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 prior to installation of the flooring. 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 irregu-larities shall be filled or smoothed with latex patching or underlayment compound recommended by the resilient flooring manufacturer. The patching or underlayment compound shall be moisture, mil-dew, and alkali-resistant, and do not use Portland based patching compounds over gypsum toppings. For commercial installations, it must have a minimum of 3000 psi compressive strength after 28 days. When using Portland cement thinset mortars, a chemical reaction known as ettringite may oc-cur. The reaction can take place fairly quickly or develop years later. Ettringite has an expansive ef-fect that sometimes causes de-bonding.

This is an important standard if the finished floor will be in a commercial use; some gypsum under-layments will not meet the 3000 psi compressive strength requirement. Too much water or too much sand and the product will not perform properly. The less water required the compressive strength becomes higher. Over watering is the single most common installation problem and must be avoided. Over watering not only lowers compressive strength (PSI) it also creates a “soft” surface which can affect the ability of the flooring adhesive to bond tenaciously. Over watering by as little as 2.5 percent will lower PSI by 15 percent.
Is the underlayment dry? Depending on the thickness, drying conditions, environmental conditions, gypsum underlayment can take as long as 7 days or more to dry before floor covering can be installed. Continuous ventilation required until the underlayment is dry. Proper subfloor preparation, requirements for a minimum am-bient temperature of 50°F which must be maintained before, during and after installation and the provision of adequate ventilation necessary to assure proper drying of the gypsum concrete under-layment. Installations should be completed only by an applicator recommended by the gypsum con-crete manufacturer. Since satisfactory performance of the finished flooring depends, in part, on the condition of the thick poured gypsum concrete underlayment, it should be protected from heavy traffic via wheeled or concentrated loads by using temporary walking boards until the underlayment is dry.

Check with the manufacturer of the underlayment as to the recommended drying time and the recommended testing method for dryness. Prior to the installation of resilient floor coverings, the moisture content of the underlayment must be tested according to ASTM D4263, or with an ap-proved moisture meter containing a gypsum scale that is recommended by the gypsum concrete underlayment manufacturer. Two such moisture meters are the Delmhorst G-79, which is strictly a gypsum concrete moisture detector, and the Delmhorst BD-2100, which offers three scales for wood, concrete and gypsum. The calcium chloride test method is not a way to test gypsum underlayments for dryness. Usually a moisture meter or even taping plastic to the gypsum is the preferred method. Check with the GYPSUM manufacturer to be sure. Is the surface finished properly? ASTM F2419 also requires that a topcoat/sealer be applied to the thick poured gypsum concrete floor underlayment in accordance with the recommendations of the gypsum concrete manufac-turer, prior to the application of finished floor goods. Gypsum underlayments must be must be primed or sealed with the recommended materials if a glue down flooring material is to be in-stalled. This procedure should be done by the installer of the underlayment, but usually is not. This extends the adhesive’s working time, improves bond strength and results in greater transfer (no soaking in). Coating also results in greater adhesive coverage. Regardless of the brand of un-derlayment used, follow the manufacturer’s instructions for covering the underlayment with ad-hered floor coverings. If not, the surface will be far too porous and the floor covering adhesive will not work correctly.

Special Thanks to JJ Haines for the Information