What makes wood shrink

The amount of bound water in the wood continually changes with the amount of moisture in the surrounding atmosphere. On the average, wood gains or loses about 1 percent moisture content for every 5 percent change in the relative humidity.

In green wood, free water fills the cell cavities and bound water saturates the fibers in the cell walls. As it dries, the free water evaporates first, then the bound water. The moisture content of the wood varies with the relative humidity of the surrounding air, as this chart shows. Once the wood has been dried, the moisture content never again rises above 28 percent its fiber saturation point from the effects of humidity alone.

For this to happen, the wood must be immersed in water. The relative humidity is the ratio of the actual moisture in the air absolute humidity to the maximum amount of moisture the air will hold at its present temperature. The warmer the air, the more moisture it will hold. The wood fibers swell as they absorb moisture and shrink as they release it, causing the wood to expand and contract. In the Northern Hemisphere, relative humidity increases in the summer and decreases in the winter.

And due to the effects of heating and air conditioning, the relative humidity is generally different indoors than out. Additionally, the relative humidity may vary from one building to another if the indoor temperatures differ. Consequently, wood tends to move with the seasons or whenever you change its location. The grain structure causes it to move differently in three different directions. And there are other forces that may cause a board to move or change shape. Internally stressed wood called reaction wood moves when you cut it.

Cutting relieves some of the stress, and the wood reacts by changing shape. This is quite different from normal wood movement, however. Another consideration as regards equilibrium in wood is moving a piece of furniture to a significantly different climate. A finely crafted hardwood chest of drawers that has achieved balance in a humid environment such as in Louisiana but is then moved to a dry climate such as Arizona could actually crack as the wood releases a substantial amount of moisture.

A skilled woodworker knows how the wood being used will expand as it equalizes with the moisture in its environment. The movement in lumber created by gaining or losing moisture occurs across the grain and not along the grain. In other words, an 8-foot-long 2 x 4 will usually always remain 8 feet long. The width and thickness of the same 2 x 4, on the other hand, could vary significantly. In any case, the grain of the lumber should be oriented in the same direction.

Expansion can have an effect on drawers, however, making them difficult to open. For this reason, plywood is used to build most cabinet carcasses. The following is a tip for dealing with potential cupping of boards when gluing boards together to make a table top: As you place the boards side-by-side, alternate end grains facing up and end grains facing down. Some wood varieties literally drip with moisture when they are cut. Wet wood is not very stable, and will most assuredly shrink as it dries out.

This is the reason why freshly-cut lumber does not work very well for fine woodworking, which requires great precision and fine tolerances. The lumber used in precision woodworking must be dried considerably before it is stable enough to use. This is why lumber manufacturers either store lumber for long periods to air-dry it or bake it in ovens, called kiln-drying. Ideal stock for woodworking should be at a state of equilibrium with the environment in which the finished project will reside.

There are a few considerations to keep in mind here:. Knowing that wood will naturally attempt to equalize with the humidity in its environment, a woodworker must know how the wood will expand. Movement in a piece of stock caused by shifts in moisture will occur across the grain, as opposed to along the grain. That is to say; a 1 x 6 that is 4 feet long will almost always stay 4 feet long.

However, depending on the moisture content of the stock and the air and the variety of wood used , the width and thickness to a lesser extent may vary considerably. When building a carcass for a cabinet, each of the four sides of the box should have the grain-oriented in the same direction.

As such, all four sides should grow relatively equally mainly if all four come from the same piece of original stock. This, however, can cause issues when drawers are used in the cabinet, making the drawers difficult to open and close.

This is why most cabinet carcasses are built using plywood , which isn't affected by humidity nearly as much as dimensional lumber.