, when we stripped it, shoot, I've found walnut under the paint several times. When this generations kids strip that MDF it's going to be a little less impressive.

The closer it is to raw the greener it is and a real person made the wages not a robot.

[Old Greenhorn]

Don, you poked up a story in my head. Years ago I met an older fella that, in his heyday, made a good living restoring very old homes built in the times prior to 1850 into new modern homes to suit the elite folks that bought them and had the cash to make it happen. When I first met him, those days were over and his health was declining and the folks that sang his praises and invited him to big parties stopped calling. Still I found him a neat guy with a lot to share and things to teach. I enjoyed going and helping him out as more things became difficult just to spend time with him. He now lives in a small cabin ne rebuilt for the owner who allows him to rent it for cheap. He is nearing the end of his time. But I digress.
 As you might guess he got/took a lot of odds and ends out of these houses in the demo process which he saw value in and wanted to use in the rebuild. Too many times, the client would say "Take that to the burn pile, we want all new inside."
 So I went over to help him one day when he was having a rough time medically and he invited me in to warm up and have some coffee. As I stood in the doorway and looked around inside his cabin, my eye was drawn to the floor. "Gary, holy cow, how wide are these floor boards?!" He said "they vary from 18-22" and I took what I could get". 'Where'd you get these?' He smiles and says "Well when I was doing _______'s house those were the floor boards and I wanted to pull them, re-plane them and re-lay them after we replaced the sills. But ___ said 'Nope, we want something new and solid' so I put down a nice maple floor for them and took these home before they hit the burn pile." As he is talking I am now down on my knees trying to figure out the species, but it looks weird to me, so I ask. "oh" he says "this whole floor is chestnut. That house was built in 1780 and they wanted to burn all this. I put it in a barn until I had a use for it. When I die, somebody can make some money off it, probably worth more than the cabin itself."

 So all the floors in that cabin are 18-22" wide x 6/4 chestnut and man they are beauties. I left out the property owner in this story because I think just about everybody here will know who he is and I don't want to make him look foolish. You can PM me if it's really killing you. ;D

[kelLOGg]

WDH
That is excellent customer service; it will keep him as a customer and attract others. 

[moodnacreek]

Lay cupped boards round side up on the lawn until they go flat then back in the sticker pile and stick all the way to the ends with lots of weight on top.

[GeneWengert-WoodDoc]

If all the pieces are cupped toward the bark, then you are seeing the natural cupping with change in moisture content that occurs in all pieces of wood.  There is less cupping in quartersawn and rift sawn pieces, and more in flat sawn from nearer to the center of the log...within 15 rings, cupping is quite severe characteristic.

When drying, you may be able to reduce cupping by using weights, but when the moisture content changes in use, the , interior or exterior, wood will cup more than normal.


Cupping occurs whenever the wood is subject to moisture change, so, for example, can occur in an installed floor or and exterior deck.

In the case of an interior floor, if all the pieces cup upwards, but the pieces are randomly installed so the bark face is up and sometimes down, then you have a moisture issue, but the bottom side is wetter than the top side.  This can happen after installation.  Once an interior floor cups, like in the winter when the top side is exposed to drier air than the bottom, the cup cannot be removed by reversing the moisture gradient.  Sanding is the only choice.  Once sanded, future cupping after this initial cupping is very slight.

[blackhawk]

My Daughter and SIL are building a new house.  All the baseboard and window trim is MDF.  Painted white.  Floors are engineered composites that snap together.  There is little real solid wood in new homes anymore.  Everything is engineered or plied or pressed together.  Depressing, but much easier to deal with than real wood movement issues.  But, I still want the real stuff even with the challenges .

My house is full of solid wood. Paneling, doors, cabinets, trim, moulding, etc.  The style today is faux wood, white farm house style.  Everything is painted, mostly white, even cabinets.  The Woke generation wants paint.

I talk about this all the time.  Everything in new houses are painted white.  We have a very upscale housing development a couple of miles away on a golf course.  All house are between $500K and $1M.  I have been in a few and see pictures when they go up for sale.  I have never seen a single one of these houses with anything that is not painted white except for kitchen cabinets.  I don't think anyone wants to build anything where they can't hide the flaws with caulk.