The determination of moisture content is an
essential part of quality control within the flooring installation process.
Flooring installers must know the dryness of not only the wood flooring, but
its subfloor and the concrete slab beneath it, if one exists. Hand-held
electrical tools, called moisture meters, should be part of the toolbox of
every flooring contractor, for measuring moisture in wooden or concrete
subfloors and in the wood floor materials. It is equally important to check
% of relative humidity of the area of the floor installation as well as the
moisture meters Moisture meters have become one of the most critical tools of the trade, yetthey are
often neglected by those who need them most - flooring contractors. The
installer who continues to work without one is simply jeopardizing his jobs.
A single moisture-problem installation should be enough to convince him or
her to invest in a meter and make use of one of the most cost-effective
tools in the business.
meters have many purposes. They can determine if floor boards are
dry enough for an installation to proceed. They can check subfloors and
concrete for high moisture levels; they can decide when a second coat of
finish can be applied; they can assess water damage.
There are two main types of meters--Probe and
The probe type, which is the older method,
measures electrical resistance across opposed sets of pins, which are pushed
into the wood. The higher the moisture content, the lower the resistance.
One advantage of probe type meters is that
those with insulated pins can measure moisture content at varying
depths--you can tell whether the moisture content near the bottom of a board
is higher than near the top, for example.
Deciding which kind of moisture meter to buy
is a matter of sorting through the features you think you will need, and how
often you may use it, and then understanding the various features and
benefits. It is important that the meter you choose offers the following:
A wide moisture content range from at
least 6 percent to 30 percent. (The accuracy of readings outside of
these limits are generally considered questionable.)
A clear analog or digital dial.
The necessary adjustment tables for
various species; (some meters have a built in adjustment for this
For a probe meter, the ability to take
external probes, and a selection of pin sizes.
Testing Wood Subfloors ~ Wood subfloors are actually easy to check for moisture content. Just test
for moisture at several locations in the room and average the results. In
most regions, a "dry" wood subfloor that is ready to work on has a
moisture content of 12 percent or less( down to 7% in low R/H states ). If
excessively high readings are obtained, installation should not proceed
until the origin of the moisture is identified and moisture problems are
remedied. During the winter, an overly moist subfloor can be dried out by
running the heat for a few weeks. Air conditioning during the summer will do
the same thing. Before flooring can be installed, the moisture content of
the subfloor should be within 4 percentage points of the flooring that will
be laid on it.. If the moisture content between the flooring and subflooring
varies more than 4 percentage points, then the flooring should not be
Testing Concrete Subfloors ~ As concrete moves through its initial drying period, regular checking of
moisture content can start after 30 days. In most cases it will take 60 days
or more before the slab is dry enough for wood flooring installation to
proceed. Excess moisture in the concrete will cause problems such as
condensation or failure of the adhesive under the flooring.
Concrete is made up by
mixing an aggregate of varying sizes (sand, gravel, or crushed stone)
Portland cement and water. The water and Portland cement mix together to
form a new chemical compound that we refer to as paste. This paste is a
liquid when it is first mixed together which flows into the spaces that
occur between all of the aggregate. A chemical reaction also starts to
occur when the Portland cement and water are combined that causes the paste
to harden. Any excess water that is originally mixed into the concrete that
is not used in the chemical reaction remains as water droplets distributed
throughout the paste portion of the concrete. Since this chemical reaction
occurs over time some of the residual water trapped within the concrete
becomes part of the cement paste as it continues to react or cure. In fact
some of this additional water is important to the curing process that
gradually increases the strength of the concrete. During the initial curing
period (7 to 10 days), additional water must be added to the surface to
replace the water that evaporates from the surface or a sheet of plastic is
placed over the surface to inhibit surface evaporation. Curing compounds
are also sometimes applied to the surface to inhibit surface evaporation.
Any water that remains
after curing is substantially completed will continue to evaporate through
the exposed surfaces of the concrete. In the case of a floor slab that is
poured on an unvented steel pan or a 6 mil polyethylene vapor barrier, only
the exposed upper surface can accommodate the evaporation of this excessive
moisture. During the period concrete is exposed to an exterior environment
this drying is retarded by high exterior relative humidity; rain and snow.
It is not unusual to have very high interior moisture levels in a concrete
floor slab that is many months or possibly several years old. The water to
cement ratio that occurs during the initial mixing is the primary factor
that influences the amount of residual moisture contained in the concrete.
The amount of water required to chemically react all of the Portland cement
is fixed by the quantity of the Portland cement in the mix. As more excess
water is trapped in the concrete, the resultant concrete structure is more
porous due to the spaces that remain after the water has dried out. It is
common practice to add extra water making the concrete “soupy” so that it
flows and levels more easily and makes it easier to place the freshly mixed
concrete when pouring a floor. Or in the case of high rise construction
extra water is added to make it easier to pump the concrete to the upper
Moisture conditions in concrete slabs that
ultimately create moisture problems in flooring may not the flooring
contractor's responsibility, but it is the flooring contractor's
responsibility to ensure that potential moisture problems are resolved
before installation begins. Unless the flooring contractor takes the
initiative to determine the potential problems, through testing for moisture
content, he is the one who will get called by the unhappy homeowner-because
the buckling wood is the only result the homeowner sees.
A flooring contractor can begin his
determination with some subjective and logical questions: What is the
history of other homes in the area, as well as the history of the building,
the quality of the building and the quality of the slab?
Also, what is the age of the concrete? (An
installer should not accept a slab as "ready" on age alone.) What
is the concrete's visual appearance? Color of concrete surfaces: The
"proper" color of concrete should be a light yellowish-grey
(similar to limestone) not sugar white. In fact, a sugar-white surface
usually means the concrete was improperly cured and can create a severe
moisture problem later on (usually months later). Any concrete that is
blotchy, with dark spots and martyred some whitish freckling within the
darkened areas should be suspected of having contaminants, admixtures or
other problems unsuitable for flooring.
Flooring contractors should make sure to rely
on flooring manufacturers' recommendations for your definition of what
qualifies as "acceptable moisture content," as well as for which
type of moisture testing each manufacturer prefers and care should be taken
to insure there is no confusion on the units of measurement.
Testing for moisture in concrete can be
accomplished using specially designed and calibrated moisture meters, and
there are also several types of physical tests that can be used. The most
common types of tests are discussed later on.
Electrical Moisture Meter testing of concrete Some meters are designed and calibrated specifically for concrete use.
Regular checking of moisture content of the concrete slab during the drying
out period is required to ensure it has reached sufficient dryness to accept
the floor covering.
Electrical testing works on the principle of
impedance measurement in the concrete slab which is translated to a percent
moisture content reading.
When testing concrete slabs, particularly if
they are on-grade or below-grade, the moisture condition should be tested
not only on the surface, but also in the body of the slab ( some moisture
meters do this in a non-destructively). The reason for testing both the
surface and the mid-section is to ascertain if there is continuous moisture
movement toward the surface. If the flooring is installed while the slab is
in this condition, upward movement of moisture will continue and the
moisture will move into the floor. It is important the insure the moisture
meter has the capability to measure into the slab, not just the surface. The
results are costly and damaging.
As always, tests in multiple locations
throughout the slab. Moisture meters that give meaningful readings and that
unit of measurement can be related to manufacturers recommendation, should
only be used.
Moisture Detection Equipment
Recommended relative humidity levels: The recommended relative
humidity levels in concrete for wood floor installations are 1) 75% or less
for Engineered wood flooring, and 2) 60% or less for Solid wood flooring
which are adhered directly to a concrete surface. You should note that when
solid wood is installed over concrete, wood flooring industry standards
require that a very low permeability vapor barrier should be installed over
the surface of the concrete. Although adhesives are often designed to be
“water proof” this does not necessarily mean that the adhesive is a proper
vapor barrier. In fact most adhesives do not effectively function as proper
vapor barriers. The exception to this is a heavy application (at a spread
rate of no more than 30 square feet per gallon) of Asphalt adhesive combined
with 6 mil thick polyethylene.
Various physical tests of concrete~
Here are some other tests that installers employ to check the moisture
content of the concrete before starting the installation. All tests should
be done at several different locations in a room--typically along exterior
walls and walls with plumbing enclosures, as well as over mechanical chases.
THE CALCIUM CHLORIDE TEST: The calcium
chloride test is becoming one of the oldest used concrete moisture tests.
The calcium chloride test has been used most often by sheet vinyl
installers, but a growing number of wood flooring installers now employ the
test as well. Costs can run about $50 or $60 per test. As always, refer to
flooring manufacturer recommendations, since some believe other tests are
The calcium chloride test works by measuring
changes in weight of anhydrous calcium chloride crystals.
A small plastic dish of crystals is sealed
with a plastic tape. The entire dish is weighed on a gram scale prior to
exposure and the weight, date and time the test was started must be
recorded. The lid is then opened, and the dish of crystals is carefully set
down on the concrete for 60 to 72 hours. The dish is enclosed within a
7-by-10-inch cover, which is sealed to the concrete. During this time, the
only source of moisture being absorbed by the crystals is what can evaporate
out of the covered concrete surface area.
At the end of the test, the dome is removed
and the lid is placed back on the dish and sealed. Again the dish is weighed
on the gram scale and the date and time are marked. The change in weight is
multiplied by a constant and divided by hours to provide an estimated rate
of evaporation, in pounds.
Pounds is the equivalent weight of the water
that evaporates out of a 1,000-square foot surface area during 24 hours.
Water weighs 8.3 pounds per gallon. If the test reports 8.3 pounds emission,
then one-gallon of water is leaving a 1,000 square foot surface area in 24
A conservative but generally recommended
allowable amount of moisture emission as expressed by the calcium chloride
test is 3.0 pounds per 1,000 square feet per 24 hours at the time of the
installation of the flooring. A note of caution: Use care in lid dealing and
removal of the dish, and weighing as exposure to atmosphere will
dramatically effect the results.