A reader speaks out on what he feels is a misrepresentation of the properties of gypsum.

Ray Thompson’s article, “Troubleshooting Resilient Concerns,” addressed many important issues regarding resilient floor installation. However, it missed the mark in claiming that gypsum “has a tendency to soften and expand when subjected to excessive moisture.”

Unfortunately, this statement tends to reinforce certain myths about gypsum that are disproved by scientific testing. As we make both cement- and gypsum-based compounds, we feel well-qualified to recommend either, depending on the application. Certainly, in the presence of occasional standing water, cement is a more water-resistant material, and the surface of lesser-quality gypsum patches may soften.

However, gypsum does not expand as much as concrete, or most cement-based materials, when completely immersed in water. When it is subjected to “excessive moisture” (vapors), gypsum does not soften or expand.

A quality, latex-modified gypsum patching compound is very tolerant to evaporative vapor emissions within the 3- to 5-pound calcium chloride test result accepted by resilient floor covering producers. Emissions above that level will tend to soften and cause failure in modern latex-based adhesives and de-bond the vinyl before they even begin affecting any patch.

Ray Thompson mentions fast-track construction. The characteristics of gypsum-based patching materials are ideal for use in this application. Gypsum products go through a fast drying process and, when dry, they are highly dimensionally stable and at maximum strength. Cement-based products, however, go through a hydration process – a chemical reaction. These materials are not stable for weeks, until they have properly hydrated. On the surface, the cement-based product may quickly appear dry; however, it is nowhere near final or ultimate strength. This is very important to the installer as well as the owner.

When a failure does occur, however, and gypsum-based patching compound is present, it is falsely blamed for the failure with blatant disregard for the true cause of the failure (i.e. moisture levels above the tolerance of adhesives and floor coverings).

We wouldn’t recommend using gypsum patch as a setting material for mosaic tile in a shower floor, but over ordinary wood or concrete substrates that pass moisture testing, we continue to believe that gypsum floor materials actually provide the superior floor preparation to receive resilient floor coverings.

Homer S. Taft


Dependable Chemical Co.