Water and Wood Basics by National Wood Flooring Association The easy explanation that students learn in grade school - trees grow with roots in the ground and leaves in the air - still serves as the basis for understanding the never-ending relationship between water and wood. The roots collect moisture and nutrients from the soil and ship them through vessels or fibers up the trunk and branches to the leaves. Wood is hygroscopic - meaning when exposed to air, wood will lose or gain moisture until it is in equilibrium with humidity and temperature of the air. When wood is neither gaining nor losing moisture, an equilibrium moisture content (EMC) has been reached. Wood technologists have graphs that precisely tie EMC and relative humidity together, but as a rule of thumb, a relative humidity of 25 percent gives an EMC of 5 percent, and a relative humidity of 75 percent gives an EMC of 14 percent. The seasoning of lumber Freshly sawn lumber begins to lose moisture immediatelly. In practical terms, the process works this way: 1)A standing oak tree is felled and sawn into a board 1-inch thick, 10 inches wide and 8-feet long. Placed on scale, board weighs, say 36 pounds. 2) The board is placed in stack of boards seperated from the next by stacking strips of uniform size to keep the board straight. The stack is aimed at prevailing breezes to accelerate drying. After two or three months of air drying, the board now weighs 25 pounds. It is also 31/32-inch thick, 93/4-inches wide and 8 feet long, with 25 percent moisture content. 3) This 25-pound board is trucked to the flooring mill and loaded into a dry kiln, a building large enough to hold three or four railcar-loads of lumber. After six or seven days, this same board is now 15/16-inch thick, 9.2-inches wide, 8-feet long. It weighs 21.6 pounds with an 8 percent moisture content. If all the moisture were removed, the board would weigh 20 pounds. Dimensional Stability When flooring manufacturers and distributors talk about relative stability of various wood flooring species, they are referring to how a floor "moves" once it is out down. Quatersawn wood will usually be more dimensionally stable than plainsawn. Wide Plank Boards How much can temperature and humidity affect the dimensions of a hardwood floor? Take a look at one 5-inch red oak plank board: 1)Within "normal living conditions" (say, an interior temperature of 70 degrees and relative humidity of 40 percent), the board has a moisture content of 7.7 percent and is 5-inches wide. 2) If the relative humidity falls to 20 percent, the moisture content of the board will be 4.5 percent, and the same 5-inch board will shrink by .059 inches. Across 10 feet of flooring, that could translate t oas much as 1.4 inches of shrinkage. 3) If the humidity rises to 65 percent, the board's moisture content would be 12 percent and the same 5-inch board would expand by .079 inches. Across 10 feet of flooring, this could translate to 1.9 inches fo expansion. In actual practice, however, change would be diminished in a complete floor, as the boards' proximity to each other tends to restrain movement. To sum up, gaps between 5-inch boards and wider will be larger then the gaps for strip flooring 2-1/4 inch wide.

The dimensional change coefficient can be used to calculate expected shrinkage or swelling. Simply multiply the change in moisture content by the change coefficient, then multiply by the width of the board. Example: A red oak (change coefficient = .00369) board 5 inches wide experiences a moisture content change from 6 to 9 percent - a change of 3 percentage points. Calculation: 3 x .00369 = .01107 x 5 = .055 inches

Dimensional Change Coefficient for 23 common wood species .00129 Mesquite .00158 Merbau .00162 Australian Cypress .00180 Padauk .00186 Teak .00201 Wenge .00212 Purpleheart .00238 Santos Mahogany .00248 Black Cherry .00263 Heart Pine .00265 Southern Yellow Pine .00267 Douglas fir .00274 Black Walnut .00274 White Ash .00300 Brazilian Cherry .00315 Pecan .00338 Yellow Birch .00353 Hard Maple .00365 White Oak .00369 Red Oak .00396 Jarrah .00411 True Hickory .00431 American Beech