Q: We replaced an old yellow-pine ceiling with T&G hickory last winter. This summer, the boards warped and literally started popping off. We don’t want to replace them until we understand the problem.
A: Clayton DeKorne, editor of JLC, responds: Hickory is not the most stable wood, so it moves a lot with changes in humidity. In winter, when you installed the ceiling, the air (and wood) were very dry, but as indoor humidity climbed in summer, the boards swelled and buckled. The “problem” is likely in the material selection. True hickory is strong and hard, but its cell structure reacts to moisture.
First, the basics with any solid-wood paneling or flooring: Nail one side only through the tongue to allow each board to move without splitting. Before installation, bring the wood indoors for a few days to acclimate to indoor conditions. Test the moisture content (MC); hickory should be installed at 6.5 to 7.5 percent MC. But unless indoor humidity levels are stable, dry wood will swell as it takes on moisture from the air. This is more pronounced in humid-summer climates, especially if there is no AC in the home. (And even if there is, the HVAC system may be oversized and not run long enough to dehumidify. Maintaining a stable indoor climate in a humid climate usually happens only with dedicated dehumidification.)
Wood does not change much in length (parallel to the grain). Hickory expands about 0.1%, or about 3/32 inch in 8 feet. A shorter board moves less, but the expansion is cumulative across the total length of the ceiling. Wood moves a great deal more in width (perpendicular to the grain). Space must be left on all sides of any wood-paneled area to accommodate total expansion.
The width of that space can be calculated using the wood’s dimensional-change coefficient: board width x annual change in MC x dimensional-change coefficient, which for hickory is .00411. (For a list of other woods, see Table 13-5 in the USDA Wood Handbook; free from the Forest Products Laboratory; fpl.fs.fed.us.) Expansion across the width of a single 5-inch hickory board can be 3/32 inch (almost 1 3/4 inches over 8 feet). By comparison, a 5-inch yellow (long leaf) pine board, with a coefficient of .00263, expands less than 3/64 inch, or half as much.
These numbers assume flat-sawn boards and an annual change of moisture content at 4%. If your indoor climate isn’t stable, you should probably choose a wood other than hickory with its relatively high dimensional-change coefficient. To find an alternative flat-sawn wood in the Handbook table, look for a dimensional-change coefficient in the Ct (tangential) column in the range of .0025. Quarter-sawn wood (in the Cr (radial) column) has lower coefficients and is even more stable.
