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
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. 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
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 some detail.
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
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
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 oven dry state, wood will only change by about .1 percent
of its dimension along the grain (lengthwise in a
flat sawn 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
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
(Note: Engineered 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
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
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
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
Dimensional stability 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.
Calculation: 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.
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.