Expert: Jamie Yocono - 5/17/2006

Question
Thanks for the help on previous question. I would also like to now if it is possible to screw into the edge of 3/4 birch or oak plywood. I am planning on using fixed shelves of plywood,with plywood sides. Would predrilling help.
Thanks FRED







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Followup To
Question -
    I am planning to build an entertainment center. Is there a rule of thumb as to the size of the screws I should use? I will be using primarily 3/4 inch plywood and 1 inch thick pine.
What size screw wood be best ussing on a 3/4 inch board?
Answer -
Hi Fred,

Good question, and one most people forget to ask. When using screws, there are a few rules. Mostly, the one I follow is that the screw should be roughly 3 times the thickness of what it's going thru. So if you're going though 3/4" plywood, it should roughly be 2 1/4" long.

Another train of thought says that at least 2/3's of the length of the screw should go into the second piece. If you do the math, it means the same thing. Roughly one-third would be taken up with the first board, and 2/3 of it would go into the second board.

Of course, you have to use your head. When I'm screwing 2 pieces of 3/4" plywood together, I use 1 1/4" screws. I don't even want to take a chance of coming out the other side, so I give myself that extra 1/4" of cushion for when I make mistakes and drive something a little deeper than I had hoped to.

Anyway, it's common sense- measure what you're putting together, and make sure the screw isn't longer than those 2 combined dimensions. And as a rule of thumb- always remember the "3 times the thickness" rule.

If you're taking about the screw size, I almost always #8 and #10 screws for furniture making. Number 6 screws are too delicate, and can break. And #12 screws are overkill. I am sure there is a rule of thumb about the screw shank diameter to use, but I just looked in two of my old textbooks and can't find anything definitive. It seems like it's one of those common sense areas. If the screw is going to have a lot of stress on it, I would go beefier. And if it's not, you can probably use the smaller size, like a #8.

Good luck, I hope this helps. Feel free to write back with any other questions you might have after reading this.

Jamie Yocono
Wood It Is! Custom Cabinetry
www.wooditis.com
Las Vegas, NV


Answer
Hi Fred,

Putting screws in the edge of plywood isn't really a great idea. There isn't any grip to the edge grain, and the screw can pull out relatively easy. Even worse, it can cause the layers of wood to separate, and therefore, split the plywood apart. That's not to say I haven't done it, though- in certain instances, of course I've screwed into the edge of plywood, but those usually involved a dado joint, where the plywood wouldn't really be able to split too easily.

There's a better way to screw into the edge of plywood, and I'll try to explain it. It involves putting a solid wooden plug of wood into the plywood, where your screw will be located. You drill through the face of the plywood, and insert (with glue) the solid wood plug. I don't drill all the way though, but instead, just up to the good veneer. Then I sand it flush, so you barely know it's there. If you use the same wood as the plywood veneer, it's hard to see it.

So the edge now has an area of solid wood, which will hold the screw much better than the layers of wood can ever hold. Predrill you screw hole, and then insert the screw. Going into solid wood really helps add strength.

Good luck, I hope this made sense! Feel free to write back with any other questions you might have after reading this.

Jamie Yocono
Wood It Is! Custom Cabinetry
www.wooditis.com
Las Vegas, NV