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TESTING CONCRETE SLABS:
Moisture meters are an effective tool to check comparative moisture conditions in concrete. They
can tell you where there may be excess moisture and help determine if you need to conduct
further testing, and identify specific areas on which that testing should be performed. Meters do
not provide quantitative results as a basis for acceptance of a slab for installation of moisture-
sensitive flooring systems. ASTM Test Method F2170 (RH using in situ probes), F1869 (calcium
chloride), and F2420 (RH on surface using insulated hood) provide quantitative information for
determining if moisture levels are within specific limits.
It is important to test both the surface and mid-section of the slab, especially if the slab is on or
below grade. This will help determine if there is continuous moisture migration toward the
surface. If this condition exists, the moisture movement may be so slow that once it reaches the
surface, moisture evaporates and causes a “dry” reading when a surface test is made.
However, if a sub-surface test is made, the meter may read “wet” indicating the presence of
moisture. When the slab is covered and the upward movement of moisture continues, moisture
will move into a hygroscopic (wood) floor, or build-up pressure under a non-breathing synthetic
floor, causing delamination.
Taking a surface reading:
Drive two hardened-steel masonry nails about 3/4" apart into the finish coat of concrete floor.
Drive them about 1/8" deep so they make firm contact with the concrete and do not move when
touch
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Touch the nails with the electrode’s contact pins.

Press the select key and read the meter, set to ‘concrete’. If the meter reads less than
“68”, the surface is dry. However, sub-surface tests should be made to verify if the slab
is dry throughout.
Subsurface test:

Drill two 1/4" holes, 3/4" apart and 1/2" to 2" deep.

Drive the masonry nails into the bottom of the holes and make the tests as described
above. Nails must not touch sides of drilled holes.
If the meter still indicates a “dry” condition, the floor is ready for covering. Tests should
be made at several points, especially when the slab is thick (4” or more) and air
circulation is poor. Make tests only in newly drilled holes.
Even readings in the “wet” range can indicate relatively low moisture content in Concrete.
For example, readings in the “68 to 77” range indicate approximately 2% to 4% moisture
content.
When evaluating a slab for readiness, always consider its age, thickness, whether the
slab is on grade or suspended, whether a vapor barrier is present and the drainage
condition of the ground.