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Shrinkage
usually begins at 25 to 30 percent moisture content, the fiber saturation point. Shrinkage continues to zero percent moisture content, an oven-dry state.

Swelling occurs as wood gains moisture, when it moves from zero to 25 to 30 percent moisture content, the fiber saturation point. Different woods exhibit different moisture stability factors, but they always shrink and swell the most in the direction of theannual growth rings (tangentially), about half as much across the rings (radially) and only in miniscule 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.

Generally, flooring is expected to shrink in dry environments and expand in wetter environments.

Between the fiber saturation point and the ovendry state, wood will only change by about .1 percent of its dimension along the grain (lengthwise in a flatsawn board). It will change by 2 to 8 percent across the grain and across the annular rings (top to bottom), if quartersawn; and 5 to 15 percent across the grain and parallel to the annular rings (side to side), if plainsawn.

Wider boards tend to move more than narrowerboards. Movement in a 5-inch-wide plank is more dramatic than in a 21/4-inch strip. The ideal moisture content for flooring installation can vary from an extreme of 4 to 18 percent, depending on the wood species, the geographic location of the end product and time of year. Most oak flooring, for example, is milled at 6 to 9 percent. Before installation, solid wood flooring should be acclimated to the area in which it is to be used, then tested with a moisture meter to ensure the proper moisture content.
(Note: Laminated wood flooring tends to be more dimensionally stable than solid flooring, and may not require as much acclimation as solid flooring prior to installation.)

A wood's weight and moisture content

Wood is hygroscopic - meaning, when exposed to air, wood will lose or gain moisture until it is in equilibrium with the humidity and temperature of the air.
Moisture content (MC) from 5 to 25 percent may be determined using various moisture meters developed for this purpose. The most accurate method in all cases, and for any moisture content, is to follow the laboratory procedure of weighing the piece with moisture, removing the moisture by fully drying it in an oven (105 degrees C) and reweighing. The equation for determining moisture content is
weight of wood with water - oven-dry weight MC% = x 100 oven-dry weight

Equilibrium moisture content

The moisture content of wood below the fiber saturation point is a function of both relative humidity and temperature in the surrounding 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.
A 50 percent swing in relative humidity produces an EMC change of 10 percent. How that affects wood flooring depends on which species is being used. However, let's say the width variation is just 1/16 inch for a 21/4-inch board. That's a full inch over 16 boards in a floor. Over the width of a 10-foot wide floor, that amounts to more than three inches of total expansion or contraction.
Protective coatings cannot prevent wood from gaining or losing moisture; they merely slow the process.

The seasoning of lumber

Freshly sawn lumber begins to lose moisture immediately. Its color will darken and small splits or checks may occur. Movement of moisture continues at a rate determined by many factors, including temperature, humidity and air flow, until a point of equilibrium is reached with the surrounding air. The shrinking and swelling of wood are dimensional changes caused by loss or gain of water. In practical terms, the process works this way:

  1. A standing oak tree is felled and sawed into a board 1-inch thick, 10 inches wide and 8-feet long. Placed on a scale, the board weighs, say, 36 pounds.
  2. The board is placed in a stack of boards separated from the next by stacking strips of uniform size to keep the board straight. The stack is aimed at the 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.

The milling of lumber

Most hardwood lumber is dried to an average of 6 to 9 percent moisture content before milling is begun. Mill inspections conducted by the National Oak Flooring Manufacturers Association, allow 5 percent of the wood outside this range, to a maximum moisture content of 12 percent. The 6 to 9 percent range is likely to be the average of all types of wood products used in a normal household environment, assuming usual heating and cooling equipment is used to ensure human comfort.