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Water & Wood

For wood flooring professionals, it's important to inform end users about the normal behavior of wood in relation to moisture. Most solid wood flooring will contract during periods of low humidity (usually during the heating season), sometimes leaving noticeable cracks between boards, or else expand during periods of high humidity. To help minimize these effects, users can stabilize the environment of the building through temperature and humidity control.

This is an overview of how water and wood don't mix-and what to do if they do. Spotting any potential moisture problems, and taking the proper steps to avoid them, is the path to the most-serviceable floor. Fortunately, many of the instances that involve moisture can be mitigated before, during or soon after installation.

Water and Wood Basics
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. These vessels are similar to the "strings" in a stalk of celery. They are similar, too, to a group of soda straws gathered together, running up and down the tree.

That's the simple version of how a still-standing tree is made up of vertically-aligned fibers. Cut the tree down, and the fibers are horizontal. Saw it and manufacture strip flooring, nail the floor down and most of the fibers are still horizontal, running the length of the boards.

In the live tree, the fibers are loaded with moisture, as sap. After being cut, the tree begins to dry out, just like a rose will wilt after it's picked. As the tree's fibers dry, they shrink in thickness or diameter, but almost none lengthwise. This shrinkage, characteristic of all woods, is critical in understanding the effect of moisture on wood flooring.

Moisture content in solid wood is defined as the weight of water in wood expressed as a percentage of the weight of oven-dry wood. Weight, shrinkage, strength and other properties depend on the moisture content of wood. In trees, moisture content may be as much as 200 percent of the weight of wood substance. After harvesting and miring, the wood will be dried to the proper moisture content for its end use.

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. Here are some quick points about shrinking and swelling:

  • 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 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.

  • 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 narrower boards. Movement in a 5-inch-wide plank is more dramatic than in a 2 1/4-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 MC% = weight of wood with water - oven-dry weight / divided by oven-dry weight X 100.

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 2 1/4-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, 9 3/4 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 5~inch thick, 9.2 inches wide, 8 feet long. It weighs 21.6 pounds with an 8 percent moisture content. If aH 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.

Wood Flooring has a comfort level, too
Wood flooring will perform best when the interior environment is controlled to stay within a relative humidity range of 30 to 50 percent and a temperature range 60 to 80 degrees Fahrenheit. Fortunately that's about the same comfort range most humans enjoy. The chart below indicates the moisture content wood will likely have at any given combination of temperature and humidity. Note that equilibrium moisture contents in the recommended temperature/humidity range (shaded area) coincide with the 6 to 9 percent range within which most hardwood flooring is manufactured. Although some movement can be expected even between 6 and 9 percent, wood can expand and shrink dramatically outside that range.

All the way to the floor
Flooring is usually dried to the national average moisture content expected in use so that shrinkage and swelling are minimized and buckling or large gaps between boards does not occur. However, the careful drying and manufacturing of wood flooring cannot entirely prevent an unsuccessful installation.

Many manufacturers record moisture-meter readings before the flooring leaves the facilities, and such readings are attached to invoice and packing lists.

Growing 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 a 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 to as 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 .O79 inches. Across 10 feet of flooring, this could translate to 1.9 inches of expansion.

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 The Hardwood Company
Main Office: 550 North Main Street
PO Box 126, Granite Falls, NC  28630
Phone: 800-771-5006  Fax: 828-396-8182
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