Let it move!

A introduction to wood expansion and warping

Last week, I wrote an article about seasonal wood movement. It was full of interesting things, including pictures of several articles around my house that have moved around and changed shape. Some of them were falling apart, while others withstood the test of time.

Why? Well, wood moves in mysterious ways. Luckily, we can predict enough about wood movement to either deal with it or use its movement to our advantage. (Though mostly just dealing with it.)

Types of Movement

The wood moves in two ways regular expansion/contraction and warping.

Expansion and contraction occur across the grain. The more grain the more it will expand and contract. This means boards get wider and a little thicker but not longer. The simplest way to deal with expansion is to just orient the grain in the same direction. This is why the grain on a box wraps around the sides. The sides expand and the box gets taller, but since boards don’t get longer the box doesn’t get wider or longer. (A clever reader might think to ask how we can safely put a top or bottom on a box… more on that later.)

Warping is less predictable and less easy to work with. It is also harder to explain. The easiest way to think about it is the rings in the wood really want to be straight. If you look at the end grain of a board and the rings arc across the board, then the board might cup such that the rings tend to straighten out. If you look across the face of a board, and see arcing rings, then the board might bow. However the rings in boards do all kinds of crazy things, so we find boards doing all kinds of crazy dances, but the most common and prominent type of warping is cupping. After a board has warped, it is pretty much warped for good. At that point, the easiest fix is to get out your jack plane and make it flat again. Still, there is some really important joinery to prevent warping. Dovetails will keep cupping in check and continue to work as the boards expand and contract.

The Basics of Handling Expansion

There are basically two ways to handle expansion. You can either provide space to allow the wood to safely expand, instead of pushing other parts of the project out of place, or you can put the boards together so that their expansion happens at the same time and in the same direction. Sometimes you have to do both of these things. Table tops are made by gluing boards together along their side faces. This results in all the boards of the table top expanding in the same direction. Then the top is attached to the apron and legs. While top could be screwed directly to the apron, but when the top expands it will put a lot of pressure on the sides of the table. This can be done successfully, but the screw holes in the apron need to be oblong to allow the screw to move with the expanding table top. Alternatively, a grove can be put along the inside of the apron and L shaped brackets screwed to the table top. In this way, the brackets slide in and out (though not completely out) of the groove.

The Basics of Preventing Warping

The two types of movement happen together, but luckily most warping be prevented through clever joinery. The trick involves capturing the end of a board such that when it expands, the expansion is force along an axis so the board remains in the desired shape (usually straight).

Since boards always cup so that the side of the board that faced the inside of the tree cups toward the outside of the tree, sometimes the board only needs to be constrained along one side. Imagine a box made such that the sides were all configured with the inside of the tree facing out. If you rabbeted the sides of two of these boards, then added a single nail or screw in the center of each joint. The nail would hold the centers of each board against its neighbor. This combined with the rabbets and neighboring edges would keep the boards from cupping. Another couple nails could be added to add structural integrity and you’ve got a solid box that will last a long time. Since the grain on the sides of such a box is all running the same direction, the expansion will also run about the same. This means that glue could be used instead of nails and screws.

We can’t always orient the faces of the boards so that we only need to constraint one side, and some times constraining both faces gives us other advantages. When using frame and panel construction, the panel has both faces constrained, and the panel is held in place by the frame. This way the panel won’t warp; the expansion is accounted for; and the panel doesn’t need any fasteners or glue. (In fact, in such a construction, glue or fasteners hurt, rather than help with handling expansion.)

More to come…

The first draft of this article was over 1900 words long. The second draft was over 2700 words long. That is eleven pages of text. Everyone I talk to says that they don’t want to read a blog longer than 750 words (and the final draft of this article still weighs in at over 1000 words). This article is the introduction to a topic that easily expands to cover a variety of woodworking joints, techniques, and example projects. So my plan is to punch up the first quarter of this article to make it an interesting and useful article in its own right, and since you are reading the conclusion, I hope you’ll agree that it was just that.

Next week we’ll be talking about mortise and tenon joints. How to make them, how they are used, how they react to expansion, and prevent warping.

A very interesting look at woodworking joints and a lot of their variations can be found in this small book Woodwork Joints: How to Make and Where to Use Them by “A Practical Joiner”. This small book was published in 1915, and shows all kinds of interesting joints and explains how they are used. That said, It doesn’t really talk about wood movement very much.