WATER AND WOOD
HOW MOISTURE AFFECTS WOOD FLOORING PRODUCTS
When Water and Wood
When the phone rings for the flooring
installer, its often a prospective customer who got the name of the
business from a friend. Give an estimate for a new floor? Sure. Getting
those kinds of phone calls are good for the business.
But sometimes the caller is a previous
customer with a complaint. Perhaps the floor that fit so well when it
was first installed now shows cracks, cupping or buckling.
Those are the most common changes that
moisture can inflict on a floor. They do no favors for the customer, nor
for the hardwood flooring industry. Tales of how floors were damaged by
water, for whatever reason--improper installation or careless
maintenance by the owner--leave the impression that wood floors are more
problematic than other flooring choices.
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
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. A
well performing wood floor is often the result of an installer taking
the proper time and care necessary for a successful installation. It
involves a knowledge of:
- the expected moisture content of wood
flooring in a particular area after acclimation;
- the moisture content of flooring at
the time of installation;
- and the expected "in use"
changes. Moisture is a large part of the reason for how wood
behaves, both during the machining process and after installation.
Installers would do well to understand moisture's effect on wood in
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
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
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.
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 24-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
Protective coatings cannot
prevent wood from gaining or losing moisture; they merely slow the
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
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
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.
Manufacturers who have controlled
storage may control the moisture content of the wood up until the
point it is placed on the truck for delivery. Various parts of the
country have EMCs that range from the dry, desert areas of the Southwest
(under 5 percent EMC) to the moist areas along the Gulf of Mexico (over
10 percent EMC). Additionally, a wide range of relative humilities can
be experienced between individual job sites in the same locale, such as
an ocean-front or lakeside home versus one that's a few miles inland.
Many manufacturers record
moisture-meter readings before the flooring leaves the facilities, and
such readings are attached to invoice and packing lists. The use of
moisture meters, from manufacturing to distribution to installation, is
discussed further on.
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 put down.
The numbers in the
accompanying chart were developed by the Forest Products Laboratory of
the U.S. Department of Agriculture. They reflect the dimensional change
coefficient for the various species, measured as tangential shrinkage or
swelling within normal moisture content limits of 614 percent. Quartersawn
wood will usually be more dimensionally stable than plainsawn.
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.
3 x .00369 = .01107 x 5 = .055 inches.
In actual practice, however,
change would be diminished in a complete floor, as the boards' proximity
to each other tends to restrain movement.
How much can temperature and humidity affect the dimensions of a
hardwood floor? Take a look at one 5-inch red oak plank board:
"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
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.