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Help With Table Top Bowing

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Forum topic by Lenny posted 06-15-2021 02:51 PM 483 views 0 times favorited 12 replies Add to Favorites Watch
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Lenny

1707 posts in 4681 days


06-15-2021 02:51 PM

I’m looking for thoughts/ideas on why this table is bowing. My instinct and experience tell me it’s a wood movement issue but I am not certain. The bowing seems most prominent at both ends so I believe it is linked to the legs. I routed to a depth of ¼ inch and to the exact dimensions of the top horizontal leg member to inset the legs (snug fit). Eight screws hold each leg in place.

The table top is a glue-up of about seven boards of quarter sawn white oak (QSWO). While QWSO is known to be quite stable, it will of course expand and contract. Are the legs restricting its movement? If so, do you think lengthening the inset 1/8 inch past the ends of the legs and perhaps elongating the screw holes will solve the issue?

If not an expansion/contraction issue, other thoughts?

UPDATE: I neglected to mention that the finish is Top Boat Spar Marine Varnish. About 4 coats on underside and 6-7 on top and edges.

-- On the eighth day God was back in His woodworking shop! Lenny, East Providence, RI


12 replies so far

View Robert's profile

Robert

4631 posts in 2634 days


#1 posted 06-15-2021 03:35 PM

I thin kit is a moisture imbalance issue. The top is shrunk more than the bottom due to drying effects of sun & wind while the bottom is protected.

I don’t know what you can do to fix it when white oak moves, its stronger than steel!

I don’t think this is cause, but the flat steel cross plate should be fastened to allow for movement – dado longer than steel/screw holes elongated.

My suggestion is to remove all the steel, flip the table over on some sawhorses and see what happens. I’ll bet a donut it will go flat.

It this is it, then re-evaluate the design and attachment of the leg structure. Many people use C channel or angle iron imbedded in grooves to hold the table flat.

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

View Rich's profile

Rich

7060 posts in 1743 days


#2 posted 06-15-2021 03:36 PM

In the second photo it appears that the two middle boards are actually two boards glued face-to-face to make a thicker one. That’s bad news. Those pieces will move differently and any number of board defects will occur.

I can’t tell from the photos, but I’d bet that the bowing is in the middle, where those two boards are, and the rest is relatively flat.

I did the same thing a few years ago. I was making interior doors and was short on the 8/4 stock I needed to do the last one. I decided to glue two 4/4 pieces since it was a hinge stile for a closet door and wouldn’t be visible. In no time that board looked like a pretzel.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View David's profile

David

79 posts in 3181 days


#3 posted 06-15-2021 03:45 PM

Lenny
Looks like the boards in the middle of the table are the problem. It seems that they are not soil, they have been laminated to make the thickness off the top. The grain is not in an opposing direction, which will bow. The legs cross member seams to be bowing with the top. So the cross member may not be heavy enough to counter the force of the wood. You could try to router in some cross stabilizers, every 8-10 inches, and elongate the screw holes in the cross members of the legs. Hope this helps. If you want to give me a call.

-- http://littleredshop.net "A man that works with his hands, knows his soul" " Have Fun, Go Fast, and Take Chances for Christ's Sake!!"

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Aj2

3941 posts in 2952 days


#4 posted 06-15-2021 05:37 PM

I agree with Robert the top is gaining moisture at night then losing it during the day. Swell shrink swell shrink on and on.
Solid wood table do not do well outdoors unless they are very very thick.
A good outdoor table is the common picnic bench style. It’s a proven design that stands the test of time.

-- Aj

View pintodeluxe's profile

pintodeluxe

6437 posts in 3967 days


#5 posted 06-15-2021 05:55 PM

Lengthening the slots that accept the legs and slotting the mounting holes will help. I’m not certain it will stay dead flat, but it will probably be good enough.

I would expect some cupping / warping / twisting in a wood top exposed to the elements (even if the boards were all solid 8/4).

Hope you can make some minor tweaks to improve things.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View vjc's profile

vjc

22 posts in 1540 days


#6 posted 06-15-2021 06:36 PM

That leg design does nothing to counter the tendancy to warp. The embedded plate just bends along with the wood. If you can get it flat again by flipping it over for a while you might prevent it from happening again by welding some braces from the cnter of the horizontal plate down to the legs. Even better might be another plate at 90 degrees to the horizontal plate, maybe 2” wide, from one leg to the other.

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CaptainKlutz

4655 posts in 2648 days


#7 posted 06-15-2021 07:01 PM

+1 above comments:
The laminated sandwich in middle will always be issue that you need to deal with. The glue joint moves less than normal, and can create strange twist/warp forces. If you want to save the top, only choice for sort of flat top is brute force restraint.

- Plate mounted to table should be u-channel or box shape if you want it to resist bending. But Oak is as strong as steel, and you would need ~1.5×1.5” minimum structure to resist the wood movement during normal RH changes.

- Your leg mounting design needs to accomidate the movement.
Screw slots in plate must be slotted, and dado for plate needs to be larger so the wood can more.
Try using the wood movement calculator on Woodweb to see what happens as Oak moves from 8-15%, which is normal range for indoor furniture. Outdoor stuff has higher range. :(

- The sag in table is normal. Check out the sagulator to see how much a span of wood can move. If you want to stop the sag, will have to add 2 braces running between legs. They need to be same thick structure. Even though wood movement end to end is small, they should have slotted screw holes too.

Best Luck.

-- If it wasn't for bad luck, I wouldn't have no luck at all, Doom, despair, agony on me… - Albert King - Born Under a Bad Sign released 1967

View Oldtool's profile

Oldtool

3263 posts in 3344 days


#8 posted 06-15-2021 07:42 PM

Hello Lenny,
Sounds like all the above is good advise, common theme being moisture problems, and there’s not much im could add to correcting the problem.
I would add however, this is an outdoor table, so if it does goes flat again with a complete drying, you might want to completely coat it with clear epoxy to prevent future moisture invasion. Just a thought, it seems to work for the wood boat craftsman on YouTube.
Good luck.

-- "I am a firm believer in the people. If given the truth, they can be depended upon to meet any national crisis. The point is to bring them the real facts." - Abraham Lincoln

View Lenny's profile

Lenny

1707 posts in 4681 days


#9 posted 06-15-2021 10:10 PM

Thank you to all who have posted a response. There is much to digest and consider.

-- On the eighth day God was back in His woodworking shop! Lenny, East Providence, RI

View Madmark2's profile

Madmark2

2955 posts in 1742 days


#10 posted 06-15-2021 10:56 PM

The top will do what it wants. No amount of 1/4” steel will stop it. If the steel is strong enough then the top will split. You MUST accommodate the motion or the wood will fail.

White oak can move 5% from green cut to overdry. On a 36” table that’s over 1-3/4”!

Obviously the finish isn’t moisture tight. Flip it until it goes flat and the seal it up tight with multiple coats of a good uv resistant finish.

Grind slots in the metal stand on both ends. Leave the center hole as-is. Tighten the center screws but leave the outer screws a hair less than tight so the screws can slide in the slots.

Seal the patio it sits on to reduce moisture/humidity from the concrete wicking up.

-- The hump with the stump and the pump!

View squazo's profile

squazo

245 posts in 2799 days


#11 posted 06-16-2021 12:17 PM

Its sagging because there is no grain running 90 degrees to the top.

View Lazyman's profile

Lazyman

7385 posts in 2541 days


#12 posted 06-16-2021 12:37 PM

It can’t hurt to elongate the holes in the metal but that may not have much affect on the warping since it is probably caused by the bottom and top drying at different rates, especially in the sun. One thing to try as part of diagnosis is to turn the table upside down onto the lawn in the sun for a few hours and see if that at least temporarily flattens it out. The soil will add a little moisture to the top side while the sun will dry out the bottom. If that helps even a little, then differential drying from the sun is the culprit. Short of bringing it inside or building a covered patio, there may not be much else you can do other than a design change where you rip it into separate pieces and then screw it back down leaving gaps between the boards for shrink and swell.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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