|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
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
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
A wood's weight and moisture
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
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
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.
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
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
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,
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.
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.
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.