Screws for securing back of plywood bookshelf

[Yasha Renner]

I've been following a good set of bookshelf plans published by Popular Woodworking (see here). When it comes to the fasteners, however, the only detail the author gives is that he used #6 1-5/8" screws. I went to my local hardware store, and the only screw I could find in this length were all-purpose bugle head construction screws, which doesn't seem like the right choice. The #6 wood screws measure 1-3/4" which is close enough, but the threads were fine, and I wasn't sure if that is a good choice for plywood, versus the coarse threading on the bugle head screw.

So what do you all recommend to secure the back of my bookshelf? The back is 3/4" plywood that sits in a 1/2" rabbet.

Thanks everyone!
Yasha

[Lee Schierer]

First question is why 3/4" ply? Is the back of the cabinet supporting the entire cabinet? I normally use 1/4" plywood for the backs of cabinets and attach it with #4 x 1/2" flat head wood screws.

[Yasha Renner]

I was just following the plans. The author said it "produced a box that is very strong." If I use a traditional wood screw, wouldn't the unthreaded shank need to be about 3/4" in length, so that it would pass through the back via a clearance hole?

[Jim Becker]

I personally prefer the coarser threads for this kind of application. You should pre-drill with countersink and then install the screws. Sometimes you need to source fasteners from places other than the local 'box stores when you want something specific and appropriate for woodworking. I happen to buy my screws from McFeeley's but there have been many discussions over the years about various sources for different kinds of screws.

And yea... a 3/4" back on that book case is going to make it very, um...stout...and heavy...

[lowell holmes]

Well, it should not tip over.

[Yasha Renner]

You should pre-drill with countersink and then install the screws.

Thanks, Jim. That is my plan, and so your suggestion seems to confirm that I should not use bugle heads, as they won't seat flush in the countersink. By the way, what is the ideal application for bugle head wood screws, soft woods? I know McFeeley's sells them. Also, do you suggest that I drill a clearance hole in the back? (I was going to use a tapered drill bit.)

[scott vroom]

Bugle head screws will seat flush in a countersink. 1-1/4" coarse drywall screws will work fine. You're overthinking this.

[Yasha Renner]

You're overthinking this.

As usual! Lol. You'd probably agree that "anything worth doing is worth doing badly." One of my favorite quotes by GK Chesterton, but a lesson I'm still learning.

I know bugle heads will sorta kinda seat, even though obviously they are a different shape than the 82 degree angles cut by the countersink. I can't bring myself to use drywall screws; I'm sure Mr. Becker would agree with me on that one.

[Mike Kees]

Drywall screws work real well for drywall. I end up using square drive yellow deck screws for a lot of these type applications.

[Jim Becker]

While there certainly are many folks who use drywall type screws for non-drywall applications, they are not the best fastener for furniture and cabinet building because they are quite brittle...by design. They are so easy to snap off, especially given most folks drive with an electric powered drill/driver that can exert a lot of torque when the fastener sinks home. In a pinch, well...you do what you gotta do, but I decided a long time ago to just bite the bullet and buy quality screws more designed for woodworking and cabinet making. I use mostly #8 screws, but also keep #6 and #10 for when they are most appropriate. I use # 7 trim heads for my tack trunks which use 12 mm (1/2") plywood as they are primarily for "clamping" while glue does what glue does.

[Thomas McCurnin]

I have made super heavy duty cabinets that support heavy tools and have never used anything thicker than 1/2 inch for the back. Waste of lumber and makes it heaver than needed, if the bookshelf is going to be hung. Now if the back was going to support shelves, that might be another story.

I use gold/brass colored construction screws for most work, but honestly grab anything at first reach at the hardware store. While I am aware that drywall screws are more brittle, they generally have a 200 lb pull rating and between 50-150 lb shear rating each. I am guilty of using drywall screws for all sorts of unintended uses and have never seen a failure in a cabinet context.

[Terry Wawro]

If you've already cut it, go ahead and use the 3/4 ply. If not, then 1/2" is more than strong enough. Heck, most of us use 1/4" with brads and that still works for about 95% of applications. A lot of us are also guilty of using drywall screws too. I've used them for decades and never had a failure. Drywall is pretty heavy and you don't see it falling off the walls do ya. :-)

[Yasha Renner]

Thanks everyone. After all the raised eyebrows about the thickness of the back, I picked up some 1/4" birch this morning at the BORG. I did intend to anchor the cabinet to the wall using a french cleat, as shown in the plans I posted above. Unfortunately, the plans failed to alert to the fact that using a 3/4" back with a 3/4" flush rabbet would make using the cleat impossible, as it would stick out too far and leave a gap between the cabinet and the wall. Now with the thinner 1/4" back, I can use a 1/2" piece of hard wood for the cleat, thus bringing all the pieces together flush. So all's well that ends well....

[Yasha Renner]

Now if the back was going to support shelves, that might be another story.

Just curious, when would this sort of design ever be appropriate? I'm having trouble visualizing it.

[Mark Hockenberg]

Sounds like you landed on the right solution. For cabinets anchored to the wall, I put a 1/4" back in a dodo set in 1/2" from the back edge. Then I run horizontal nailers across at the VERY top and near the bottom. I normally use particleboard or MDF when it doesn't show - hardwood isn't necessary. When you run a screw through from the inside it captures the back and nailer. This provides a very stable attachment and the cabinet stays where you put it.