Question on straightening a redwood slab

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Forum topic by rotherpj posted 03-11-2016 04:22 PM 954 views 0 times favorited 11 replies Add to Favorites Watch
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4 posts in 831 days

03-11-2016 04:22 PM

Topic tags/keywords: slab warp redwood table top

I was able to obtain a 42”x84” x3” redwood slab from northern California that i want to use for a dining room table. It was shipped to southern California where it had 3 coats of tung oil and the top had a couple of preliminary coats of water based poly. The finishing attempt was aborted and it was then transported to CO. On the way, it spent a couple of days in AZ. When the slab arrived, it has about a 3/8” downward cup from the middle towards the ends.

I’ve put the slab on 2”x4”s top and bottom and put some weight on the top 2”x4”s. The cup was length ways as the board with the weighs is laid. I’ve also cranked the humidifier in my house to the max and its been sitting like this for 3 weeks. The cup has decreased to about 1/4” but hasnt changed in the past week.


I’m a complete newbie on this so i would really appreciate some advice on how to get this slab straight and flat.

thanks much

11 replies so far

View martyoc's profile


42 posts in 940 days

#1 posted 03-11-2016 07:37 PM

That is a fine looking piece of redwood. I’d suggest sanding the finish off of the one side so both can absorb the ambient air moisture at about the same rate. It will probably take quite some time to to get an more or less uniform moisture content throughout the slab and may not make it completely flat. Do you know how flat it was prior to the finish being applied?

-- Marty O'C

View Kazooman's profile


1025 posts in 1976 days

#2 posted 03-11-2016 07:56 PM

If I understand your description of how the finish was applied I think that the problem is the result of having poly on just one side. As Marty said, you need to allow the wood to achieve uniform moisture content. When you do get it flat again and apply the poly be certain to apply it to both faces.

View rotherpj's profile


4 posts in 831 days

#3 posted 03-14-2016 01:31 PM

Thank you both for your replies. I’m not positive on how flat it was when it shipped from northern CA. I’ve been told it was flat after they planed it.

Any guess on how long it needs to sit once i sand off the poly for it to get uniform moisture throughout the slab?

View Snipes's profile


177 posts in 2268 days

#4 posted 03-14-2016 04:38 PM

Your wasting your time with the weights. What are you using for a base, can you fix it then? As far as time, that depends, i’ve put them in the sun or by wood stove and seen the cup change sides in an hour.

-- if it is to be it is up to me

View AZWoody's profile


1346 posts in 1247 days

#5 posted 03-14-2016 05:04 PM

Your wasting your time with the weights. What are you using for a base, can you fix it then? As far as time, that depends, i ve put them in the sun or by wood stove and seen the cup change sides in an hour.

- Snipes

Why is he wasting time with weights? I’ve had boards cup straight off the mill and adding weight flattened them out as they were stickered on the drying pile.

But, getting the coat of poly off one side is critical. The humidifier is a good idea but only if the moisture can equally hit both sides. If you don’t, it still might help a little because more than likely, the non poly side has dried faster than the other but getting the slab back to an unfinished state will be important to get the slab to equilibrium to your climate and location for the table.

View rotherpj's profile


4 posts in 831 days

#6 posted 03-14-2016 05:45 PM

I’ll get on taking off the poly tonight. Is the consensus to reset with the weights or not?

thanks again

View Snipes's profile


177 posts in 2268 days

#7 posted 03-14-2016 08:33 PM

ok put the weights on for two years and report back. did this board just come off the mill? is it in the drying pile? you don’t want to add moisture equally unless you want it to stay the same. you need to add moisture to the concave side, or vise versa.

-- if it is to be it is up to me

View JBrow's profile


1361 posts in 944 days

#8 posted 03-15-2016 02:40 AM


I have a little different take on the approach. I agree that removing poly from one side will help. However, I believe that removing the finish on both sides to expose bare wood will allow moisture to enter and leave both faces at an even rate. Were it me I would also cut thin strips from each end and off both edges, just enough to expose raw wood – however I am not sure that exposing raw wood on the ends and edges is necessary. My concern is that Tung oil, even though a penetrating finish, will impede the movement of moisture perhaps just enough so that the cup reverses. Leaving Tung oil on one face with bare wood on the other, and on the edges and ends makes applying finish to the entire flattened slab more difficult and could lead to problems down the road.

As I understand it, the crown side of the slab is the side which holds more moisture than the opposite face. Moisture on the crowned face caused the wood fibers on this face to swell more than on the opposite face. The board can return to flat, assuming it was flat at one time, by either ridding the crown face of the extra moisture or by introducing moisture to the face opposite the crown.

Once both faces of the slab can take in and give up moisture at the same rate the moisture in the slab should equalize and flatten on its own. A downward force applied to the crown side to force the crown out of the slab can squeeze the swollen wood fibers and drive moisture out a little quicker. Adding weight to the crown face is one way to apply this anti-crown force so long as the weight does not block moisture from leaving the slab. It is difficult to say much time is required to flatten the slab by allowing moisture to equalize naturally or by applying a downward force.

Moving air across the crown face, while the opposite face, edges and ends are protected from moving air, should in principle, speed the loss of moisture on the crowned face. Since I have not tried this method, I cannot say how well it might work, if at all. If this approach is taken, the slab should be carefully monitored to avoid driving out too much moisture and thus reversing the crown. The more evenly the moving air is distributed across the crowned face, the better the results I would think could be achieved. However, ensuring an even distribution of moving air across your slab would be a challenge.

Alternatively, slowly and carefully introducing moisture to the face opposite the crowned face along with some flattening force can speed the process. Over the course of about three hours I succeeded in eliminating a crown in a ¾” thick walnut glue-up using this method. Redwood, being a softer wood and the slab being thicker, the time to re-flatten may take more or less time.

The way I re-flattened the glue-up was to place the crowned face down on a flat surface. I used straight cauls laid across the width of the glue-up and applied clamping pressure to carefully flatten the glue-up. I placed the cauls at the ends and, in your case probably 3 additional straight cauls evenly spaced down the length of the slab. When I began applying clamping pressure I was careful to ensure that I did not crack the panel. Then I spread water over the entire surface of the glue-up. I used a damp cloth and just wetted the surface. I did not flood the surface with water. Once the face was dampened, I laid damp clothes over the glue-up to keep it from drying out. I checked progress about every hour. I did not want to add too much moisture and reverse the cup. Once the glue-up was flat, I removed the damp cloths and allowed the slab to air dry. I then stored the glue-up so that moisture could enter and leave the wood evenly to keep the slab flat.

View rwe2156's profile


2962 posts in 1504 days

#9 posted 03-15-2016 12:25 PM

You’ve got a natural plank of wood there and its going to behave/misbehave for a while. Could take a 4 or 5 years to totally acclimate to your house. If it was flat to begin with, it may go back given enough time.

If I’m reading you right, this is a cup not a bow, correct? Most moisture related issues will manifest as cupping, not bowing. Bowing is usually due to internal stresses in the wood fibers which are released during sawing.

The cup could have been there all along or could have happened during its trek thru the desert. All depends on whether is was wrapped in plastic or not. I’m with Snipes. Trying to mechanically wrestle warped wood into submission with weights, cauls, clamps, etc. is usually an exercise in frustration IME. I simply springs back with you release them. But you’ve got nothing to lose.

I would take some moisture readings on both faces and see what that is first.

-- Everything is a prototype thats why its one of a kind!!

View Aj2's profile (online now)


1425 posts in 1821 days

#10 posted 03-15-2016 01:48 PM

It’s a 3 inch thick slab.Id rip that sucker down the middle flatin one face.Run the two halfves thru the planer.Joint the two edges back and glue.You will lose a little thickness and length.
As other mentioned coat both sides equally.
Forcing the wood back flat is probably a waste of time.

-- Aj

View rotherpj's profile


4 posts in 831 days

#11 posted 03-15-2016 08:17 PM

Thanks for all of your responses. There is a lot of expertise here.

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