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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
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
(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
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:
- 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.
- 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.
- 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