For the problems and questions that arise as you work through new projects, count on finding the answers in Woodworking FAQ (Storey Publishing, 2012) by Spike Carlsen. This woodworking guide offers the know-how and experience to answer common questions and provides handy tricks of the trade and simple instructions that will improve your woodworking and building skills. The following excerpt on wood shrinkage and expansion is taken from Chapter 2, “Wood & Plywood.”
Movement in Wood: Dealing With Wood Shrinkage and Expansion
One challenge all woodworkers face is dealing with wood’s predisposition to shrink and expand in response to changes in humidity and temperature. This may eventually result in cracks, gaps, and weak joints. This section offers some tips for meeting that challenge.
Q: Why is it so necessary to use dry wood?
A: Wood shrinks and changes shape as it dries. You want the bulk of that shrinkage and change of shape to occur before you start working with it. Freshly cut wood is also extremely heavy — in some species, over twice as heavy as when it’s dry. Plus, wet wood is more susceptible to decay and rot. That said, there’s an entire field of woodworking called “green woodworking” that focuses on building furniture and other items with wet or unseasoned wood. This furniture is often “rustic” in nature.
Q: Does wood shrink and expand equally in all directions?
A: No. The amount of shrinkage varies from species to species, but generally wood shrinks 8 to 10 percent tangentially, 4 to 5 percent radially, and close to zero percent lengthwise. In other words, the surface of the board where the grain intersects it perpendicularly, or close to perpendicularly, shrinks the most. This means woods of different shapes will shrink differently based on how they’re cut from the tree.
Q: A friend told me I should use quartersawn boards for a tabletop because they’re more stable than flatsawn boards. Is that true?
A: Yes. Due to the orientation of the grain, the boards will tend to expand and contract less across their width and be less prone to cupping.
Q: How much can wood expand and contract?
A: Some kiln-dried wood can change 1/8" to 1/4" in width for every foot. This may not seem like much, but when you add it up, a 4'-wide table can vary in width by as much as an inch from dry season to wet season.
Q: I’ve heard wood dries out in stages. What does that mean?
A: Picture wood as a bundle of paper drinking straws filled with water. If the water is emptied from the straws, the bundle will remain the same size. That’s the first drying stage. If you were to extract the moisture from the straws themselves, the bundle itself would start shrinking. That’s the second stage. In stage one, the moisture held in place by the cell walls dries out, bringing the wood down to a moisture content of about 30 percent. In stage two, water leaves the cell walls themselves. This is when noticeable differences start occurring. The wood shrinks, and as it shrinks it becomes denser, harder, and stronger.
Q: How long does it take for wood to dry thoroughly enough to stop shrinking and expanding?
A: Wood never stops moving. It’s always trying to reach its equilibrium moisture content (EMC), the point at which moisture is no longer entering or exiting the wood. A board’s EMC changes as the relative humidity changes. So unless wood is kept in a controlled environment where temperature and humidity never fluctuate — and even museums have difficulty doing that — the EMC is constantly changing, and the wood shrinks and expands accordingly.
Q: How much fluctuation can there actually be?
A: In dry areas, environments, or seasons where the relative humidity is as low as 20 percent, the EMC can be as low as 4 percent. In damp areas where humidity levels reach 80 to 90 percent, the EMC can reach nearly 18 percent. As a compromise, most kiln-dried lumber has a 6 to 8 percent moisture level.
Q: How can I tell if my wood is dry enough to build with?
A: The best way is to check it with a moisture meter. There are two types. Pin-style meters have pins that you push into the wood to measure electrical resistance, which in turn is expressed as a percentage of moisture. Pinless meters use radio frequencies to measure moisture levels. Pinless meters generally are more expensive, but they don’t leave pinholes in your wood and often can give more accurate readings in dense woods that are difficult to penetrate with pin-style meters. Prices range from $50 to $200 and more.
Tip: A Scrap Wood Hygrometer
Throughout the year, the humidity in your workspace, and therefore the dimensions of the wood you’re working with, can vary appreciably. One way to know how much to compensate when you’re building is to use “moisture boards.” Glue up 12"-wide planks of your most frequently used woods, measure their widths during both the driest and the most humid months of the year, and write those measurements on each board. When it’s time to perform an exacting task, such as fitting an inset door or drawer, consult the moisture board made of that type of wood to see how much it’s expanded or contracted, and adjust the fit of your pieces accordingly.
Excerpted from Woodworking FAQ © Spike Carlsen. Used with permission of Storey Publishing.