LumberJocks

Cupped Slab

  • Advertise with us

« back to Wood & Lumber forum

Forum topic by lblankenship posted 12-04-2019 07:17 PM 674 views 0 times favorited 31 replies Add to Favorites Watch
View lblankenship's profile

lblankenship

51 posts in 882 days


12-04-2019 07:17 PM

Topic tags/keywords: slab lumber twist cupping wood movement humidity moisture content live edge wood

Hey guys, have a quick question but first the backstory. I purchased a slab online that was preflattened prior to shipping and was cautious of it twisting or warping on me once I got it. Prior to getting it I got a humidity/temp reader to check the info for the room it will live then took it to my garage to compare. My garage is heated 24/7 and insulated with a dehumidifier always running.

I was at about 68 degrees with 30% relative humidity for both the room and garage. I needed to turn the temp up a bit for some epoxy I needed to pour so I turned it up slightly.

Well checking today the garage was 86 degrees and at 20% relative humidity. And now my slab has cupped by about a quarter inch. Prior to this the slab was perfectly flat on the bottom side and I had checked very often because I was trying to prevent this. I was already planning to add c channel to the underside to help prevent this but hadn’t gotten to that step yet.

My question is, if I lower the temp and let the humidity go back to 30%, like it was previously, will the slab move back and remove the cup? Or will it potentially just make it worse? I dont really want to go through flattening it because it was already at a finished thickness from the supplier. Which is why I was trying to be extra careful to keep it from warping on me.

Any other tips on trying to flatten it would be greatly appreciated. Could I add the channel and crank down on it to pull the center down?

Currently having a mini meltdown. Ha


31 replies so far

View John Smith's profile

John Smith

2135 posts in 771 days


#1 posted 12-04-2019 08:15 PM

the questions I would ask are:
how thick is the slab, size, etc.
and it was shipped from where to where ??
age of the slab would also come into play.
a photo or two would be of interest.

.

.

-- I am a painter. That's what I do. I paint things --

View tomsteve's profile

tomsteve

988 posts in 1827 days


#2 posted 12-04-2019 08:21 PM

what was/is the MC of the slab itself is another question

View Aj2's profile (online now)

Aj2

2651 posts in 2406 days


#3 posted 12-04-2019 08:23 PM

Nothing wrong with your idea getting the wood pre milled to finished thickness. It’s worth a try.
From my experience it’s pure luck if it didn’t cup.
You don’t mention what species or the thickness you working with. These all matter.
I’m not believer in trying to force wood straight because i see it as a wrong approach to woodworking.
Buy over sized and sneak up to your finish thickness but don’t be too set on what that is.
There is a way to bring out the cup across the width. You will need to remove material from the hump a 1/4 inch is a lot but some woods will respond quickly.

See the flat sawn pic the wood cups across the width. As if it’s trying to straighten out the growth rings. Removing some of the hump will help bring it back flat.
Good Luck

-- Aj

View wildwoodbybrianjohns's profile (online now)

wildwoodbybrianjohns

712 posts in 156 days


#4 posted 12-04-2019 08:44 PM

I have read somewhere? that if you get the convex side rather wet and lay the board in the sun wet side up, as the wood dries this MIGHT straighten it out? I havent tried this, nor am I recommending that you do either. This technique just as well might cause the slab to twist or bow or check, or all of those?

From my experience, the slab will do what it wants, and the only way to flatten it is by removing more of it.

-- Wildwood by Brian Johns: If you tell the truth, you dont have to remember anything (S. Clemens) Edit: Now where is that darn pencil/ tape measure!

View Kazooman's profile

Kazooman

1411 posts in 2561 days


#5 posted 12-04-2019 08:47 PM

You have nothing to lose by trying just what you suggested. Move the slab into the 68 degrees and 30% humidity environment and see what happens.

View lblankenship's profile

lblankenship

51 posts in 882 days


#6 posted 12-04-2019 08:52 PM

The slab was shipped from NH to OH, the slab is English Elm and is 1.75” thick. Not sure the exact age but I know they air dry for close to a year then kiln dry it the rest of the way. They did have the slab for sale for about a year before I bought it.

I didn’t think to check the MC when I opened it up but checking now it reads anywhere from 2 to 7%. 2% is on the cupped ends and the center of the cup is around 5%. The cup is worse on one end and gradually gets less as you go down the slab. I scanned various places across the entire slab.

I too was expecting the worst prior to purchasing thinking I should buy slightly thicker than I need and flatten myself even though they flattened it but they assured me they haven’t had any issues with movement and I had asked another woodworker who has purchased multiple slabs from them before with no issues.

I had the slab opened up for about 4 days and throughout that time it showed zero movement so I felt good about that, but I’m assuming after the drastic temp and humidity change the moisture from the top side got sucked out and caused the cup.

I’ve taken a few pictures to show as well. Just to mention, the slab is in the crate it was shipped in but I have it sitting up on some blocks so air the underside is still exposed to the air vs sitting flat on a bench.

Any other time I would have no problem flattening with a router or hand plane but I really dont want to take off 1/4 from both sides because I dont have that much material to work with. So I’m trying to figure out another method if flattening it without removing material. I saw stumpy nubs recommended you could steam the concave side with a damp rag and iron, but that was just for a small board and nothing this big. So I’m not sure if that would work.

Thanks again in advance guys. This is always my first place to go for advice and I haven’t been disappointed so far.

View avsmusic1's profile

avsmusic1

564 posts in 1293 days


#7 posted 12-04-2019 09:03 PM

I’d definitely put it back into an environment like it was in when flat – it’s very possible it self corrects. If it does, i’d get a couple quick coats of shellac on it just to seal it up. You can sand them off easily as part of your finish prep.

additionally, i think you should expect this is gonna happen with this slab once it’s finished. hopefully you get it flat and can get everything finished while it is, but it’s likely gonna move seasonally for the rest of it’s life. That said, i wouldn’t worry to much if a little cup manifests from time to time on a table like this personally

View Aj2's profile (online now)

Aj2

2651 posts in 2406 days


#8 posted 12-04-2019 10:03 PM

Too me it looks like the end that’s cupped is very close to the center of the tree. I can see a very short cathedral grain on that end.
If I’m see right there not much your going to do to change that part.
Elm is a beautiful wood but very stubborn.

Good Luck

-- Aj

View AZWoody's profile

AZWoody

1477 posts in 1832 days


#9 posted 12-04-2019 10:17 PM

Was it sitting flat on the table?
To acclimate it, it needs to be on stickers even on the table. Humidity has to acclimate evenly on the top and bottom faces.

Sometimes, just turning it over and leaving it flat on the table will be enough to even out the moisture differential and then turn it back over and put stickers under it again to acclimate on both sides.

View lblankenship's profile

lblankenship

51 posts in 882 days


#10 posted 12-04-2019 10:42 PM

So far I got the temp back down and have a small clothes steamer running to increase the room humidity. I’m almost to 30% and once it gets there I’ll see what happens after a few hours. It’s currently stickered and has been since I moved it in my shop.

View pottz's profile

pottz

7705 posts in 1593 days


#11 posted 12-04-2019 11:13 PM

if it does go flat again at 68 degrees and 30% humidity are you going to have that condition when it’s finished inside your house,i dont think so and that means it’s gonna move again.normally the idea is to get wood dry not reverse it once it is.it’s not cupped horribly looking at your pic’s,id say get it dry then plane to flat,youve got 1-3/4 to work with.good luck.

-- sawdust the bigger the pile the bigger my smile-larry,so cal.

View therealSteveN's profile

therealSteveN

4655 posts in 1182 days


#12 posted 12-04-2019 11:52 PM


Too me it looks like the end that’s cupped is very close to the center of the tree. I can see a very short cathedral grain on that end.
If I’m see right there not much your going to do to change that part.
Elm is a beautiful wood but very stubborn.

Good Luck

- Aj2

Agree with all of that. Elm is beyond stubborn. Plus that last pic looks like the pith, and that isn’t going to uncup anytime soon. If you didn’t remove any wood from either side, and it just did this on the arrival to it’s new home, you are probably just getting a weather report from NH to Ohio.

NH, flat and sunny

Ohio, cupped with a bout of nausea.

Buy fat, wide, and long. Chop to near what you want, check for damage. No damage in sight, proceed. If damage shows attend to it as you need to. Not to do this, you get the nausea.

Best of luck with it.

-- Think safe, be safe

View pottz's profile

pottz

7705 posts in 1593 days


#13 posted 12-04-2019 11:56 PM

your makin me queesy TRSN-lol. i agree always buy bigger than needed to allow for flattening.

-- sawdust the bigger the pile the bigger my smile-larry,so cal.

View Aj2's profile (online now)

Aj2

2651 posts in 2406 days


#14 posted 12-05-2019 12:16 AM


Too me it looks like the end that’s cupped is very close to the center of the tree. I can see a very short cathedral grain on that end.
If I’m see right there not much your going to do to change that part.
Elm is a beautiful wood but very stubborn.

Good Luck

- Aj2

Agree with all of that. Elm is beyond stubborn. Plus that last pic looks like the pith, and that isn t going to uncup anytime soon. If you didn t remove any wood from either side, and it just did this on the arrival to it s new home, you are probably just getting a weather report from NH to Ohio.

NH, flat and sunny

Ohio, cupped with a bout of nausea.

Buy fat, wide, and long. Chop to near what you want, check for damage. No damage in sight, proceed. If damage shows attend to it as you need to. Not to do this, you get the nausea.

Best of luck with it.

- therealSteveN

Thanks Steve, For the nod in my direction. I hope the op can work it out somehow or find a acceptable use for the wood.

-- Aj

View lblankenship's profile

lblankenship

51 posts in 882 days


#15 posted 12-07-2019 01:32 AM

Just an update for everyone, after getting the temp/humidity back to normal I took a wet paper towel and wiped the concave side of the slab. I repeated this multiple times since originally posting and checking the moisture content each time before wetting it down again. This afternoon I have almost removed it completely with the cup being around 1/16th of an inch which I can work with.

However, I flipped the slab during all of this just to check the other side because I was worried of twist when I set up some winding sticks and found the other side appeared to have been effected much worse with almost 1/4” on both sides of the slab. I could place a level and teeter it like a see-saw. I also had noticed the original chalk markings and stickers were there from when the original pictures were taken almost a year ago which tells me for sure the top side had not been reflattened prior to shipping. I was told they checked it, so maybe it was flat and something else happened. They use a wide belt sander so suspect they might have only ran the bottom through prior to shipping or only checked the bottom and not the top.

I haven’t had a chance to flip it back over to see if it’s no longer an issue now the cup is almost completely gone from the underside because I have epoxy curing. That third picture I posted earlier was a bark inclusion and I had cleared it all out to fill. I’m working on making my c-channel supports tonight and am hoping it’s smooth sailing from here.

Thanks for the help and advice. I appreciate it.

showing 1 through 15 of 31 replies

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics

HomeRefurbers.com