A solid wood tabletop changes width with the humidity of the room, and the USDA Forest Products Laboratory puts numbers on it.
The figures below come from Chapter 4 of the Wood Handbook, FPL-GTR-282. The worked example is an estimate built from those figures.
What the Wood Handbook Says
- Direction: Wood is anisotropic. It shrinks and swells most along the growth rings, about half as much across the rings, and only slightly along the grain.
- Length: Shrinkage along the grain is generally small, between 0.1 and 0.2 percent from green to ovendry for most species.
- Threshold: Wood changes size only below the fiber saturation point, which averages about 30 percent moisture content.
- Equilibrium: Wood settles at a moisture content set by the relative humidity and temperature of the surrounding air. Seasonal changes in humidity change that moisture content.
- Coatings: Varnish, lacquer and paint retard moisture changes but do not prevent them.
The Handbook lists warping, checking and splitting as results of shrinking and swelling. It names loosened tool handles and gaps in flooring as examples.
A Worked Example
Table 4-2 of the Handbook gives the equilibrium moisture content at 70 degrees Fahrenheit. At 30 percent relative humidity it is 6.2 percent. At 65 percent relative humidity it is 12.0 percent. The difference is 5.8 points.
Equation 4-9 estimates shrinkage by assuming it is linear below 30 percent moisture content. The Handbook says to use tangential values for plain-sawn width and radial values for quartersawn width. For a 36-inch-wide top of white oak, the math runs as follows:
- Plain sawn: 36 inches x 10.5 percent x (5.8 / 30) = about 0.73 inch.
- Quartersawn: 36 inches x 5.6 percent x (5.8 / 30) = about 0.39 inch.
The 10.5 and 5.6 percent figures are white oak’s tangential and radial shrinkage from Table 4-3.
This is a hypothetical top, not a measurement of any Saline River Workshop piece. The Handbook warns that average values suit estimates that do not need great accuracy. It also says the shrinkage of an individual piece cannot be predicted exactly.
Plain Sawn and Quartersawn Tops
The Handbook’s Figure 4-4 caption says tangential shrinkage is about twice as great as radial. That ratio explains the gap between the two results above. Read the shrinkage figures for each oak group in White Oak vs Red Oak for a Dining Table.
Questions to Ask a Furniture Maker
- Attachment: Ask how the top connects to the base and whether it can move across the grain.
- Cut: Ask whether the boards are plain sawn or quartersawn.
- Moisture: Ask what moisture content the boards held when the build began. The Handbook says the aim of drying is a moisture content close to what the finished piece will have in service.
- Room: Ask the maker to account for the room where the table will stand.
- Repair: Ask what to do if a seam opens in a dry winter.
Tables From Bradley County and Wilmar
Saline River Workshop lists tables in solid white oak and walnut, including a Solid White Oak Coffee Table and a 72-Inch Round Wooden Pedestal Table. Its contact page lists a workshop in Wilmar, Arkansas.
Bradley County timber history is in Bradley Lumber Company of Warren, Arkansas. Joinery is covered in Mortise and Tenon Furniture Built in Arkansas.
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