Moisture detection is one of the most important aspects of any pre-installation inspection. The interaction between the selected flooring and substrate is as important as any other factor in achieving a successful installation. There have been countless articles on concrete moisture testing, moisture meters and the affects of moisture on plywood and OSB but few on the more unique underlayments such as Thick Pour Gypsum Cement.

As always the best place to start is by understanding the applicable consensus documents. In this case ASTM F2419 – 11 Standard Practice for Installation of Thick Poured Gypsum Concrete Underlayments and Preparation of the Surface to Receive Resilient Flooring which states:

“12.2 Prior to installation of finished floor goods, thick poured gypsum concrete underlayment shall be tested in accordance with Test Method D4263 (Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method) or with an electronic moisture meter with a gypsum scale recommended by the manufacturer of the thick poured gypsum concrete underlayment. Acceptance criteria for the suitability of the substrate for installation of resilient floor covering shall be provided by the manufacturer of the resilient floor covering.”

gypsum underlaymentSeveral different companies make gypsum cement underlayments such as Maxxon and USG.

The Maxxon web site at states:


Follow ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slab Using in situ Probes, by using the Delmhorst HT-4000F or TotalCheck meter packages or the Wagner Rapid RH system. Both are accurate relative humidity probe systems that allow for easy and continuous monitoring of the drying process… When a moisture meter is required, use a pin invasive type such as a Delmhorst model G-79 or Delmhorst BD2100. (On the model BD2100, make sure you are on Scale 3 ‘gypsum,’ then use the digital display only, do not use the color coded LED indicators). In pours over 1½” thick use insulated pins in drilled holes at various points and thicknesses.”

The USG Web site at states:

“Sheet vinyl and vinyl composition Tiles

Follow the Surface Requirements instructions to prepare the underlayment. As a general rule, prior to application of the vinyl, the moisture level on the floor should be less than 1%, as measured in a variety of locations throughout the underlayment surface using a Protimeter moisture meter. However, the vinyl floor manufacturer’s recommendations will always supersede the recommendations provided here. Carefully follow and adhere to floor goods manufacturer’s recommendations.”

As stated above in the three references:

  1. The gypsum underlayment MUST be tested for moisture content prior to installation.
  2. The moisture content allowable is directly related to the floor covering being installed.
  3. The Meter required varies from manufacturer to manufacturer.
  4. Thickness of the pour affects how to use the given meters.

gypsum underlaymentThe moisture content of the underlayment must be determined. Excess moisture in the substrate can cause adhesive failures, damage to the flooring and mold issues. The less permeable the flooring the more impact excess moisture will have. Carpet and cushion with a permeable scrim slows moisture to pass from the substrate through the flooring system so the process of evaporation is continued. When wood flooring is installed over a substrate with excess moisture it is absorbed into the wood and the resulting dimensional changes cause damage to the flooring and substrate. Resilient flooring is a vapor barrier because it has little to no permeability. When the moisture comes out of the underlayment it is stopped at the bottom of the plank or sheet goods. In the right conditions the moisture can condensate weakening the underlayment, damaging the adhesive and creating an environment conducive to mold growth.

Using the meter and process required by the manufacturer protects the installer from taking responsibility of a failure. Documentation of the moisture testing using a camera with date stamp is further evidence the moisture testing was done.

Finally, the thickness of the underlayment affected the reading. Any none invasive meters measure ¾-1” maximum in depth which is insufficient for thicker pours. The Maxxon Instructions state “In pours over 1½” thick use insulated pins in drilled holes at various points and thicknesses.” This method will give a moisture reading specific to the depth of the pins up to and including a wood subfloor. Although the top inch of the underlayment may be dry there is likely higher moisture content deeper in the pour that will equalize when covered with any sort of vapor retarder or barrier.

With modern flooring and adhesives, measurements of moisture content are not optional. The high cost of replacement versus the low cost of pre-installation inspection make proper testing imperative. The value in knowing the variables in acceptable moisture content of concrete, gypsum, OSB or Plywood and how it affects the floor covering that is being installed makes education essential for all floor covering professionals. Knowing the pitfalls and how to deal with them is 90 percent of the battle in preventing failures. The only thing more damaging to your company than providing your employees an education and having them leave, is not educating them and having them stay.