Dry out the job site, prep the subfloor, and don’t
skimp on nails
Even after 20 years in the wood flooring industry, I still
get immense satisfaction when my tools are packed up in the van
and I can admire the natural beauty of a newly completed job.
For that wood floor to perform flawlessly throughout its
projected 50- to 80-year life span, however, there are many
crucial steps I have to carefully perform.
Moisture Control Is
Accounting for moisture is the single
most important factor when laying a hardwood floor.
Understanding how moisture affects wood is not a mystical art
taught only in the inner sanctum of the Temple of Wood
Flooring, it’s common sense. Simply put, wood reaches a
state of equilibrium with the relative humidity (RH) of the
surrounding materials and environment. When RH is low, wood
loses water molecules and shrinks. When RH is high, wood gains
water molecules and swells.
Wood scientists and lumber manufacturers long ago figured
out the relationship between the amount of water in wood and
the surrounding RH levels (see Figure 1).
1. As relative humidity (RH) in the air rises,
so does wood’s moisture content (MC). Wood swells
in size until it reaches its fiber saturation point, at
an MC level of 28%.
We call the amount of water contained in a wood sample its
moisture content (MC), which is calculated for each type of
wood as a percentage of its weight when oven dry (OD). A sample
is deemed oven dry when it has been baked in a 200°F oven
for 24 to 48 hours and all of the water molecules have been
A wood sample shrinks to its minimum dimensions at 0% RH and
0% MC. Maximum swelling occurs at 100% RH and 28% MC, at which
point wood reaches its fiber saturation point (FSP). The FSP is
the total amount of water molecules that can be absorbed within
the microstructure of wood.
At the Mill
Because of the
shrinking and swelling that accompanies changes in RH, it makes
no sense to manufacture any wood product to its final
dimensions and shape until it has been dried. The first step in
lumber processing is to remove the excess moisture from the
green wood. After the lumber is rough-cut, it’s dried to
an MC falling in the range of RH levels to which the wood will
be exposed as a finished product.
During the 1930s, a nationwide survey of interior RH and MC
by the USDA found regional variations based on local climatic
conditions (Figure 2).
2. Seasonal ranges in RH vary across the U.S. Optimally,
both the subfloor and the strip flooring should be at mid-range
MC levels shown on this map during installation. This means
drying out new construction sites before delivering materials,
and acclimating materials only if you are in extremely dry or
moist areas of the country.
Today most manufacturers kiln-dry wood building materials to
an MC of 7.5% at a corresponding RH of 40% — roughly the
Excessive moisture at the job site is the leading cause of
problems with wood floors. All of the expense and effort to
properly kiln-dry and precisely manufacture wood strip flooring
are for naught if it is later exposed to excessive moisture and
swells before, during, or after it is installed.
Much of the wood flooring industry literature is misleading
because it emphasizes acclimating wood flooring to job sites.
In reality, the reverse is true: A job site needs to be dried
out before any wood flooring arrives. There is nearly always
excessive moisture on new construction sites and major
remodeling job sites.
Wood flooring should never be delivered to the job until all
excessive moisture has been eliminated. The quickest and most
effective method for removing job-site moisture is to run the
heating system and increase fresh air ventilation.
Wood flooring should be installed only after the interior MC
level of a structure has been reduced to within the range that
will prevail during the life of that structure after it is
occupied. This prevents the excessive moisture present during
any major renovation or new construction project from being
absorbed into a kiln-dried wood floor. If the wood subfloor
over which the wood floor is to be installed contains excessive
moisture, then the wood flooring will absorb the moisture and
Three of the biggest lies I’ve been told in my 20
years as a flooring contractor are, "We’re from the
government and we’re here to help you;" "The check is in
the mail;" and "This job is as dry as a bone — you can
start laying the floor next Monday."
Even if there is no intent to deceive, you need to confirm
that a job site is dry by measuring the moisture content of the
subfloor. If you’re acting as the flooring sub,
don’t rely on the contractor to control site moisture. In
the end, it’s the installer who needs to take
responsibility for checking MC levels.
Water vapor is colorless and odorless, so it can only be
reliably measured with moisture meters (Figure 3).
3. Using a moisture meter is the only accurate
way to check for excessive moisture in subfloors and
finish flooring strips. A pocket-size meter costs less
than $200 and is an essential tool for good wood
Electrical resistance meters are the simplest type to use
and provide the only practical non-destructive way to determine
moisture content in wood frame construction. Two pins are
driven into a wood surface parallel to the grain and the meter
gives an MC reading. Small pocket versions are available from
several manufacturers for less than $200.