Our picket fence was built to perfection during a dry Oregon summer several years ago, but in the winter, it goes out of plane. The pickets are still vertical, but it’s as if they’re nailed onto a ribbon. And it’s because of the wood quality of the horizontal part of the fence, the crosspieces, technically called “rails.” In the rainy season, the wood in the rails swells asymmetrically, causing the rails to warp. And it’s fascinating to understand why.
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Why would rails warp in the rainy season? If you take a cross-section from a tree trunk, the dot of cells in the middle is called the pith. In softwoods (like pine, redcedar, and Douglas-fir), the structure of the wood cells changes gradually from the near the pith to farther out. That central wood is “core wood” (sometimes called “juvenile wood”), as opposed to the “outer wood” (mature wood). The properties of the wood also change gradually from the pith outward, with the changes related to warp occurring about 15 to 20 growth rings from the pith (7 to 10 growth rings from the pith for many pines). When moisture gets into the dry wood, core wood swells much more than outer wood. That means if a rail has core wood along one side and outer wood on the other, when it rains, the board will bow.
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Why does core wood swell more than outer wood? It has to do with the structure of the tracheids (the water-conducting cells that make up about 95% of the wood). These cells are roughly the size of an eyelash, they’re hollow, and their walls have three layers. All of the layers have a material in them called cellulose—it’s what the strings in celery are, and it’s what cotton thread is. In the thickest layer of the cell wall, long cellulose bundles are oriented in the direction of the grain in outer wood, but in core wood it’s at an angle of about 30 to 45° off of the direction of the grain (think of a barber pole).
Think of a bunch of vertical parallel lines. Those are the bundles of cellulose in outer wood. When outer wood becomes wet, it swells a little and the cellulose strands get farther apart, so an 8-foot 2”x4” rail might get a tiny bit wider (say, 2.05” x 4.1”) but it won’t get any longer.
Now think of a bunch of parallel lines that are all at an angle of 45°, spiraling around a cylinder like the stripes on a barber pole or a loose spring. When core wood gets wet, those bundles get a little farther apart and the board gets longer, but it doesn’t get any wider. Because of the angle of that cellulose, the rail will stay about 2” x 4”, but gain as much as an inch in length, going to 8 foot 1 inch.
So if my fence has core wood on one side of the rail and outer wood on the other, the core wood side will swell an inch in length and the outer wood side will stay the original length, and the rail will bow. And given that that the ends are nailed to the posts, their swelling is confined, so the bowing in the middle is accentuated even more.
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What’s the advantage to a tree of this structure, of having wood with a cellulose orientation near the pith different from farther out? The answer is surprising: it has nothing to do with being wet or dry—the wood inside a live tree is always wet. The orientation lets a narrow tree trunk bend in the wind without breaking. If you bend your pinky finger, notice that the skin stretches over your knuckles and is compressed on the side of your finger where the fingerprints are. Your skin is stretchy, flexible, and allows bending without breakage. If your skin were plaster of Paris, when you bent your pinky, the surface would crack on both the knuckle side (where the skin has to become longer), and on the fingerprint side (where the skin piles up). The bending pinky is like a small-diameter tree bending to a tight angle in the wind. The small tree needs to be able to get much longer on one side and shorter on the other without breakage in either the elongating or the compressing side. The orientation of the cellulose fibers in core wood lets the young tree have that “high strain,” as it’s called, without breaking. As an extreme example, think of each core wood cell having a wall structure like a slinky; the cell can easily bend to the side without breaking.
Why doesn’t an older, large-diameter tree also need to have that high flexibility? A good physical model might be a metal paperclip and a big cylinder of the same metal. You can bend the paperclip but you can hardly bend the metal cylinder. It’s the size itself of the metal cylinder that deters the bending. (If that analogy didn’t work for you, imagine a skinny plastic coffee stirrer. You can bend it. The near and far sides of the coffee stirrer are close together. But in the case of a plastic drinking straw, the near and far sides of the straw are much farther away from one another, so if the straw is given the same force as the coffee stirrer was, the near side of the straw would have to stretch much more and the far side would have to compress much more than those sides on a coffee stirrer for the same bend. They both have the same amount of plastic, but in the straw, that plastic is farther from the center, so when you try to bend it, the diameter itself keeps it from bending much.) Similarly, a large-diameter tree trunk won’t experience much bending in its trunk (the whole thing can tip, but the trunk itself can’t bend much because the size, not the wood material’s flexibility, prevents it). On top of that, the large-diameter tree benefits from having the cellulose in a vertical orientation, which improves the wood’s strength.
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Why did my fence rails have both types of wood in it? The wood harvested in the USA in former times would have grown more slowly than the wood harvested today, so even a modest-sized tree would have only a small core-wood core. Core wood wouldn’t have been a very sizeable proportion of the wood in a lumber yard. As managers have gone to techniques that allow trees to grow faster, the trunks reach a harvestable size at a younger age, and we end up with a lot of boards that have both core wood and outer wood in them. This decrease in “wood quality” due to younger harvest age has been a prime motivator for the development of improved wood adhesives and the rise of wood composites. Like plywood, these other composites (such as oriented strandboard (OSB), glulam beams, cross-laminated timber, laminated veneer lumber (LVL), and wood plastic composites) can use a mix of “good” and “bad” wood qualities, and give a product that benefits from all the wood but minimizes the effects of the “bad” or “defective” wood.
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Which brings me back to my picket fence. Do I wish it were vinyl or something that didn’t shrink or swell? No, not at all. I like wood, and it has a relatively low environmental cost. And those other materials have other downsides I don’t like, besides aesthetics.
Do I wish the carpenter had bought outer wood for my rails? Yes, but not at the expense of harvesting old growth (self-planted, old) trees. Plantation trees are fine. The carpenter chose the best wood from what was available in the lumber yard. He would have estimated how far from the pith each rail was, by looking at the curvature of the growth rings on one of the cut ends, and choosing boards that were sawed from the farthest out.
Do I mind that the fence becomes waggly in the rainy season? No, as long as the movement doesn’t work the nails out.
So what’s the big fuss? Nothing, in the case of a fence. (I’d have a different opinion if this were a finished wall!) I just enjoy watching the change of seasons in my fence, and knowing why it happens.
Barb Lachenbruch 
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