Wood Movement Considerations for a Large Table Top

      Swelling and shrinkage place stress on wood table tops, especially if both sawn lumber and plywood are involved. Here's advice on construction details. March 13, 2007

Question
What are the chances of joint failures on a 4'x8' 4/4 Goncalo Alves top? The 4/4 boards will sit on a substrate and the whole thing will be wrapped with 2" boards to give the appearance of thickness. Any help is appreciated.

Forum Responses
(Furniture Making Forum)
From contributor F:
I think you're looking for big trouble! The 4/4 has to be able to move with changes in humidity etc. If the substrate doesn't ruin your top, the edging will. Use thicker stock for your top or veneer over a suitable substrate.



From the original questioner:
I would agree that veneering is the way to go.


From contributor A:
To contributor F: How much bigger stock would you suggest for this project?


From Gene Wengert, forum technical advisor:
I agree that the chance of success is close to zero. Further the skirt you have running around will not be changing size (in the lengthwise direction) while the top shrinks or swells and that will create even more problems. In any case, make the top float so it acts as one piece. Do not fasten the skirt unless you make sure that it can move independent of the top.


From contributor F:
I would use 8 or 6 quarter stock.


From contributor B:
I would agree in that I think the 4/4 is a little light for the table top. I typically use 6/4 stock, although I have used 5/4.

If you want to use the 4/4, figure on using the boards the 8' way so that the grain is moving across the width. You could glue a two inch wide edge with the grain the 8' direction.

For the ends, you will have to mill a two inch breadboard end that stands a little proud of the table edge. Fashion a tenon on the table blank and a dado on the bread board end. You can glue and pin the center, but you will have to use an elongated slot on the tongue on the ends to allow the table to expand and contract.

To help keep the table top flat using 4/4, you could use three cleats across the width on the bottom of the table that have slotted holes to receive a truss head type of screw. The screws will keep the top flat to the cleat, but the slotted hole would allow the top to expand.

You could probably make the 4/4 work as long as you are careful to let the top expand and contract. I have seen a lot of old antique tables that are no more than 3/4" thick and they have held up over time because the builder was careful to utilize joinery that allowed the top to move.



From Gene Wengert, forum technical advisor:
Can you explain a little but more on what you mean by the 4/4 boards will "sit on a substrate" as this may be a problem. I think the recent posting all considered that you did not have a substrate, but merely a frame.

Contributor B's comments about construction techniques to minimize movement problems are great procedures. But I do want to emphasize that the first step is to get the correct MC so any movement is slight.



From the original questioner:
Basically we were considering using 4/4 boards of Goncalo Alves to make a 4' x 8' top. We attached that top to a 4x8 sheet of ply by screws through elongated holes. Then a final sheet of 1/2" ply was glued to the bottom of the 3/4 to give us roughly our 2" thickness and to hide the elongated screw holes. We then considered nailing our 2 " skirt to the ply only. The customer wants this table to appear 2" or more thick.


From Gene Wengert, forum technical advisor:
The plywood will certainly be more stable across the width of the lumber than the lumber. So MC changes in the table will result in more movement of the lumber than the plywood across the lumber's grain, but the reverse along the lumber's length. As the plywood is shrinking in two directions, you would seem to need slots that run in both directions. Not sure how to do this.
Plywood does often cup when the MC changes.

In any case, put the lumber and the plywood together in a humidity controlled environment (perhaps 33% RH) for a week or so to allow all the pieces to achieve uniform starting MC. Overall, I would prefer an all lumber top, using 8/4, as suggested earlier.



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