In a live tree, the fibers (running up and down) carry nutrients and moisture up the trunk and branches to the leaves. After being cut, the tree begins to dry out, just like a flower will wilt after it is picked. As the tree's fibers dry, they shrink in thickness or diameter, but almsot none lengthwise. This shrinkage, characteristic of all woods, is critical to understanding the effect of moisture on wood flooring.
Wood fibers are dimensionally stable when the moisture content is above the fiber saturation point (usually about 30 percent moisture content). Below that, wood changes dimension when it gains or loses moisture. Different woods exhibit different moisture stability factors, but they always shrink and swell the most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially) and only in minuscule amounts along the grain (longitudinally). This means that plainsawn flooring will tend to shrink and swell more in width than quartersawn flooring, and that most flooring will not shrink or swell measurably in length.
Because wood flooring will shrink in dry environments and expand in moist environments it is important to consider the relative humidity at all stages: milling, storing, transporting, and the site or installation. Once milled, the flooring should be stored in dry, well-ventilated warehouses before shipment to jobsites. Flooring should be loaded and unloaded in good, dry weather, never in damp, rainy weather in which it will absorb moisture and swell.
Before wood is delivered, the jobsite must be checked to determine if it is ready. The structure should be fully enclosed, with doors and windows in place, and interior climate controls (heating or air conditioning) should be operating for at least 48 hours to stabilize the moisture conditions of the interior. Wood flooring should NOT be delivered until plastering and painting are completed and dried. Moisture evaporates from damp walls and the flooring will absorb some of it. Exposing the wood to unwanted moisture will cause it to expand. If the flooring is installed in this 'expanded' state, unacceptable cracks may develop as it dries.
Once at the jobsite, the wood should be set indoors and spread over the sub floor. The wood should be allowed to acclimate for at least four days. Moisture contents of both the flooring and the sub floor must be checked and recorded before installation begins.
Wood flooring will perform best when the interior environment is controlled to stay within a relative humidity range of 40 to 50 percent and a temperature range of 60 to 80 degrees Fahrenheit. Fortunately that's about the same comfort range most people enjoy. The best way to maintain moisture during the heating season, and thus minimizing normal cracks, is by installing a humidifier to your furnace controlled by a humidistat set at 40 to 50 percent relative humidity. Alternatively, you can add some humidity by boiling a pan of water on the stove, opening dishwasher after rinse cycle, turning off bathroom exhaust fan, and just live with small cracks during the heating season.