# A question about the tongue-and-groove corner joint and why it is only suitable for sheet goods



## HarveyDunn (Aug 29, 2013)

I'm working my way through Gary Rogowski's "Joinery" book. On Page 101 he discusses "the tongue-and-groove corner joint", illustrated thusly:










The text says "remember that the joint described here is useable only for plywood or other sheet-good material".

I'm wondering why that is - the answer is not supplied in the text.

I supposed the answer is "movement" (isn't it always?) but I can't work out why.


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## Oldtool (May 27, 2012)

Pure speculation on my part: since the outside of the joint - plywood to solid wood - is flush, any swelling of the wood sides would stand proud of the corner piece. My best guess. You should email Gary Rogowski for confirmation.


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## jdh122 (Sep 8, 2010)

If you replace the plywood with a wide glued up piece of solid wood, it will expand and contract seasonally, while the corner block will be almost totally stable along its length. This will eventually cause the glue to fail or the wood to crack/buckle. With plywood, on the other hand, there is very little dimensional movement because the plys are alternated 90 degrees to each other. To do this kind of joint with solid wood you'd need to only glue the center and then pin the ends in elongated holes like you do for a breadboard table.


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## CharlesA (Jun 24, 2013)

How is that different than tongue and groove frame joint (for movement, that is)?


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## Smitty_Cabinetshop (Mar 26, 2011)

Harvey, I'm with you.

Following along to see what comes of this post, it's a good question.


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## a1Jim (Aug 9, 2008)

IMO Jdh122 hit the nail on the head ,it's all about wood movement.BTW I think Gary Rowgowski's book "Joinery" is the best book out there on the subject of Joinery.


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## PittsburghTim (Jan 16, 2012)

Boards expand and contract across the width of the grain, not along their length. The wider the board, the more movement that takes place. Smaller frames may never see enough movement to cause a problem. A wider joint may cause problems is the temp and/or humidity varies seasonally.

Think of a wide joint, like the breadboard edge on a table. If you don't install it to allow for movement, you will see cracks down the road. That is why they are not gled along the entire length.

Again, in a well-regulated environment, you may not see the problem, but let the AC off while you're on summer vacation and you could have an unpleasant situation when you return home.


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## jdh122 (Sep 8, 2010)

Charles,
I think it all depends on scale. It's true that if you make a frame with tongue and groove (or a mortise and tenon or even a lap joint, for that matter), the pieces are in cross-grain orientation, but the glue joint is never more than a few inches wide, so movement is not a problem. But if you get beyond about 6 inches or so it could start to cause problem.
In terms of the diagram above, you could use solid wood as long as the grain was oriented in the same direction as the post (imagine a blanket chest with this joint: it would be fine as long as the sides all had grain that was running up and down rather than along the front and sides, although I'm not sure how you'd attach a bottom so it wouldn't either crack or fall out). But in the picture the grain on the plywood is running perpendicular to the grain on the posts.
Just my 2 cents, house-bound on this snowy Sunday.


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## Loren (May 30, 2008)

You can probably glue up something 6" wide that way
out of solid wood, but wider is asking for trouble. With
breadboard ends we don't glue all the way across, we 
use slotted screw holes or accept a little seasonal gapping 
of the joint at the shoulder.


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## CharlesA (Jun 24, 2013)

I guess I was taking the pick too literally and not imagining a joint over 4-6".


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## Shanearoonie (Dec 28, 2020)

So, I know this is a super-old thread, but I'm considering this same approach to joining corners of a large box (tack box, about 24"x24"x36"). I want to use the corner hardwood both as a decorative element, and to make the joint, and therefore the box, very strong.

However, the exposed interior corner would be a forearm killer as people reach in for stuff. I'm curious how the strength of the joint would be affected (if at all) by cutting the the little interior triangle off at 45 degrees? In theory it could weaken the inside face of the grooves on each side, but I'm hoping it would be negligible. But I'm no structural engineer…. Opinions?


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## Unknowncraftsman (Jun 23, 2013)

I'm thinking taking out some of the pointy inside corners are not going to affect the strength of the joint.
Too me the original post of the op shows short grain on the out side that could be easily split along the grain.
What's the box for that your making just curious.
I like making boxes
Good Luck


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## Shanearoonie (Dec 28, 2020)

It's a tack box for my daughter, intended to hold a saddle and related horse and grooming supplies to take with her to horse shows. I'm guessing the sum total weight of contents isn't likely to approach 100 lbs, but pulling it (on wheels) across rough terrain in and out of barns and frequent loading/unloading from vehicles could put a pretty good mechanical strain on it, so I want to make sure the corners are as strong as they can be. The design I'm working on originally had just decorative trim covering up lock rabbets or something, but then after seeing this joint, it was appealing to go this route if I could diagonal cut the inner corner without jeopardizing strength. Thanks for the feedback!


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## bilyo (May 20, 2015)

> So, I know this is a super-old thread, but I m considering this same approach to joining corners of a large box (tack box, about 24"x24"x36"). I want to use the corner hardwood both as a decorative element, and to make the joint, and therefore the box, very strong.
> 
> However, the exposed interior corner would be a forearm killer as people reach in for stuff. I m curious how the strength of the joint would be affected (if at all) by cutting the the little interior triangle off at 45 degrees? In theory it could weaken the inside face of the grooves on each side, but I m hoping it would be negligible. But I m no structural engineer…. Opinions?
> 
> - Shanearoonie


If you plan to use plywood for the sides, you should be good to go. However, if the sides will be solid wood, take special note of the discussion above regarding wood movement. 24" cross grain dimensions will be particularly susceptible to cracking and/or splitting if you don't allow for it.


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## SMP (Aug 29, 2018)

I don't think I would use this joint for box corners for a box that big full of 100 pounds of gear getting hauled back and forth. Not a lot of pull resstance.


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## tvrgeek (Nov 19, 2013)

T&G has been used with solid wood for probably over 1000 years. ( look at the history of Oriental joinery) 
Just because someone put it in a book does not mean it is true.

Now for a tack box, I think I would reinforce the corners on the inside with corner blocking.


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## splintergroup (Jan 20, 2015)

I've used this joint and versions of it many times with all solid wood with good results. It makes for some clean looking corners and is simple to implement.

This was all on jewelry box sized items however. Something the size and required strength of a tack box really needs either blocking or more traditional joints (DTs or box joints)


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## Shanearoonie (Dec 28, 2020)

I feel a consensus forming that this is a perfectly good and strong joint, but perhaps not for this particular application. Maybe I'll to a normal lock-rabbet (there will be pull stress on at least one short side to roll it around), and reinforce with an inner block. Then just stick with thin decorative hardwood trim glued on the corners.

Thanks everyone for your input!


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