Shelf movement in a plywood bookcase

Question

I'm building some 7' by 3' bookcases with mix of plywood and solid oak for the sides and back (mostly plywood), but full solid oak shelving for strength. I'm worried that the ~9" depth to the shelves will allow up to about 1/8" of movement. If I used the online formula correctly. Since the sides and shelves are constructed so differently, and will thus move differently, I'm assuming I can't just secure the fixed shelves to the sides and back.

My primary question is: am I right about the movement, and if so, is it sufficient to place the fixed shelves in a dado and only glue the front couple inches? (I want the front of the fixed shelves to look solidly attached throughout the year.) Secondarily, can I "hide" the movement at the back, for instance by running a 1/4" dado and only insert the first 1/8" inch during construction (it's winter, so it's probably near my low humidity point)? I didn't plan quite far enough ahead to select my boards for grain direction, so I'm assuming the worst.

Here's a rough look at my design, for reference; green is plywood, and the rest is solid. The topmost and bottom visible shelf, as well as one not visible near the very bottom are the fixed shelves I'm asking about attaching. (The rest of the shelves will be adjustable, so I just need to allow room for their largest size, and accept some gap.)

Answer 1

My primary question is: am I right about the movement, and if so, is it sufficient to place the fixed shelves in a dado and only glue the front couple inches?

Yes and yes. You could also fix the shelves at the front by toenailing them in from beneath or using pocket screws (could also be used in addition to glue, although there's no benefit to doing both).

Secondarily, can I "hide" the movement at the back, for instance by running a 1/4" dado and only insert the first 1/8" inch during construction (it's winter, so it's probably near my low humidity point)? I didn't plan quite far enough ahead to select my boards for grain direction, so I'm assuming the worst.

Yes this is a good solution, much like the allowances allowed for in some things made from tongue-and-groove. Be generous in your spacing

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But, you made an error at the outset in assuming that the solid-wood shelves would be stronger than plywood. Broadly speaking quality plywood will be comparable to most solid woods, of course without any attendant worries about seasonal movement.

If you hadn't already bought your materials plywood would presumably be significantly cheaper as well, so normally this would all be upside. The oak can be put aside for use in a future project of course.

One other reason for going with plywood in your case is I'm not sure there is sufficient allowance for movement. Doing a few quick sums with some guesses on my part I'm getting figures between roughly 1/5" and as high as 1/4". And if your seasonal humidity swings are wider than my assumption the internal moisture change in the wood will be greater than the 8% I've allowed for, obviously meaning that movement could be slightly more than this.

Answer 2

Looks good! Your calculation and assumptions about movement appear to be spot on. The plywood faces with solid oak edges will have almost no movement over the year, but the shelves will. Your solution to leave a gap in the dado joint at the rear should work fine. Glue them up on to the front styles and the first several inches of the side panels. Leave a small gap in the side dadoes as well (1/16") so that the shelves move freely in those slots as well.

Answer 3

After getting general confirmation from the answers that others provided here, here's what I did to actually size the bookcase. And after going though the low and high points of our year's humidity cycle, I'm satisfied with the results.

Step 1: Identify the amount of wood movement expected

This first requires identifying the wood in question (solid oak), and the expected range of humidity swings in the area. I searched for something like "moisture content of interior woodwork on usda maps", and found this chart. From that I was able to estimate a 5% swing between the extremes.

But how much movement does that mean? In a search for something like "wood movement oak" I found this Wood Movement Chart, where you can see that 12 inches of red oak will move 0.9" radially to 0.22" tangentially across a 5% relative humidity range. Since my dimension was only about 9 inches, I only need to allow for 3/4 of that, and I'll assume I chose lumber really badly so will get the larger movement. So that means my shelves' depth will move by up to 3/16".

Step 2: Figure out where my lumber is right now

I was able to use an old moisture meter and take a reading of my lumber's moisture content. This old meter only reports a number on an analog readout, and you need to then adjust that number per a correction table. But it turns out that red oak is a direct mapping.

Unfortunately I lost my notes on exactly where I was. I think it was about 1/3 of the way through the humidity range I expect to see, nearer the low end. So that would mean I could expect up to 1/16 shrinkage or 1/8 growth. I adjusted my cut according to those calculations.

How did it work out?

So far so good. Of course real life dictated that as soon as I finished the bookcases, the shelves in question were completely covered. Thus any gap will never be seen. That may be the most important lesson to take away: as much as you want something to be perfect, it's worth considering how visible it will really be, and err on the side of whatever can't make your project fall apart.

Also, it's worth noting that my calculations here are very much worst case for my design. Since it has some solid lumber in the sides as well, those could effectively cut the depth of the shelves by another couple inches. So I may have overcompensated in multiple ways.

Regardless, I'm happy with the result, and so is my wife, and that's what matters.