Advice on Wood Movement

[Brian Kent]

I want to add thickness to the back of this coffee table. (Pictures show the back).

The trim is 4/4 walnut. The center is 1/4" burl on 1/2" birch ply. So far so good. The legs will be a very sturdy maloof inspired pedestal. I realized there was a problem because I need to attach the legs using 4 screws but they will be attached to only 1/2" ply. Very short screws.

So I was going to build up the center of the table with 4/4 walnut, covering the plywood and extending about an inch onto the trim.

Late last night I woke up and realized that if the extra layer of walnut moved, it could crack the whole top apart.

How can I add a little thickness support to the back of the table top without creating wood movement problem?

[Mike Cruz]

As long as the only place that you attach the bottom piece is near the center (lengthwise), allowing the extremes to move freely, you should be okay.

Or, you can attach the bottom piece all the way accross with screws, BUT you NEED to make your pilot holes in the bottom peice slightly elongated to allow for the movement. Make sense? If not, I can try explaining that a different way.

[Brian Kent]

Yes, that does make sense. Up until now I was thinking of gluing the extra supports to the underside of the table and screw the legs to the support, but I could just as easily screw the support to the table as well.

[George Bregar]

Isn't that the table top? Not sure what the back is. Seems like you have your answer, I'm just confused...

[Brian Kent]

George, yes that is the table top. It was just too thin and flimsy for what I want so I am beefing it up underneath.

[Howard Acheson]

Isn't that the table top? Not sure what the back is. Seems like you have your answer, I'm just confused...

I think Brian really means the bottom, not the back. Had me confused too.

Let me throw another stone in the water. Gluing 1/4" thick solid wood to a plywood substrate could end up being problematic. The general rule is to have veneer no thicker than 1/8" glued to a substrate. Might get away with it if the relative humidity remains fairly constant.

[George Bregar]

I think Brian really means the bottom, not the back. Had me confused too.

Let me throw another stone in the water. Gluing 1/4" thick solid wood to a plywood substrate could end up being problematic. The general rule is to have veneer no thicker than 1/8" glued to a substrate. Might get away with it if the relative humidity remains fairly constant. Says he using 4/4. If I understand correctly he is gluing up a 4/4 panel of walnut and adding it to the bottom. It will be slightly bigger then the exposed plywood. Am I right Brian? That being said I would just glue another ply and edge it also.

[Brian Kent]

Let me throw another stone in the water. Gluing 1/4" thick solid wood to a plywood substrate could end up being problematic. The general rule is to have veneer no thicker than 1/8" glued to a substrate. Might get away with it if the relative humidity remains fairly constant.

Howard, I have already seen that problem as I glued the 1/4" walnut burl to the plywood in the winter and then the Riverside dry heat came. I have chased a lot of cracks until I fed the burl with some Formby's Tung Oil Finish.

I am inexperienced with veneer and I think I sliced the burl too thick. It is now closer to 3/16 to 1/8" after leveling and sanding.

[Brian Kent]

I have been gluing together my 4/4 walnut to screw to the bottom of the table. Even so, it is not too late to switch to a plywood piece glued to the other plywood section and then edged.

Too many good ideas, folks. that's another reason I love this place:D.

[Brian Kent]

I like the 4/4 walnut "bottom", "bottom of top", "backing", or whatever it is.

I'll screw it into place and then screw the pedestal to the "bottom", "bottom of top", "backing", or whatever it is.

Thanks everyone.

Brian