The American National Standard Specifications for the Installation of Ceramic Tile, ANSI A 108, lays out the method for installing ceramic tile with organic mastic. In section A-4.2, ANSI points out that use of organic mastic over gypsum board or gypsum plaster is not approved if used in wet areas. In wet areas, water-resistant (WR) board must be used.
Adhesive Application. Though seldom done in actual practice, ANSI specifies that the adhesive be applied with the flat side of the trowel to obtain a mechanical bond and proper coverage. Then the notched side of the trowel should be used to comb the adhesive to obtain an even setting bed.