Cementitious Backer Units

TecTips #893

Cementitious Backer Units

What Is Necessary for A Proper Floor Installation
A satisfactory tile installation that includes a cementitious backer unit such as UTIL-A-CRETE begins long before the arrival of the tile contractor. Requirements for the assembly of new or existing plywood subfloors include:

Maximum spacing of joists to be 16" on-center.
Subfloor to be minimum 5/8" exterior grade plywood.
The American Plywood Association recommends the spacing of plywood subflooring to allow room for expansion.
Maximum variation in plywood surface should not exceed 1/8" in 10 feet from the required plane.

Fastening of cementitious backer units to plywood subfloor:

Stagger the joints of the cementitious backer units so as not to coincide with the plywood subfloor.
A bond coat of dry-set mortar should be used between the plywood and cementitious backer units.
Sheets should be spaced approximately 1/8" apart and that gap should be filled with dry-set or latex-portland cement mortar.
Taping of the joints with 2" fiberglass tape embedded with bonding mortar is always recommended.
Use corrosion-resistant galvanized nails or screws long enough to penetrate through the plywood subflooring.
Fastening pattern varies with cementitious backer unit manufacturer but should be a maximum spacing of 6"-8" apart.
Deflection in the floor should be no greater than 1/360 of the span including live and dead loads.

Caution: Plywood that has not been properly kiln-dried or been saturated with rain during construction may cause a job failure due to shrinkage of the plywood as it drys out.

Following these recommendations will not guarantee a 100% success rate; however, this method of tile installation has proven to be a highly effective method for many years.

Guidelines have been established from TCA Method F144-93 and ANSI 108.11-1990 American National Standard for Interior Installation of Cementitious Backer Units.

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WAIVER OF GUARANTEE: Because site conditions, material and other circumstances vary, ConestogaTile cannot guarantee satisfactory results of the above recommendations. TecTips represents what ConestogaTile considers the best recommendations at the time of publication.