Installer Update
January 2nd, 2008

Higher PSI’s Make Stongest Case Yet For Poured Gypsum

January-February 2008

By Tim McDonald

The newest overnight sensation in the flooring underlayment industry has been used in some form of construction for about 6,000 years. So this relatively abundant mineral is really no newcomer at all. But it has recently piqued the interest of commercial and institutional builders, residential builders, remodelers and tile installers as never before.

We’re talking about gypsum or, more precisely, its present version as poured gypsum underlayment, an increasingly popular flooring choice because of its easy, cost-efficient installation, self-leveling properties, convenient light weight and outstanding performance record. Gypsum underlayments also provide an ideal thermal mass for radiant heat applications where they create uniform heat distribution across the entire floor.

The superior acoustical properties of a poured gypsum floor have always been well-known. Its considerable mass and its ability to resist shrinkage can deliver Sound Transmission Class (STC) ratings in the 60s, in wood frame construction and with a properly installed sound mat product, Impact Insulation Class (IIC) figures of 55 or more, well above the minimum requirements.

Featuring UL-certified fire ratings of up to two hours, poured gypsum has also earned high marks for its resistance to flames. In situations of exposure to fire, the product emits steam, because of the water molecules attached to the gypsum particles in the cured mix. This is the mechanism that deters the transmission of heat and the spread of the flames when it is exposed to fire.

Bulking Up in All the Right Places

The need for faster construction at lower costs and the rapid expansion of the multi-unit residential market have created the escalating demand for poured gypsum, especially now that new advancements are making it more feasible due to its added compressive strength. Minimum gypsum concrete underlayment standards have increased dramatically in the past year or so from about 1,000 psi to 2,500 psi or, in some cases, 3,000 psi or higher. Many leading brands, including Maxxon and others, even offer compressive strengths of 6,000 psi or more.

The new, higher strength gypsum underlayments overcome formerly perceived weaknesses such as powdering, dusting or chipping, while still providing good resistance to shrinkage cracking as in the past. At roughly one-fifth the cost of cementitious underlayment, poured gypsum appears to be here to stay…at least for the foreseeable future.

Tile advocacy organizations such as the Tile Council of North America (TCNA) are taking a closer look at the product, also known by the scientific name of hydrated calcium sulfate. They are weighing certain criteria such as thickness requirements, load-bearing capacity, types of thin-sets or adhesives being used, required curing time and any other incidentals or peculiarities of their specific project such as the on-site design mix and application methods.

New Guidelines Add Stability

Results have been very promising, and now the new TCNA guidelines F-180-05 and F-200-05 recommends that crack isolation membranes be placed over poured gypsum underlayments prior to the installation of tile or stone, increasing the surface stability even more. This is helping to make the product even more attractive within the tile industry.

The application of a crack isolation or waterproofing membrane over poured gypsum in preparation for setting tile ensures a protective barrier between the cement mortar and the gypsum. Without this protection, the mortar and gypsum together can create a negative chemical reaction leading to the formation of ettringite, a crystalline structure that can weaken the bond between the mortar and the underlayment. Equally important, a crack isolation or waterproofing membrane protects against water, which could soften the gypsum underlayment. In fact, this inclusion of a membrane is an essential step for a successful installation and an integral requirement set by the TCNA for setting tile or stone over gypsum floors.

Less Time and Effort

Poured gypsum technology made headway in the 1950’s and ‘60’s, thanks to its development by companies such as U.S. Gypsum and Georgia Pacific. Through the years, it has gained an increasing number of followers, especially among those involved in fast track construction and renovations. Facing increasingly tighter completion schedules, today’s contractors appreciate the speed and simplicity of poured gypsum. Its ease of application, with no special preparation other than standard cleaning and priming, requires less skilled labor. And because it dries so quickly…usually within two to three hours…workers can get back on the job the very next day. Poured gypsum demonstrates better self-leveling properties than concrete, once again reducing labor and minimizing application time.

Because it is a simple one-step process, installing a crack isolation membrane such as Fracture-Guard 5000 with poured gypsum is fast and relatively simple. The membrane is installed in a semi-fluid state and can be applied to any form or irregular shape. In most cases, a primer is applied to the subfloor, then the gypsum is poured. As part of its routine installation process, Maxxon requires a primer/overspray before applying the Fracture-Guard 5000 on top of the gypsum, enhancing the adhesive bond. A good waterproofing membrane with anti-fracture properties can be substituted for the crack isolation product if an even higher degree of water protection is required.

Keep in mind that the gypsum underlayment should be applied to between 3⁄8 and 1⁄2-inch thickness with a concrete subfloor, and approximately 3⁄4 of an inch to an inch with OSB or plywood. The subfloor must be able to tolerate live and dead load with a deflection limitation of L/360 for tile and L/20 for stone.

Maintaining the Quality Component

Despite the ease with which poured gypsum can be applied and the minimum experience required, a strong network of trained applicators has developed and rigid quality control procedures are in place. Maxxon, for example, hires installers that typically have 10 to 30 years of experience, and the company conducts an extensive training program at its main facility. Various manufacturers such as Maxxon and USG have created extensive product lines to meet the many unique demands of specific projects. This combination of special application products, quality controls and stronger minimum compressive strengths bodes well for the product’s long-term appeal, especially considering the increasing need for speed in flooring installations of all kinds.

Today, everyone is looking for that special edge in delivering a quality product with speed and simplicity. The popularity of multi-family dwellings creates the added need for engineering that is resistant to sound and flame. So the popularity of poured gypsum could continue to accelerate as a viable floor base alternative to cementitious underlayments. Used in combination with a proven crack isolation membrane, it makes a strong case for itself.

Tim McDonald is Vice President of ParexLahabra, Mer-Krete brands. Mer-Krete is a leading provider of thin-set mortars, underlayments, grout, waterproofing and crack isolation products since 1971. Mr. McDonald has co-developed a number of the company’s leading products, including those designed for inhibiting mold and improving the environment. Mr. McDonald can be reached at 800-851-6303 or

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