Subfloor Moisture Testing: The Checklist for Slabs and Wood Subfloors

Subfloor moisture testing measures how much water is in the slab or the wood panels before a new floor covers them. Concrete gets a calcium chloride test (ASTM F1869) or relative humidity probes set into the slab (ASTM F2170); plywood and OSB get a pin-type moisture meter, read against the moisture content of the flooring itself.

A reading only counts when it comes from the test the flooring manufacturer names, taken in enough spots, under living conditions, and written down. Every warranty behind the floors we sell excludes damage from subfloor moisture, so these numbers are what protects the floor. The checklist below follows the manuals of the lines we carry: PermShield, Chesapeake, Denali and Hallmark.

Before any test: the house has to be at living conditions

• Heating and air running. Chesapeake wants the HVAC operating for at least 14 days before its engineered floors go in, holding 60–80°F and 35–55% relative humidity; its laminate guidelines ask for normal living conditions for 14 days. PermShield wants 65–85°F during and after installation.
• Tests timed with acclimation. Chesapeake’s Points East guide asks for the moisture tests to be verified and documented before and after the wood has acclimated for at least 48 hours.
• Wet work finished. Plaster, paint, masonry and tile are done before the flooring arrives, as both Chesapeake and Denali require.
• Slab old enough to test. Points East says concrete must be at least 30 days old before testing, even though installation waits longer; the installation minimums by brand are in engineered hardwood on a concrete slab.

Concrete slabs: the testing checklist

1. Find out whether your floor accepts a meter at all

The same meter reading can be valid for one brand and meaningless for another. Denali does not recognize electronic concrete moisture meters for deciding whether a vapor retarder is needed; it wants the calcium chloride test.

Chesapeake’s engineered guidelines accept a Tramex concrete meter reading up to 3.5 on the upper scale. For Points East and for Chesapeake’s laminate, the meter is a screening tool: above 4, Points East calls for a calcium chloride or relative humidity test, and the laminate calls for 6-mil polyethylene under the floor.

2. Screen cheaply, if you like

Points East describes two quick checks. In the poly film test, you tape 2-foot squares of clear plastic in several places and wait 24 to 48 hours; condensation under the plastic or a darkened patch of concrete signals likely excess moisture.

In the phenolphthalein test, you chip about 1/4 inch into the concrete and apply a few drops of a 3% phenolphthalein solution; a color change means the same. Either result makes a calcium chloride test mandatory. Treat both as screening only: they tell you to test properly, not whether the slab passes.

3. Run the test the manual names

Calcium chloride (ASTM F1869) measures the moisture vapor the slab gives off, in pounds per 1,000 square feet over 24 hours. Relative humidity probes (ASTM F2170) read the humidity inside the slab; Chesapeake sets the probes at 1.5 inches deep or 40% of the slab’s thickness.

According to Hallmark and Points East, the kit or meter maker’s instructions are to be followed to the letter.

4. Run enough of them

Chesapeake and Points East ask for three tests in the first 1,000 square feet and one more for each additional 1,000. Chesapeake counts any fraction of 1,000 as a full one, so a 2,200 sq ft ground floor takes five tests, not one in the middle of the living room.

5. Add a pH reading

PermShield wants the concrete between pH 5 and 9, Points East between 6 and 9, and Chesapeake’s laminate guidelines ask for a pH-neutral slab.

6. Read the result against your floor, not a general rule

The ceilings differ by product: PermShield SPC allows up to 8 lb (F1869) or 90% relative humidity (F2170), while Chesapeake’s laminate and engineered floors stop at 3 lb or 75%. We line up the floor types against those limits in our guide to the best flooring for a concrete slab.

7. Don’t treat a pass as permanent

Chesapeake states that its tests don’t guarantee a dry slab year-round, and Hallmark notes that vapor coming through concrete can change over the life of the floor. That is why the 6-mil poly and vapor retarder rules still apply after a good reading.

Wood subfloors: the testing checklist

Plywood and OSB are read with a meter made for wood. Hallmark calls probe-type pin meters the best method, Denali asks for a calibrated pin-type meter, and Chesapeake for a meter approved for testing wood. The limits are where the manuals part ways:

• Measure the flooring, too. Under a 2% rule, a subfloor reading 9% passes under boards at 10% and fails under boards at 6%. Acclimating the boards is its own routine, covered in how to acclimate engineered hardwood.
• Read the underside where you can reach it. Hallmark’s 2% top-to-bottom rule means a crawl space reading matters as much as one from above.
• After a leak, start over. Points East warns that a subfloor that has been flooded or rained on may not be suitable for flooring, and Chesapeake requires water-damaged or delaminated subflooring to be replaced. What to do in the first days is in flooring after water damage.

Write it down: the record sheet

Every manual on this list asks for paper, not memory.

PermShield wants the test locations and results recorded; Chesapeake asks installers to document subfloor moisture readings before installation; Denali wants all results recorded and given to the homeowner before work begins, plus a daily log of room temperature and humidity and periodic readings of floor and subfloor during the installation. Print this and fill one line per test:

A filled-in sheet also makes the floor choice easier, because it tells you which products your slab or subfloor can take today.

Send it to us with the room sizes and we’ll check it against the limits of the engineered hardwood lines we carry in North Charleston and Richmond, or of our vinyl and laminate if the numbers point that way.