The more moisture a board absorbs or releases, the more it swells or shrinks. However the surface of a board moves differently depending on the grain direction and type of grain. Wood movement along the grain is almost negligible. From 0 to 28 percent moisture content, a typical board will move only 0.01 percent of its length. However it will move about 8 percent across flat grain and 4 percent across quarter grain. This is why woodworkers consider quartersawn lumber more stable. It’s also why boards with mixed grain (and mixed expansion rates) tend to cup.
So how do you predict how much a board will move and in what direction? That depends on the grain direction, type of grain and time of year. In most areas, the relative humidity climbs as the weather turns warmer. This causes the wood to expand. The rule is to allow for 1/4″ of movement across 12″ of plain grain and 1/8″ across the same amount of quarter grain. If you’re working in the summer, the wood shrinks as winter approaches. When building in the winter, count on the wood expanding when summer comes. (Thus the old saw, “Work tight in summer, loose in winter.”)
The wood grain in the legs of this pedestal table runs parallel to the longest dimension, making the legs as strong as possible. Were the grain to run parallel or perpendicular to the pedestal, the legs would be weakest at their narrowest point, the ankles.
Wood is Strong Along the Grain
The wood cells are made from long, tough cellouse fibers, bound together by a glue-like substance, lignin. The cellulose is a lot tougher than the lignin. Consequently, it’s much easier to split a board along the grain (separating the lignin) than it is to break it across the grain (snapping the cellulose).
This botanical trivia plays an enormous role in woodworking design. Can you imagine what might happen if you cut mortise-and-tenon joints in which the grain ran across the tenons? They’d snap if you just looked at them sideways. Yet tenons cut parallel to the grain will far outlast the woodworkers who cut them.
But wait there’s more — when strength is paramount, grain direction may not be your only consideration. Some species of woods are naturally stronger than others. Windsor chairmakers, for example, typically use hard maple, birch and hickory for legs, rungs and spindles. Because these parts are fairly slender, weaker woods won’t do.
A good indicator of a wood’s strength is its density — the weight of a given volume of substance. Wood density is measured by calculating its specific gravity — the weight of a volume of wood compared to the same volume of water. Generally, the higher the ratio, the denser — and stronger — the wood.
Specific gravity, unfortunately, doesn’t predict when a wooden board will break, sag or dent. For this, there are measurements of strength.
• Compressive strength tells you how much load a wood species will support parallel to the grain. If a corpulent relative sits in the chair, will the legs buckle?
• Bending strength shows the load wood can withstand perpendicular to the grain. How many kids can stand on that chair rung before it’s firewood?
• The stiffness indicates how much the wood will deflect when loaded perpendicular to the grain. How far will those shelves sag when you display your collection of cannonballs?
The hardness reveals how resistant the surface is to abuse. How hard can you pound when taking your frustrations out on the workbench?
A Parting Thought
Too often we approach our craft as if it were a collection of recipes. Take two boards, chop them up on a table saw, add a dash of glue and — presto! — a birdhouse. Or a Chippendale highboy, depending on how many boards and how finely you chop. But woodworking is more than knowing how to use a tool or follow a plan. It’s the accumulated insights and inspirations of 5,000 years of craftsmanship. And at the heart of this craft is a surprising material that has yet to reveal all of its mysteries. PW
Nick Engler is contributing editor for Popular Woodworking.