Typically, gypsum concrete underlayment is mixed with water and poured or pumped while in a liquid state over suitable substrates. (Photo courtesy of Hacker Industries.)

Before you begin to install ceramic tile or natural stone over gypsum concrete underlayment, you need to know a few things about the underlayment itself. Ignorance of these facts ultimately can result in an expensive installation failure.

Gypsum concrete underlayment is a mixture of gypsum, Portland cement and sand supplied in bag form. Typically, it is mixed with water, and pumped or poured while in a liquid state over suitable substrates. The underlayment mix then is poured over a structurally sound concrete or precast floor at a minimum depth of 1/2 inch, or a depth of 3/4 of an inch over a structurally sound wood subfloor.

At this point in time, there are approximately 150 certified applicators in the field. The primary reasons for using gypsum underlayment are to meet accepted sound and fire ratings, and to provide a suitable surface for the installation of flooring.

Minimum compressive strengths of gypsum concrete underlayments should be 2,000 pounds per square inch (psi) over wood subfloors and 3,000 psi over concrete subfloors. Compressive strengths may go as high as 5,000 psi.

Because of its unique characteristics, lightweight gypsum concrete underlayments are often used for radiant heating applications, and in residential, multifamily, commercial, and renovation projects.

Many installers are unfamiliar with poured gypsum underlayments and, unfortunately, cannot find much installation help from flooring manufacturers. However, if a few simple steps are followed, a successful installation can be achieved even by those who haven't had experience working with such underlayment products.

First and foremost, the gypsum must be dry. Under normal environmental conditions, and with adequate ventilation, the poured underlayment will dry in seven to 10 days. Just as with Portland cement concrete, the underlayment must be properly dried before a floor covering installation may safely proceed.

One recommendation for testing dryness is to tape down a 24-by-24-inch sheet of plastic to the surface of the underlayment. Another is to lay flat a 24-by-24-inch high-density, smooth rubber mat atop the underlayment and weigh it down. If no condensation or darkening develops within 48 to 72 hours of placing the plastic sheet or rubber mat, the underlayment is considered dry.

Among the most popular and effective dryness test involves use of the Delmhorst pin meter for gypsum. This instantaneous test yields extremely accurate results. The readings should show a moisture content of 5 percent or less.

DO NOT use the ASTM 1869 calcium chloride dome test to check the dryness of poured gypsum underlayments. This test method has proven inaccurate for such purposes and, at a minimum, the results can be misleading. Because gypsum concrete underlayment is porous, and the calcium chloride crystals used in the dome test attract the moisture surrounding the test application, incorrectly high moisture values tend to be produced.

Since I just brought up the subject, it's worth noting that the porosity of the underlayment may be an obstacle to achieving the strongest and best bond with tile or other floor coverings. A weaker bond can develop if the adhesive too quickly penetrates the underlayment and loses open time, and then skims over with subsequent loss of proper transfer to the flooring material for this extreme porosity.

To guard against this possibility, I recommend the use of an acrylic sealer such as Specco S-55. A product specifically manufactured for sealing gypsum underlayment, Specco S-55 also helps prevent degradation from topical moisture penetration that may occurs as the result of poorly constructed sheet vinyl seams. Specco is supplied in full strength with a solids content of 55 percent. It must be diluted at one part Specco to four parts potable water, and may be applied by brush, roller or spray. (Editor's note: Not all gypsum concrete underlayments require the use of sealers. Check with the manufacturer of the underlayment to determine if a sealer is required.)

I usually limit my discussions to tile and stone applications, but most gypsum underlayments are poured in apartments and multi-unit residential buildings where, in many instances, vinyl flooring installations are most prevalent. When installing vinyl, pay particular attention to seaming because most problems occur as a result of cleaning water and dirt penetrating at the seams to cause disbonding.

And with the increased popularity of wood flooring, more and more wood is being installed over gypsum underlayments. Because wood is hygroscopic, and will expand and contract continuously, I recommend the use of 100 percent single-component urethane adhesive. This type of urethane does not contain any water, as most flooring adhesives do, and remains flexible to help compensate for any wood movement. The most commonly used urethane of which I am aware is Bostik-Findley's, but there are likely others available in the marketplace.

For ceramic and natural stone installations, I recommend the use of an anti-fracture membrane. This serves two purposes -- one being the prevention of tile cracking from movement, and the second being the prevention of water penetration to the underlayment through the grout joints. Anti-fracture membranes are available at any ceramic tile or stone distributor. The use of an anti-fracture membrane also eliminates the possibility of Portland cement thinset expanding and popping lose, due to a chemical reaction called ettrengite.

Installation of glued-down carpet should be accomplished by using a premium carpet adhesive. Don't use the low-end adhesives. If you do so, you'll be asking for trouble especially if there are any carts or chairs with rollers used in the building. Always use the proper trowel (a new one) and check for adhesive transfer constantly.

Tackless installations require careful observance of the instructions provided by the underlayment producer. Be particularly aware of directions that relate to nail penetration. Check with the manufacturer for proper procedures.

Glue-down tack strips may be applied by following manufacturers' instructions as to proper adhesive.

Literally billions of square feet of poured gypsum concrete have been used in floor installations with a ratio of flooring failure, I would guess, similar to that of Portland cement concrete. All underlayments require understanding if the finished installation is to endure for the long term.