Pilot holes into plywood edges – sizing?

[Tyler Bancroft]

I'm building a bookcase that will have plywood shelves seated in dadoes in the carcass. I plan to run screws into the end of the plywood to reinforce the glue joint. How should I size the pilot holes? Same as for hardwood? Larger? I'm a little concerned about the pressure from the screw delaminating the shelves. I'm using maple-veneered PureBond plywood, if it matters – I think (but am not certain) that the core layers are aspen.

[Thomas McCurnin]

For a No. 6 Screw drywall type screw, which is what I would use, I would drill three different size holes--one for the shelf which would simply be a small pilot hole, one for the carcass, which will be fairly loose, and a countersink on the carcass.

I would probably experiment, but I would start with 3/32 for the pilot hole, 1/8 for the carcass hole, and 5/16 for the countersink. Add a plug to the countersink and the connection will be invisible.

[Dan Cameron]

Screws into the edge of plywood is not a good idea. Make a well fitted, well glued dado joint and a plywood back panel to eliminate racking.

[Cameron Wood]

Try different bits in scrap, & select the largest size that will still hold the screw threads.

[Michael Schuch]

It sounds like the edges of the shelves are going to be captive in a dado? In that case I would just use the same pilot hole that you use for hardwood. For the most part the screw is just there for holding everything together while the glue dries but it does provide some strength to the joint I suppose.

I actually have some shop shelves with 1" thick plywood shelves that are captive in dados and I added reinforcing screws. They have held up to heavy tools on them fine over the past 30+ years. I would not expect a 5/8" thick shelf that is glued and screwed with captive dados to have problems over the years either.

[Lee Schierer]

For a No. 6 Screw drywall type screw, which is what I would use, I would drill three different size holes--one for the shelf which would simply be a small pilot hole, one for the carcass, which will be fairly loose, and a countersink on the carcass.

I would probably experiment, but I would start with 3/32 for the pilot hole, 1/8 for the carcass hole, and 5/16 for the countersink. Add a plug to the countersink and the connection will be invisible.

Drywall screws should only be used for drywall, they are too brittle for other uses.

As far as the pilot holes go, Measure the thread root diameter at the mid point of the threads and use a drill with that diameter for the full depth hole, then drill a clearance hole for the non-threaded portion of the screw for the total depth of wood that matches the length of the non-threaded portion of the screw.

I recommend using Spax #8 screws for their strength and ease of being driven. They are also easily removed if necessary and they don't rust as easily as dry wall screws.

[Osvaldo Cristo]

I made my library shelves 30 years ago using 20 mm naval quality plywood veneered by mahogany. Lots of them: 50 linear meter of shelves.

On that time, I had intention to have those shelves only for two or three years, so I used only butt joint, no glue and only two fine threads 50 mm wood screws for each joint. I made pilot holes in the vertical supports and no pilot holes in the shelves. I had no splitting, and it is very solid up to now.

As you can see in the attached pictures the shelves are full loaded.

You are using dados, so I think it is yet safer than mine to use no pilot holes in the shelves.

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[Maurice Mcmurry]

I use the same guidelines that Grandpa used. A proper pilot hole for a wood screw involves at least two sizes. I generally use 3 sizes.

This is a little TMI but covers the basics.
US Wood Screws - Pilot Holes (https://www.engineeringtoolbox.com/wood-screws-holes-d_1519.html)

[lowell holmes]

I use pilot holes and dowels for such joints. Try sample joints and see if you like it.

[Richard Coers]

Did you consider confirmant screws? If I used Spax screws, I eyeball the drill bit over the roots of the thread. That way the only parts touching the plywood is the threads and wedging open with the body of the screw

[Ken Kortge]

Honestly, that dado glue joint is going to be VERY strong without using screws. If you don't want to clamp while waiting for the glue to dry, use brads to pin it together.

[Tom Bender]

I'm with Lowell on this. Dowels can be left proud on the ends for a little interest.

[Tyler Bancroft]

Lot of good takes here – thanks all.

[andy bessette]

Screws into the edge of plywood is not a good idea...

This. Extremely poor design.