The Way Wood Works

Wood is a cantankerous substance; there’s no two ways about it. Its virtues, of course, are legendary. It’s attractive, abundant and easy to work. Pound for pound, it’s stronger than steel. If properly finished and cared for it will last indefinitely. But none of that makes up for the fact that it’s a complex and often perplexing building material.

Unlike metals and plastics, whose properties are fairly consistent, wood is wholly inconsistent. It expands and contracts in all directions, but not at the same rate. It’s stronger in one direction than it is in another. Its appearance changes not only from species to species, but from log to log — sometimes board to board.

That being so, how can you possibly use this stuff to make a fine piece of furniture? Or a fine birdhouse, for that matter? To work wood — and have it work for you — you must understand three of its unique properties:

• Wood has grain.
• Wood moves more across the grain than along it.
• Wood has more strength along the grain than across it.

Sounds trite, I know. These are “everyone-knows-that” garden-variety facts. But there is more grist here for your woodworking mill than might first appear.

Wood Has Grain
As a tree grows, most of the wood cells align themselves with the axis of the trunk, limb or root. These cells are composed of long thin bundles of fibers, about 100 times longer than they are wide. This is what gives wood its grain direction. Additionally, a tree grows in concentric layers, producing annual rings. You must pay close attention to these two characteristics — grain direction and annual rings — the way a sailor watches the wind. Ignore them, and they’ll bite you big time.

Sawyers commonly use two methods to cut trees into boards, each revealing a different type of grain.

• Plain-sawn boards are cut tangent to the annual rings. The sawyer “cuts around” the log, turning it for each series of cuts so the faces of the boards will show mostly flat grain (also called tangential or plain grain).

• Quartersawn boards are cut through the radius of the growth rings. The sawyer cuts the logs into quarters or bolts, and then saws each bolt so the boards show quarter grain (or radial grain) on their faces.

Lumber doesn’t always show a single type of grain on its face. Plain-sawn boards in particular may show mixed grain — flat grain in one area and quarter grain in another. The grain between the two, where the surface is cut at a 30- to 60-degree angle to the annual rings, displays rift grain.

Each type of grain has a distinct pattern, depending on the wood species. You can use these grain patterns to enhance the design of your furniture or your birdhouses. More importantly, if you know how to “read” the patterns, you can predict which way the wood will move and how much. Because of its unique structure, wood is constantly expanding and contracting. And you must cope with this movement in everything you build. 

Wood Moves Across the Grain
Because of its unique structure, wood is constantly expanding and contracting. And you must cope with this movement in everything you build.

Wood moves as its moisture content changes. After the tree is felled and the sap has evaporated, the wood fibers continue to absorb and release water like a blotter. How much water they hold depends on the relative humidity of the surrounding environment. The more humid it is, the more moisture the fibers soak up. This moisture content is the ratio of water to wood. In extremely humid conditions, as much as 28 percent of the total weight of a board may be water — 28 parts water, 72 parts wood. The rule of thumb is that the moisture content of wood changes 1 percent for every 4 to 5 percent change in the relative humidity.