Wood Movement Destroys Table Top

      Another designer learns the hard way, as a table top cracks because of ambient moisture changes and restrained wood movement. May 3, 2011

Question
I'm a designer who recently had a tabletop made from reclaimed wood. The craftsman used reclaimed floorboards and re-enforced the underside of the table with both wood and steel. The problem is the top has been cracking since my client received it.

After the first series of cracks the carpenter who made it addressed the table-however, the table has begun to crack again. This time however he feels no obligation to address as he believes the central heat in the apartment is causing the cracking. I've consulted a knowledgeable source who tells me the problem is being caused by the metal strips affixed to the underside and how this prevents the top from properly expanding and contracting. Z-clips were suggested.

Before I have another conversation with the table top maker I wanted to get some feedback from folks out in the furniture making community. Please review the image below and let me know your thoughts. The table is 1.25" thick and approx. 36" in diameter.


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Forum Responses
(Furniture Making Forum)
From contributor N:
I would tend to agree with your source. Unless the means of fastening the steel allows for expansion and contraction the top is doomed. I doubt the steel is slotted as that would defeat the purpose served as flat bar positioned that way has very little structural integrity.



From contributor W:
The person who made that table top may be a reputable carpenter, but he knows little or nothing about furniture construction. It looks like a sloppy job; apparent glue squeeze-out around the wood scabs underneath, raw metal strips with ugly screws holding all.


From contributor M:
I would agree that those straps are definitely the culprit. Those boards have to be able to expand and contract. Table tops can be a disaster waiting to happen if they aren't put together properly. The points at which the base is fastened to the top isn't helping either.


From contributor C:
Slotted holes are an absolute must to allow the wood to expand and contract. I don't really understand the routed in wood method. This doesn't make any sense either.


From contributor I:
Wood expands and contracts across the grain with humidity changes, but not with the length. If you don't allow for the expansion the top will crack. Carpenters don't have to worry much about this but cabinetmakers run into it all the time. It's hard to tell from one picture, but if he routed in those crosspieces, then glued and screwed them without room for expansion or expansion slots for the screws, that would do it too. Unless you want a picnic table look, this will never be right.

Looks like he went to an awful lot of trouble without knowing what he was doing. He's probably thinking of all his time and effort, and you are thinking of the product being right.



From contributor J:
The whole construction was ill-conceived. The metal straps are wrong. The routed-in cross-grain pieces are wrong. This table is not salvageable. As already noted, the guy clearly put a lot of effort into this without knowing what he was doing. He seems to have meant well.


From contributor K:
The only way I can see that you can salvage this is to take all that metal strapping mess off, remove all the screws then saw the pieces apart at the glue lines and start from scratch. Because this would be a lot of trouble I would drop some stain into the cracks distress them a little bit and make it look like you wanted a very rustic top.


From Gene Wengert, forum technical advisor:
We know for 100% certain that the only reason why wood moves (shrinks or swelling) in use is because its moisture content is changing. In this case, the air in the customer's home was drier (the EMC was lower) than the wood's MC. This caused the wood to dry and shrink across the grain. Wood seldom shrinks lengthwise; it rarely does in special cases.

First, the wood should be within 2% MC of the expected EMC. For reclaimed wood, it is common to find 11% MC. The customer is likely at 6% EMC (maybe drier if it is really cold outside and they do not have a humidifier). So, first the MC must be close to the expected EMC.

Next, a table must be designed to allow for the expected movement, as several posts above have well indicated. As a rough rule of thumb, wood moves 1% for a 4% MC change. Some species move slightly more (like oak) and some move less (like teak).

Finally, although you might allow for drying, it is well to remember that in the summer the EMC might reach 9% EMC if the windows are open and a little drier with a/c units. So, the top will move initially as it reaches equilibrium and then will cycle yearly. The worst is when the MC change is rapid (such as moving from a shop to a customer, or from a new unheated home to heated, dry home).



From contributor G:
We have been building furniture for interior designers for 28 years and I have found very few who understand construction methods and wood movement. We always set a test piece of any given lumber and measure the movement. It does expand a little along the grain and a significant amount cross grain. The opposing wood pieces with different grain direction are not good as well as the metal strips. The builder needs to go back to the drawing board. He could make a smaller circle to mount the base to and attach that to the top allowing for movement. It is his responsibility to make it right if he wants to continue getting new customers.


From contributor O:
Whoever is given the job of pricing and then building this table has the responsibility of determining the best way to do so. The designer wants the look, it is the maker's job to get there, or explain why it can't be done. A competent maker should be able to have at least three solutions to building the table top responsibly.

For the maker, this is a great opportunity to educate the designer/customer and let them become aware of your particular knowledge in the area. You also want to offer at least one solution to the designer and justify the costs with each along with pros and cons.

If the potential customer only seeks the low bidder, that is his/her problem. They must be willing to take the risk that the maker has no clue. The general public has almost no knowledge of wood movement, and looks at it all as somewhere between voodoo or purposeful deception in order to wring a few more bucks from them. Education is the only way to overcome the low bid mindset or distrust that is so prevalent in the industry.



From contributor S:
Don't feel like the Lone Ranger. This notion of expansion and contraction is the least understood area of woodworking. Lots of folks (some of whom ought to know better) think that by putting in enough screws, or using enough glue, or applying a sufficient number of coats of finish, they'll stop this wood movement business once and for all!

Think of this principal as very similar to how metal reacts to changes in temperature. Metal expands and contracts (inevitably) to changes in temperature; wood does the same with changes moisture. Gene does a good job explaining the science, but bottom line is you have to design around this principal or face the consequences.

I think the guy actually building the piece; again your carpenter should have known about this challenge and should have educated you accordingly. He charged you (and your client) good money. He should make it right. But depending on his level of expertise, he could argue he was just following your instruction.

Bottom line here, if this was my good client and the carpenter wasn't willing to make the job right, I'd get someone else in to do it and eat the cost without giving it a second thought. As far as my (your) good client would know, it was just a little miscommunication, here's your nice table as ordered and sorry for the delay. But that's just me.



From Gene Wengert, forum technical advisor:
If properly glued, the glue joint will be stronger than the wood itself. This means that the cracks would be in the wood and not the glue joint. In any case, moisture is the reason the cracks opened.


From contributor N:
Moisture is the reason the wood in the top moved, the construction is the reason that the top is self destructing.



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