Joining Plywood

[tim walker]

In building cabinets or any box, using butt joints and not using pocket screws, do you pre drill both pieces or just the starting piece?

[Keegan Shields]

Clamp it up, predrill with countersunk holes, then screws.

[Richard Coers]

I use biscuits and glue. I have never considered pocket screws as structural, just consider them as clamps.

[glenn bradley]

If we are talking about screwing into plywood end grain the same rules apply as drilling pilot holes for MDF. The shaft of the screw should not apply any wedging action to the ply. Only the threads of the screws should bite into the material. The hole should be slightly deeper than the screw is long.

[Dave Zellers]

If we are talking about screwing into plywood end grain the same rules apply as drilling pilot holes for MDF. The shaft of the screw should not apply any wedging action to the ply. Only the threads of the screws should bite into the material. The hole should be slightly deeper than the screw is long.

Couldn't possibly agree more.

[Lee Schierer]

If we are talking about screwing into plywood end grain the same rules apply as drilling pilot holes for MDF. The shaft of the screw should not apply any wedging action to the ply. Only the threads of the screws should bite into the material. The hole should be slightly deeper than the screw is long.

Glenn is very correct. The same applies for thin wood applications.

[Jim Becker]

For butt joints, I clamp up, drill/countersink and then screw. I have two identical drill/drivers I use which greatly speeds things up. This is with or without glue, BTW. When clamping up, I use right-angle supports to help keep things as square as possible and then check/fix for absolute square when adding other components to the basic box. That's a lot easier if it's already square or nearly square. I do not generally use biscuits, dowels or Dominos for something like this unless there is a specific structural reason to do so.

[tim walker]

Thanks all

[Bruce Wrenn]

Use assembly screws specifically made for the task. Drill point, and nibs under the head to counter sink them.

[Curt Harms]

I use biscuits and glue. I have never considered pocket screws as structural, just consider them as clamps.

Yup that's what biscuits were invented for, to join sheet goods. They weren't intended to replace mortise and tenon joinery. I have some plywood cabinets fastened with biscuits only and we're 25 at years and counting on them. One is a microwave cabinet lag bolted to the wall and no sign of joint failure.

[Randall J Cox]

I have used biscuits for years to join plywood edges, but only for 1/2" thick or more sheet goods. And normally not where I need structural strength. Randy

[Tom Bender]

Mitered, glued and screwed is probably the strongest joint.

[Robert Engel]

Clamp, align, counter drill, screw.

Use screws with a smooth upper shank. I use a lot of Spax screws. Not really a big fan, but they are readily available. A lot of people use drywall screws but I hate them b/c they require such a large hole to accommodate the threads in the overlying piece & if you use a countersink drill big enough for that, the hold is too big for the threads to grip.

[Frank Pratt]

Mitered, glued and screwed is probably the strongest joint.

??? For sheet goods? Please do explain.

[Tom Bender]

Gladly

Mitering provides the most glue surface

None of the surfaces are strictly end grain.

Adding screws adds strength.

Screws can add clamping force to improve the glue up.

Should the glue fail, belt and suspenders.

[Frank Pratt]

Gladly

Mitering provides the most glue surface

None of the surfaces are strictly end grain.

Adding screws adds strength.

Screws can add clamping force to improve the glue up.

Should the glue fail, belt and suspenders.
A mitered glue surface is not a strong joint and I wouldn't put screws through a miter because there is way too little thickness for the head to hold well. It isn't strictly end grain, but it might as well be.

[Mike King]

The same best practices apply to screwing plywood as to any screwed assembly: Clearance hole (the diameter of the screw shank) for the top piece, pilot hole for the bottom piece with the pilot hole at least the depth that the screw will achieve. And size the pilot hole on whether the wood is softwood or hardwood.

Too many folk don't use clearance holes or pilot holes. And way too many folk use self drilling screws. But if you want to have the best outcome and not split your work, or strip your hole, then using the practice above is the one that will work and achieve far superior results.

[Curt Harms]

A mitered glue surface is not a strong joint and I wouldn't put screws through a miter because there is way too little thickness for the head to hold well. It isn't strictly end grain, but it might as well be.

A spline in the miter helps with that. A biscuit can be viewed as an easily created interrupted spline.

[johnny means]

If you use glue, it really doesn't matter how you hold it in place until the glue dries. I use glue and staples to build cabinet boxes for kitchens and bathrooms, anything more is just for fun.

[Mike King]

If you use glue, it really doesn't matter how you hold it in place until the glue dries. I use glue and staples to build cabinet boxes for kitchens and bathrooms, anything more is just for fun.

That's not quite true. If you do not bore a clearance hole for the shank of the screw in the top piece, you risk not cinching the top piece to the bottom. What can happen is that the thread starts cutting its path on the bottom piece proud of the top piece -- e.g. there's a gap. This is why it is important to return to basics: not just pilot hole, but pilot hole PLUS clearance hole...

[Gordon Stump]

You all probably know this. Gluing end grain particularly with plywood can be greatly enhanced by first "basting" the end grain or plywood edge with watered down glue. Let it dry then glue as usual. The thin glue seals the pores and inhibits the second coat of glue from bein sucked into the end grain. Some folks say it makes the joint as strong as long grain to long grain. I do not know about that!

[Frank Pratt]

You all probably know this. Gluing end grain particularly with plywood can be greatly enhanced by first "basting" the end grain or plywood edge with watered down glue. Let it dry then glue as usual. The thin glue seals the pores and inhibits the second coat of glue from bein sucked into the end grain. Some folks say it makes the joint as strong as long grain to long grain. I do not know about that!

No need to water down the glue, just lay down a thick layer & let it sit 10 minutes, then glue it up. If you let the glue dry thoroughly it will interfere with adhesion. PVA glue does not stick to dried PVA glue. the resulting joint is quite strong, but not as strong as long grain to long grain.

Biscuits are my go to for sheet goods. That's what they were invented for and they excel at it.

[Rob Newton]

Patrick Sullivan on youtube has a series called "Glue Myths" that is very interesting. He is very meticulous in his experiments and the data are surprising if you want to check it out:
https://www.youtube.com/watch?v=m7HxBa9WVis

[Jeff Clode]

Thanks for posting this - fascinating

[Gordon Stump]

No need to water down the glue, just lay down a thick layer & let it sit 10 minutes, then glue it up. If you let the glue dry thoroughly it will interfere with adhesion. PVA glue does not stick to dried PVA glue. the resulting joint is quite strong, but not as strong as long grain to long grain.


The rational for basting end grain with watered down glue is to stop the glue at some point from just being sucked into the end grain. This is not my idea but rather another youtube experiment. I cannot find it. But I tried it and it works. The watered down glue actually disappears so there is no dry glue to wet glue issue.

Cheers,

Gordon

[Frank Pratt]

The rational for basting end grain with watered down glue is to stop the glue at some point from just being sucked into the end grain. This is not my idea but rather another youtube experiment. I cannot find it. But I tried it and it works. The watered down glue actually disappears so there is no dry glue to wet glue issue.

Cheers,

Gordon

I didn't mean to argue with your method, but present my alternate way. Both work. The important point is to not let the glue completely dry and cure before doing the second coat and glue up. It's surprising how much stronger you can make an end grain glue up using these methods.

[Gordon Stump]

it's all good