Installer Update
October 2nd, 2009

Formula for Success: Installing “Large Format” Natural Stone Tiles and “Crack Isolation Membrane”

Fourth Quarter, 2009

By Thomas Duve’

Large format natural stone tiles are increasingly the product of choice in high end commercial and residential installations.

Large format natural stone tiles range in size from 16″ x 16″ to 24″ x 24″ and can weigh up to 50 pounds per tile. A high percentage, (more than 50-percent) of these tiles installed on floors is of the “sedimentary” variety (travertine and limestone) where the absorption rate is anywhere from 8- to 18-percent. Many of these installations use crack isolation membranes with varying degrees of success.

Crack Isolation Membranes help in preventing reflective cracking of natural stone tiles by isolating structural movement from the tiles. Millions of square feet of crack isolation membrane have been installed under natural stone, yet there are failures due to the lack of product quality and improper installation methods.

The goal of this update is to share some key do’s and don’ts that will help assure the successful installation of large format natural stone when using crack isolation membranes. However, respecting the following four basic guidelines will aid your next installation.


DO use crack isolation membranes over mortar beds, leveling and patching compounds. Pre-leveling the floor before installing the membrane is extremely important. A number of rapid set mortar beds, patching and leveling compounds are available to enable the membrane to be installed within 24 hours of the application of the leveling course.

The membrane must be as close to the tile as possible to protect it from reflective cracking, shrinkage and structural movement of the substrate, level coats and patching compounds. An additional benefit of using the membranes over these substrates is the protection of moisture-sensitive marble like travertine and limestone from discoloration and deterioration.

DON’T use crack isolation under mortar beds, leveling or patching compounds. Placing a poured topping or level coat over a flexible non-dimensional membrane imparts a tension when curing, resulting in shrinkage and cracking in the leveling materials. This is a recipe for failure of the assembly. Warping of the marble and delaminating of the membrane will occur in 10- to 25-percent of the installations. The wicking of moisture carrying calcium chloride up through the substrate can damage soft marble with warping, pitting and discoloration in natural stone. Much of the damage occurs when new construction is closed off and the HVAC (air conditioning & heating) is turned on for the first time, changing the dynamics of the concrete slab and forcing moisture up through the curing tile system.


DO use a quality crack isolation membrane meeting not one, but all five ANSI A118.12 tests for performance and durability. It doesn’t make any sense to save a few pennies a square foot for a low level, non-complying membrane, when the tile assembly costs 50 times more than the money saved using the cheaper products. Keep in mind that it costs about three times the original cost of the installation to repair the floor.

DON’T use products that haven’t passed the A118.12 specifications.

DON’T use membranes that are susceptible to mold, mildew and MVT, as these products can stain and damage the marble. Insure the bond of the membrane at the interface of the substrate can’t be compromised with high water vapor transmission causing a break down or re-emulsification of the primer, resulting in loss of bond and delimitation of the membrane. Choose a membrane that can resist a minimum of 6# + MVT.


DO use an appropriate latex modified, medium bed mortar for marble and granite over crack isolation membranes. These products are designed specifically for “large format” marble and granite and are installed in the traditional (thin-set) method using trowels of ½” or greater. Some of these specialized mortars can be “built up to ¾” without shrinkage or setup time problems” and can accommodate variations in the substrate to obtain a level floor.

These products are effective because of their use of coarser aggregate and water retention properties, which in turn provide excellent anti-slump characteristics that support heavy tile and help prevent damaging vapor emissions.

DON’T use non-modified or regular thin set mortar used in everyday tile installations. These products use a smaller aggregate than the medium bed mortars and do not have the antislump properties when a ½” or greater trowel is used. Logically, they won’t support a 24″ x 24″ stone weighing up to 50 pounds.

DON’T install green marble, resin-backed marble or other moisture sensitive stone without first contacting the mortar manufacturer for recommendations on these tiles.


DO use movement joints. Assure movement joints are placed in accordance with ANSI and TCA recommendations including around the perimeter of the installation. Movement joints partition large tile sections into smaller sections to accommodate structural and thermal movement of the tile assembly.

Investigations into recent marble installation failures revealed that 2,500 sq. ft. of 18″ x 18″ travertine became hollow, (tenting) because the entire assembly—marble, mortar bed and membrane—de-laminated from the substrate. There were no grout joints, no soft joints and no perimeter joints.


Membrane and natural stone floor failures are a direct result of putting other factors before quality. This is a problem that is easily solved by observing the measures of quality that the industry has established. Crack isolation membranes do work when installed correctly under natural stone and tile floors. The key to success is to carefully evaluate the system components and make sure the installer follows the proper installation procedures.

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