Gypsum technology has improved quite a lot and is a far superior product to what it was in years past. Since these products are used a lot in high rise apartment and hotel projects and also in radiant heated floors, it pays to know more about them rather than being afraid of them. Here are several questions that must be answered.

Does the product meet the industry standard, ASTM F 710?

Per ASTM F 710-05, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring, "Surface cracks, grooves, depressions, control joints or other non-moving joints, and other irregularities shall be filled or smoothed with latex patching or underlayment compound recommended by the resilient flooring manufacturer..patching or underlayment compound shall be moisture, mildew, and alkali-resistant; for commercial installations, it shall provide a minimum of 3000 psi compressive strength after 28 days."

This is an important standard if the finished floor will be in a commercial use; some gypsum underlayments will not meet the 3000 psi compressive strength requirement. Too much water or too much sand and the product will not perform properly. Many substrates need to be primed before the underlayment is installed to ensure a strong bond of the gypsum underlayment to the subfloor.

Is the underlayment dry?

Depending on the thickness, drying conditions, environmental conditions, etc, gypsum underlayment can take as long as 7 days or more to dry before floor covering can be installed. Check with the manufacturer of the underlayment as to the recommended drying time and the recommended testing method for dryness. The calcium chloride test method is not a way to test gypsum underlayments for dryness. Usually a moisture meter is the preferred method. Check with the manufacturer to be sure.

Is the surface finished properly?

Gypsum underlayments must be must be primed or sealed with the recommended materials if a glue down flooring material is to be installed. This procedure should be done by the installer of the underlayment. Regardless of the brand of underlayment used, follow the manufacturer's instructions for covering the underlayment with adhered floor coverings. If not, the surface will be far too porous and the floor covering adhesive will not work correctly.

I have had the opportunity to learn a lot about gypsum underlayments during my time as chair of the ASTM task group in charge of the ASTM F 710 Standard and in the past year when I designed a training program for a company that does underlayment work in gypsum and cement based products. While some resilient flooring manufacturers are still not comfortable with installations over gypsum underlayments, if the proper product is specified and proper procedures are followed, they may be used successfully under many resilient flooring projects.