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Tile Over Poured-Gypsum Underlayments and Leveling Materials

Gypsum based underlayments are predominantly composed of various grades of gypsum to which chemicals are added for controlling setting time and, in some cases, to provide flowability. They may be sanded or unsanded. They may be mixed with water or a latex admixture. Gypsum based materials are highly sensitive to moisture. Therefore, they are recommended by gypsum manufacturers for use in dry areas only. These materials are normally used by the resilient flooring mechanic for patching small cracks and holes or for correction of thickness variations of adjacent flooring materials. Larger areas may be leveled with gypsum-based products, which requires a minimum thickness of 3/4 in. (19 mm) over wood and 1/2 in. (12 mm) over concrete substrates. Poured-gypsum un derlayments are not recommended for direct use under tile because of their sensitivity to water. In addition some types have poor strengths that can contribute to cohesive failure within the gypsum product itself. Formulation changes of some gypsum-based materials, including Portland cement, have prompted claims that they are suitable to receive tile. Another recommendation is the application of specific waterproofing membranes over, and bonded to, the gypsum materials to which the tile is applied. Extreme caution should be exercised by general contractors and tile contractors when substrates of gypsum materials are encountered. The gypsum material manufacturer should be contacted for specific tile installation procedures to use over their products, supported by satisfactory performance test data. Caution

Tile Over Poured-Gypsum Underlayments and Leveling Materials – Letter

NOTE TO THE TILE CONTRACTOR

Date

Name Address City, State Zip

The tile contractor should require in writing a letter from the product manufacturer relieving him of responsibility for the performance of any tile installation over poured-gypsum products.

Re: Project Tile Over Poured-Gypsum Underlayments and Leveling Materials

The substrate to receive tile on the (project name) is or is specified to be a gypsum- based material. Gypsum-based materials are not recognized by the American National Standard Specifications for the Installation of Ceramic Tile as a suitable substrate. These specifications state in Section ANSI A108.01-2.2.3 that substrates subject to damage by exposure to water require a cleavage membrane or a traditional waterproof membrane to separate that surface from the mortar setting bed and tile. The mortar setting bed referred to in this section is a minimum 1-1/4 in. (32 mm) Portland cement mortar bed. If I am required to set tile by a thin-set method over the gypsum-based material or over a bonded waterproof membrane, I shall not proceed with the installation until I have received a letter stating that I am not responsible for the performance of this installation.

Name and Title Company

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Chapter 3 | Backerboards and Underlayments