Why do lap joints work?

[dadiOH]

I used to make lots of them. Now, less because I have more tools but I still do on occasion and I wonder why they work? We all know that wood expands and contracts and lap joints have to pieces of wood doing that at right angles to each other yet they remain nice and solid. How come?

Someone might say it is because each piece is narrow and doesn't move much. That is true but I've often made fairly wide laps. For example, all our toilet seats are wood, each made with four pieces about 3" wide lapped at the corners.

Discussion is invited.

[Leon]

I make lap joints on every one of my back face frames, typically 6~8 on every one of 20 in the last 2 years. The joints are any where from 3/4" to 3.5" wide.

I use the regular water based wood glues which always have some degree elasticity to them so the wood is able to move. Basically the same goes for mortise and tenon and floating tenon joints where the grain is not matched in direction. Wood moves but typically not enough to out stretch the glue's ability to stretch until you get in to wide panels like cabinet sides, solid wood door panels, and table tops.

[Swingman]

What are you calling a "lap joint"?

What many call lap joints these days are traditionally called "halving joints", most common being what is traditionally known as a "half lap" joint.

Halving joints are mostly moderate strength and generally require some type of pinning to stand the test of time.

The modern vernacular, "rabbet joint", is actually a "lap joint", and its moderate strength is reinforced by adding a pin of some type (Leon's Domino drawers, etc), or a locking cut:

[dadiOH]

Yeah, those.

That would certainly help but some of my toilet seats are more than 25 years old and doing fine so far :)

[Swingman]

Hell, that's just a half a second in the history of wood joints. ;)

My wooden ones use both finger and half lap joints. Cross grain "lamination" is not always bad, sez the plywood manufacturer.

The "bottom" line, just finish it real well. ;)

[Leon]

Finish it real well! Heck just let the natural oils and fluids seal the wood!

I guess you would call that the "Out House" finish.

[pat]

"right angles to each other yet they remain nice and solid. How come?"

********************** They work because glue sticks. Oh really? Trivial but true, lots of surface, lots of pull and twist resistance. And even if the surfaces are not flat or the joint was poorly clamped, it still holds because some of that surface has been "welded". Moreover, to be sure, most laps are at right angles changing shape in conflict and they still hold because of mechanics and the magic of glue. Make them too big & without a fastener, and they will creep and loosen.

I agree it is a surprise they last so long. A fastener, in my view, on occasion is called for. Make a gate that kids swing on and the connection may fail; add some steel and they won't bust it. Lap the surfaces with a dovetail bit and gain some more interlock and easier assembly.

[dadiOH]

The glues we have now are a marvel!

Damned steel is going to rust though :)

Yes, I do those from time to time.

[WW]

I used to make lots of them. Now, less because I have more tools but I still do on occasion and I wonder why they work? We all know that wood expands and contracts and lap joints have to pieces of wood doing that at right angles to each other yet they remain nice and solid. How come?

Someone might say it is because each piece is narrow and doesn't move much. That is true but I've often made fairly wide laps. For example, all our toilet seats are wood, each made with four pieces about 3" wide lapped at the corners.

Discussion is invited.

[joeljcarver]

Amen .... As someone who grew up in the 1940s and 1950s, I remember what an ordeal it often was to glue things. It seems the choices were mainly the o ld Duco for model airplanes and other light gluing, and "Iron Glue", a feti d, fish-based stuff in a small bottle.

After WWII, we saw better and better adhesives available.

I would also add the same is true of lubricants. What a difference!

[Puckdropper]

"dadiOH" wrote in news:l6ip23$8gn$ [e-mail]:

Sometimes, that's considered a good thing. If the fastener is thick enough (like a 16D nail), rust occuring on the fastener serves to lock it in place. Of course, they do occasionally rust through, and that's a bad thing.

Puckdropper

[Larry W]

I won't try to get into the "why" of it too much, but let me just throw this thought out there: A piece of plywood is basically one big set of multiple lap joints...

[Larry Blanchard]

I think a lot of it is the sealing effect of the finish and the climate control in modern houses.

Either dewaxed shellac or oil based polyurethane do a good job of retarding moisture exchange. And with a lot of houses having air conditioners, hunidifiers and or dehumidifers the seasonal humidity changes are almost nullified. And the glue itself seals the unfinished surfaces of the joint.

Someone who's made lap joints for unfinished outdoor stuff, or who lives in an area with extreme humidity changes might have a different experience.

I live in the west. Humidity is low in the summer and high in the winter, but the house heating in the winter lowers the indoor humidity quite a bit so the swing is lessened considerably.

[Swingman]

Good point. Wood does not shrink or swell unless its moisture content changes . Ship a piece of furniture cross country and movement becomes a detrimental fact, as many furniture manufacturers have learned. Modern furniture production is quite concerned with, and spends a lot of money keeping factory/shop RH and wood EMC as close to 7.0 percent EMC and 35 percent RH as possible.

[Puckdropper]

"Mike Marlow" wrote in news:l6k6kv$qal$ [e-mail]:

Wouldn't a finish tend to retard the movement of moisture into and out of the wood? If it's moisture changes causing the movement, and not something else like temperature, then could the finish play a part in allowing pieces to be transported with little difficulty?

Puckdropper

[Swingman]

A finish indeed has a slowing effect on EMC, but does not stop wood from moving toward it.

[Leon]

Most of the wood in ikea products is a man made wood product. And IIRC most of what they sell is assembled at the purchasers house. Wood movement mostly affects fit when it is used in large and or wide panels. To combat that tendency a substitute for solid wood is used, ie. plywood, MDF, particle board..

[Larry W]

The particle board, MDF, etc. used for most Ikea furniture doesn't have the cross-grain/with-the-grain diffences in expansion rates like solid wood does. :wq

[Bo]

It has to do with the better bonding of long grain faces versus the end grain which basically sucks up the glue and has poor bonding.

I think it is like no on prefers butt faces. ;)