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
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.
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.
TESTING WOOD 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 the following:
Testing Wood Subfloors
- 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.
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.
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
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.
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
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.
THE PHENOLPHTHALEIN 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