Wood Movement Basics
Wood Movement Basics
Stability and strength of furniture is effected by wood movement. Wood movement is effected by humidity, and differently in different types of wood. Understanding wood movement will allow you to make better, stronger pieces of furniture.
If you were to inspect wood under a microscope, you would discover a structure that resembles a multitude of tiny parallel tubes. When the wood was a part of a tree, these passageways were used to transmit nutrients. Once wood is cut out of a log, these tubes tend to expand and contract in conjunction with temperature and humidity changes.
Wood expands and contracts. This movement has to be taken into account in the design and building of any projects. Typically, wood moves twice as much in the tangential direction as it does in the radial direction. For woodworking purposes, we ignore movement in the axial direction. Wood moves because of the change in moisture content (amount of water molecules) in the wood. Wood that is freshly cut, i.e. green, has the maximum amount of water it can hold,called the fiber saturation point (FSP). The FSP runs from about 25 percent to 31 percent,depending on the wood. On average, we figure 28 percent.
When a woodworker like me refers to wood movement, what we’re actually talking about is the fact that wood is not dimensionally stable; that is, its dimensions change with humidity. This is all related to the cell structure of wood, and it can be a genuine pain to deal with in some instances.
The 2x4 that you buy at the lumberyard began as a living tree. As a living tree, the trunk of the tree provided a mechanism to transport water (in the form of sap) from the roots to the leaves. When this tree trunk is cut down, the sap stops flowing, but the sap remains in the trunk. This raw wood is then stripped of bark and branches, and then cut into various sized blanks. Traditionally, these blanks are air dried to approximately 15% moisture content, giving up its moisture to the atmosphere. Current practice involves using a kiln to dry wood to an even lower moisture content, approximately 7-10% moisture content.
The largest amount of shrinkage happens after a tree is cut down and is initially dried. Drying out lumber can take a long time. For thick stock, the process can take years. According to my dad, a rule of thumb for air drying white oak in Germany was one year for each centimeter of thickness, so 5 cm thick stock would take five years. But drying time is also a function of wood type and climate.
It’s a natural fact that wood moves. You can nail it, glue it and reinforce it but you will never stop the wood in your projects from shrinking and swelling with seasonal changes in humidity. So the secret to dealing with wood movement is to work with it, not against it.
The following table presents average shrinkage values, from green to oven-dry, for a number of commercially important woods. The values are expressed as a percentage of the green dimension for radial, tangential, and tangential to radial (T/R) shrinkage.
As the moisture content in wood changes, wood expands or contracts, and this in turn causes a variety of problems. The moisture content of wood is measured as a ratio between the weight of the water in the wood and the weight of the wood itself. This ratio is stated as a percentage.
There's nothing more frustrating than putting a lot of time into a project only to find out six months later that a panel split apart, doors won't close right, or joints have opened up. It's all because wood moves. It expands and contracts with changes in relative humidity, and there's nothing you can do to stop it. But what you can do is take wood movement into account when designing and building a project.
This is the second part of a two-part series on Wood Movement. Last week, I mentioned that wood "moves" because it acts like a sponge -- when the surrounding air is damp, wood absorbs moisture and expands. When the air is dry, it releases moisture and contracts.
Wood-drying refers to reducing the moisture content of wood prior to its use. It is the process of removing its moisture contents to an average equilibrium with the atmospheric conditions of the locality of use. Ideally, wood is dried to that equilibrium moisture content as will later (in service) be attained by the wood so that further dimensional change will be kept to a minimum.