Why Glass Tops Can Warp Tabletops

A cautionary tale of a table that cracked because of glass resting on it. October 19, 2005

I'm looking for professional agreement on this. I have a client who put a glass top on a 72" round, solid 8/4 table. The table warped severely and binded the clips, and it cracked. The wood was kiln dried around 8% MC. I am a well-educated woodworker looking for backup from others so my client doesn't think I'm nuts. You cannot put glass on solid wood of any significant dimension, especially a 72" cross grain width, without trouble. Even a veneer table at this dimension is susceptible to moisture changes and, although much safer, will warp - it's just a question of how much.

Forum Responses
(Furniture Making Forum)
From contributor P:
Yes, it is definitely the glass causing the warping - the same thing happened to us once. Once, that is. Since then, I routinely warn clients who order solid conference tables that glass will cause warping and destroy the table. I hope that you gave the client a heads up on this possibility prior to accepting the order.

From the original questioner:
No, I didn't warn them, as I've never had a client put glass on our tables. They are all polished oil surfaces, generally bought for the wood feel. It never entered my mind to think of the glass. I have offered to rebuild this for free, however, if the client picks up delivery. This is a very good and nice client, but they are not convinced that it wasn't the table or craftsmanship. Any agreement that even a veneer table should sit in its new home for several months before taking a glass top, and even then one summer with no air conditioning or similar circumstances would cause warping?

From contributor A:
Yep - it is the glass. For the large tables and conference tables that we do, if the client wants glass, we insist on the clear spacers that the glass company can supply. The small, often 1/16" rise in the glass gives enough air movement to help prevent warping.

From contributor B:
You need to explain to your client why the top warped after they installed the glass. They need to understand that the glass sealed off moisture penetration to one side of the solid wood, causing moisture only to penetrate from below the table. This caused the lower side to swell while the upper, sealed off side remained the same. The result is the warp that caused the glass to break. Without the glass on top, both sides would have absorbed moisture evenly and the table would have expanded and contracted with minimal (or no) warping. All of us here on the forum, of course, know this, but your client obviously does not, since they are not a woodworker. I've always found that a bit of education on wood movement can go a long way in helping customers understand our construction issues. I feel you are being more than generous in offering to remake the table for free. Also, I think you should consider printing out these responses and letting the client read them.

From the original questioner:
Thanks for your response. I explained this clearly in a written email, to even greater detail than you did. But, of course, we all know that we didn't learn all this in one conversation, and most of us probably learned about wood movement the hard way when it was our fault. To clarify, the glass didn't break - the glide clips got caught due to the warping and the table cracked. These are very nice folks, but they have seen glass on tables before and I don't want them to think I'm avoiding taking responsibility. I'm not convinced the little plastic protectors (probably 1/32 lift after they squish) would be enough air movement for a solid wood table. Are your conference tables on engineered panels? It would help, but a radiator leak or a room with ten watered plants or any drastic changes in humidity (or heat) would cause problems on a surface this big. Fine on a
sideboard or even a 36" wide table in veneer. But I am convinced the only way to do it is glass both sides on a big round table.

From contributor B:
We don't make conference tables, although I've worked on a few over the years. Wood movement, though, is universal to all woodworking endeavors. I agree that you would have to seal both surfaces in order to stabilize the wood. Also, I too think that a 1/32" spacer might not be enough air space in this situation. If the customer is going to insist on a glass top, I think you need to use a veneer panel (ply or MDF core) and seal the bottom with 1/32" laminate. Even that, though, could cause an opposite problem if the laminate is a tighter seal than the glass.

From contributor P:
The 1/16 spacers are not enough to prevent warping. Warping can happen in a matter of hours if the heat is turned on or even if bright sun falls on top of the table. This is especially true of oil finishes, which don't provide much of a moisture barrier. I make a lot of conference tables and no longer make any of them out of solids. (And I make a lot of dining tables as well - at least half of them are all solid, but no one ever puts glass on them.)

I don't think that the OP is making a mistake by offering to replace the top for free - sure, it hurts, but if you value the client, then it's the only way to go. Incidentally, when we had a similar problem with a client (who I never met - the entire transaction was done via email), I also replaced the top for free. The client then ordered another table base and decided to use the warped tabletop as a workroom table. So I got a little something for doing the right thing. It's been my experience that going above and beyond in order to solve customer problems, even when it isn't my fault, has a long term payoff in enhanced reputation and helps provide a steady stream of referrals. Plus, it's good karma and helps with restful sleep. Good luck on salvaging this situation.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor T:
Remember finish balance. Top and bottom the same. The addition of the glass to the top changed the balance. Top became, essentially, impermeable (spacers mean nothing), bottom still had the exposure of the oil finish, which as mentioned before, is not much of a finish at all. I think you are taking big chances with an only oil finish on 8/4 material. I've been at this 30 years, and we do lots of solid wood glue-ups for contractors and other woodworkers, and I hate this time of year.