Butt-jointing plywood with glue and screws

I guess I am not sure if you are screwing it on the face, or the edges. I use a pocket hole jig for 3/4 ply. It seems to work fine. I havent tried to glue ply thinner than that though.

-- Wayne - Plymouth MN

Butt joints have almost no strength, even if the plywood does not split.
Could you possibly rabbit the edges so you would have a lap joint?

Or, as an alternate, this is a good application for biscuit jointery.

I think they make special screws for pocket hole joints in plywood that have flat heads to reduce splitting.

It really depends on the overall situation.

I have used butt joints, with screws going into the edge of plywood many times on shop furniture, jigs, etc., and they work perfectly fine. However, I would not use that type of joint to support a great deal of weight, such as a shelf for a planer, unless the overall structure included some other type of reinforcement. I’d give pretty much the same advice for solid wood as well.

-- Charlie M. "Woodworking - patience = firewood"

Multiple offset biscuit joints in a “saw tooth” pattern. Or if you have the room and your biscuit jointer blade is SUPER sharp stack two at each position with the width of a biscuit between each one. The “side grain” of the plys will glue OK, it’s the end grain where the issue lies.

-- OK, who's the wise guy that shrunk the plywood?

I’m not too knowledgeable when it comes to plywood, so I have to ask…
Is the “side grain” of the plywood the edge of the plywood that is running parallel to the grain (typically the 8’ sides on 4×8?) That’s my guess, but was just making sure…

-- Mos - Twin Cities, MN - http://www.youtube.com/MosquitoMods - http://www.TheModsquito.com

I don’t think you are ever going to get a real strong joint when you put 2 pieces of plywood together. However, in some applications a real strong joint is not needed.

If I wanted a strong joint with plywood, I would run both pieces into a groove in a corner piece of solid lumber.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

You can also, if you have room, use a glue block to support the joint. (For example, a piece of 3/4×3/4 stock running the full length of the joint.)

-- Charlie M. "Woodworking - patience = firewood"

The diagram below shows a rough picture (it’s a jig for sharpening chisels on a bench grinder). The bottom and side pieces are either plywood or real wood. The gray thing across the top is threaded rod. There are other parts, but I’ve left them out.

If I use plywood for the two uprights, I’ll need to drive screws up through the base into the edges of the plywood. I can use real wood if plywood won’t work. In either case, I can probably cut some dados for the bases of the uprights to give them more stability.

Or, maybe I should mount the uprights on the sides of the base and drive the screws horizontally through uprights into the base:

The base is a big slab of 2”x12” pine, so this design would require driving screws into the end grain of the base.

-- More tools, fewer machines.

if you have the room for it
a third piece as a brace
will keep the sides from ‘wobbling’
loosening the joint
as in box construction
or cabinets

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

Brett, I would go with option #2 above, but take it a step further and rout a very shallow 1/16” rabbet on the end pieces. Not for additional strength, but to make assembly easier. With plain butt joints, things like to slip around when glue and clamps are applied.

Predrill for the screws, glue it up and everything will work great. Just use long thick screws.

If you go with option 1, I’d definitely bring the ends in about 1/2” each and cut 3/8” or even 3/4” dadoes since you’re using 2×12” pine.

-- He who dies with the most tools... dies with the emptiest wallet.

NiteWalker, if I go with option 2, should the rabbets in the end pieces be made where the base touches the end pieces? (Sorry for the beginner-level question.)

-- More tools, fewer machines.

Kreg Jig.

It’s made for this specifically.

-- Jimmy, Austin, TX - Software developer by day, woodworker by night

Brett, yep, rabbet the end pieces where they attach to the base.

And the kreg jig would work better for option 2 as well. Definitely go that route if you have one.

-- He who dies with the most tools... dies with the emptiest wallet.

I thought you were talking about a true butt joint, two pieces of plywood butted end to end.
The right angle connection you are planning should be fine.

If you need extra holding power for that joint you can put a large (3/4” to 1”) dowel through the “end-grain” piece in the path of the screw and about an inch away from the end. That makes a very strong joint.

I believe I’d go with your first example and make the base piece about an extra 1” long so the screws would not be so close to the edge.

Plywood screwed through the edge doesn’t perform as well
as most solid wood in screwholding, especially near the edges
where the plywood can split if the joint is stressed.

Confirmat screws offer the best performance in this sort
of application. The holes are best drilled with a special
bit.