|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 storage space.
Working with moisture meters
Moisture meters have become one of the most critical tools of the trade,
yet they 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.
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
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.
SUBFLOOR - WOOD FLOORING
Probe-type meters are fast and easy to
use. They come with different measurement indicators. The lower-cost
units have L.E.D. display lights indicating different moisture levels;
the better-quality units have analog or digital displays and provision
for different species and wood temperatures.
The pinless, dielectfic or independence
types, which are also referred to as "non-destructive" because
they don't leave any small holes in the wood, are quite different.
Signal penetration for pinless meters is
up to 1 inch for both hardwood and softwood. The meter can be moved
across the surface to identify pockets of moisture in a wood block or
plank. It is relatively unaffected by temperature. Rough surfaces have
very little effect on the reading. Measurements can also be taken
through coating, varnish or paint without damage to the surface. See Manufacturers
of Moisture Meters
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
- 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 installed.
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
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
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
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
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
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. See: Moisture
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
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 more
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 hours.
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.
THE POLYFILM TEST: Pieces of 24-inch squares of polyfilm are placed at
several points on the subfloor, sealed to the subfloor on all four sides
with silver duct tape. After 24 hours, the patches are removed and
inspected for signs of condensation. If beads of water are found on the
subfloor or the concrete appears darker, further testing is necessary.
If there is no indication of moisture under the polyfilm, the
installation may proceed. The reading is valid at 24 hours, but it's
even better if the test can stay in place until 72 hours have passed.
The polyflilm test can also be
"accelerated" by using a heat source (such as a 40 to 60 watt
lightbulb) 18 inches above the plastic.
TEST: This uses a 3 percent phenolphthalein
solution in water-free ethyl alcohol. Dime sized holes,1/4-inch deep,
are drilled in various areas of the slab, particularly around walls.
Then two drops of the solution are applied into each of the drilled
areas. If there's no color change in the solution, there should not be
enough moisture and the alkalinity is not high enough to affect the
installation. But if the phenolphthalein turns pink or dark red within
five minutes and the pH is 9.0 or higher, further testing must be done
with a more precise method.
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). This is due to the poor hydration of cement within the surface,
creating a disproportionately high water to cement ratio...which appears
white. A note concerning this procedure: This should NOT be the only
method used for testing moisture.