# why does plywood split apart if you drill in between one of the layers? Is the glue bad?



## Jimothy (Oct 29, 2015)

So I've wondered this. I know they use urea-formaldehyde glues in plywood to press the layers together. Heres what I dont get. Let's say I glued two pieces of wood together and tried to put a screw into the seam, if it were to break it would break literally anywhere else *but* the glue joint considering the glue is supposed to be stronger than the wood. In the case of plywood, it seems to split the joint apart, and quite easily. It's always made me wonder if the glue they use is just trash, and if plywood is really that strong overall.

Thanks


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## Knockonit (Nov 5, 2017)

gotta pre drill them


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## bilyo (May 20, 2015)

I'm no expert, but I'll tell you what I think. First, the glue is hard and brittle. Second, the plywood lamination layers are thin and weak. Plywood is strongest under bending forces and lateral forces where all of the veneer laminations are working together. When forces try to separate the layers, the brittle glue shatters and the thin wood veneers fracture. Of course, the degree to which this happens depends greatly on the quality of the plywood.

Running a screw into the edge of plywood will never be very strong. However, if you need to do it, you can better assure maximum success by clamping a block on each side where the screw will go, pre-drill the hole, and then run in the screw. This will keep the plywood layers from separating while the screw cutting it's threads into the plywood.


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## Jimothy (Oct 29, 2015)

> I m no expert, but I ll tell you what I think. First, the glue is hard and brittle. Second, the plywood lamination layers are thin and weak. Plywood is strongest under bending forces and lateral forces where all of the veneer laminations are working together. When forces try to separate the layers, the brittle glue shatters and the thin wood veneers fracture. Of course, the degree to which this happens depends greatly on the quality of the plywood.
> 
> - bilyo


So the glue is bad! haha. kinda bad. this happened to me with "furniture grade" so its a bit concerning


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## bilyo (May 20, 2015)

> So the glue is bad! haha. kinda bad. this happened to me with "furniture grade" so its a bit concerning
> 
> - Jimothy


I am assuming that you are joking, but no. I didn't say it is bad. But, that type of glue tends to be hard, brittle, and very strong for the purpose.


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## MrRon (Jul 9, 2009)

When you drive a screw into the plywood laminations, you are wedging the layers apart. I don't think plywood was intended to be fastened in this fashion.


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## tvrgeek (Nov 19, 2013)

> When you drive a screw into the plywood laminations, you are wedging the layers apart. I don t think plywood was intended to be fastened in this fashion.
> 
> - MrRon


A screw is common in putting boxes together, but pre-drill first. A "wood" screw is the worst due to it's tapered core. I have gone as far as some CA in the hole to reinforce it. 99% of the time, I glue and pin nail as modern glues are stronger than the wood.


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## Tony1212 (Aug 26, 2013)

The plies of the plywood tend to be soft wood like pine and fir, similar to the common 2×4. These woods are notorious for splitting when driving in a screw near the end of the boards where there is less "meat" to hold it together.

In cheap 5 ply 3/4" plywood, each ply would be slightly over 1/8" thick. And each ply alternates grain direction, so you'll be driving a screw into both very thin long grain and very thin end grain on very soft wood.

Then there is the tapered wedge shape of the screw itself that will also cause the wood fibers to split. Not to mention the volume of the screw that is displacing the wood and glue.

Most of the failures that I've witnessed have been due to the wood splitting and splintering around the screw. The glue is still holding most of the wood pieces in.

Pre-drilling removes a lot of the volume of wood that is replaced by the screw shaft, thus reducing the amount of splitting. Just like in the ends of 2×4's and deck boards.


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## gbarnas (Sep 25, 2021)

I try to avoid this, but when screwing into the ends of plywood, I've always pre-drilled and used (where possible) a clamp to hold the wood together, but recently have started putting a few drops of *thin *CA into the screw hole and "stirring" with a toothpick to coat the sides. A spritz of accelerator reduces the wait time - you don't want to insert the screw while the CA is wet. The CA binds the soft fibers around the hole.


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## tvrgeek (Nov 19, 2013)

Are you using a twist drill or a spur?


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## Andybb (Sep 30, 2016)

> When you drive a screw into the plywood laminations, you are wedging the layers apart. I don t think plywood was intended to be fastened in this fashion.
> 
> - MrRon


+1

And, when gluing, remember that 50% of the layers are end grain.


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