Licensed applicators pump poured gypsum underlayment onto a floor to prepare a uniform, level surface for floor covering installation.

This 10,000-sq.-ft. ceramic tile floor at the Great Mall in San Jose, CA was installed over Maxxon's poured gypsum underlayment.
Poured gypsum underlayments have assumed a place alongside plywood, Portland cement concrete and oriented strand board (OSB) in the universe of underlayment products. The poured gypsum system has grown increasingly popular. In fact, since its inception, one firm has poured more than 3.5 billion square feet of the underlayment. The increased popularity of in-floor heating systems has also been a factor in the growth of the gypsum poured underlayment producers. Despite its increased use, poured gypsum underlayments are not well understood among the installation community. The products are made of lightweight, high-strength gypsum concrete for use in residential, multifamily, commercial, radiant heating and renovation projects. With proper preparation, almost any type of floor covering can be installed over a poured gypsum underlayment.

Poured gypsum underlayment was used to prepare the substrate for this installation of Forbo resilient flooring at the Kenai Congregate Housing facility in Kenai, AK.

Background Facts

U.S. Gypsum and Georgia Pacific pioneered the development of poured gypsum underlayments in the 1950s and '60s. During the 1970s, the product's distribution channels shifted to private companies that had working relationships with applicators. There are approximately 150 certified applicators in the field today.

Gypsum underlayments can be placed at various compressive strengths, depending on the requirements of each job. These strengths range from 1,200 to 5,500 psi.

Why, and where, is poured gypsum underlayment used? It's popular for leveling condominium and apartment floors, particularly in fast-track construction projects where pre-formed concrete planks are used in the structure. Pre-formed planks require complete leveling and surfacing before any floor coverings may be installed.

Poured gypsum underlayment is also applied to level uneven wood floors. The product also enhances the fire resistance of multifamily buildings, which helps those structures meet building code requirements. In addition, the product has useful sound-deadening properties. It is practical to install over radiant-heat systems. Also, poured gypsum underlayments are economical to use, and can be installed quickly with specialized equipment operated by licensed applicators.

Most flooring manufacturers do not address installation issues relative to situations where poured gypsum underlayments are in place. To help fill this information void, I'll now point out what is required to achieve a satisfactory flooring installation.

Site Conditions & Special Considerations

Poured gypsum underlayments should be allowed to dry in an environment where the temperature is maintained at 50¿ F or greater. Continuous ventilation should also be maintained to evacuate moisture from the work area. Under these conditions, a 3/4-inch-thick underlayment will usually be dry in seven days. However, I advise you to double check before you lay a floor covering over it. Otherwise, you may be saddled with liability for installing over an underlayment that wasn't completely dry.

Also check for moisture emissions. In this case, do not perform a domed calcium chloride test, because the results can be easily distorted. Instead, use the rubber mat test or tape down a 36-by-36-inch plastic sheet. Leave the mat, or sheet, in place for 48 to 72 hours and then check beneath it for condensation. If none is present, you should be OK to proceed. A pin-type meter may also be used to determine if your underlayment is ready for a flooring installation. Look for a green or yellow LED light to determine suitability.

Adhesive performance. Generally speaking, it is recommended that the poured gypsum underlayment be sprayed or coated with an approved acrylic product. 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.

Ceramic tile. The use of an isolation membrane, or a special flexible bonding agent, is recommended. A variety of products are approved by membrane manufacturers. Two examples with which I am familiar are Noble Co.'s sheet membrane and Bostik's liquid urethane bonding adhesive.

Another rather important point worth noting is that, when using Portland cement thinset mortars, a chemical reaction known as ettringite may occur. The reaction can take place fairly quickly or develop years later. Ettringite has an expansive effect that sometimes causes disbonding.

As a safety measure before you proceed with the installation, you can apply an acrylic skim coating followed by a skim coat of Portland cement mortar. The alternative would be to use the membranes and bonding agents I mentioned above. Before you use any product, make sure the manufacturer's approval is forthcoming.

Wood floors. Laminated wood flooring, today more commonly called "engineered" wood, may be installed over poured gypsum underlayment provided urethane adhesive (which does not contain water) is used. Solvent-based adhesives are also suitable. And of course, make sure the adhesive manufacturer approves of this application of its product. Free-floating laminate floors (not laminated) pose no problems.

Resilient floors. It's a fact of life that resilient floor manufacturers do not warrant their products for use over poured gypsum underlayments. But in the real world, they do not warrant any underlayment, because they believe their warranty applies only to their product and not to the underlayment. This policy makes sense, and the underlayment supplier should be responsible for any failure due to the underlayment.

Under this set of circumstances, the secret to a satisfactory installation is to follow the procedures outlined by the manufacturer of each product you are using on a particular installation.

The few related problems that I have encountered were the result of bad seams that allowed water and dirt to accumulate in the seam area. Pay close attention to seaming when you install - regardless of the underlayment that's in place.

Carpet. With glue-down carpet, it is vital that you fully utilize your skill and expertise. It's essential that you use the correct trowel notch (no worn trowels) and a good adhesive. Make sure you get good transfer and good contact. Otherwise, disbonding may occur in traffic areas, which will allow the hard plastic backing to abrade the underlayment and create a depression in the floor.

All underlayments or substrates - be they lauan plywood, on-grade concrete, poured gypsum or some other type - present the installer with a unique set of problems. It's in the installer's best interest to develop an understanding of each underlayment's individual characteristics. Such an approach will allow you to deliver good installations, enhance your reputation and - even better - add to your bottom line.