Installation Guide

HOW DOES IT WORK? 25
Explanation of how Schluter
®
-DITRA functions
HOW DOES IT WORK?
To understand how the DITRA system works, it is important to first understand what a tile assembly is, how it functions, and how stresses
occur within the assembly. A tile installation is a composite assembly that consists of layered components (underlayments, bonding
mortars, tile, etc.). The primary sources of stress in this composite system are movements due to loading, changes in temperature,
and changes in moisture content (either in the substructure or in the components of the tile assembly, including the tile itself). When an
installation is subjected to these movements, compressive and tensile stresses develop within the assembly and interact to produce shear
stresses at the interfaces between the layered components. Therefore, a tile assembly must be able to perform well under load and, at the
same time, provide flexibility within the shear plane.
The method used to establish the overall performance of a tile assembly under loading is the ASTM C627 “Standard Test Method for
Evaluating Ceramic Floor Tile Installation Systems Using the Robinson Type Floor Tester.” The assembly is tested in cycles using a loaded,
revolving carriage. Load, wheel hardness, and number of revolutions vary with each cycle. Once a specified level of damage is exceeded,
the test is stopped. The Tile Council of North America (TCNA) Handbook for Ceramic, Glass, and Stone Tile Installation assigns performance
levels to an assembly based on the number of cycles successfully completed. The ratings include residential, light, moderate, heavy, and
extra heavy, in order of improving performance.
The TCNA conducted the tests shown below, which included a single layer of plywood (3/4" thick) over joists spaced at 19.2"
o.c., DITRA bonded using modified thin-set mortar (ANSI A118.4), 12" x 12" porcelain tile (3/8" thick) bonded using unmodified
thin-set mortar (ANSI A118.1), and modified portland cement grout (ANSI A118.7).
This flexibility is readily apparent when the overall DITRA assembly is subjected to shear testing. In the tests shown below, porcelain tile
was bonded to concrete using unmodified thin-set mortar meeting ANSI A118.1. One specimen included DITRA, while the other did not.
The results show that the amount of stress developed in the system when the tile layer is displaced is significantly reduced through the
inclusion of DITRA, which is due to the product’s flexibility in the shear plane.
The two installations produced ratings of heavy and light, according to the TCNA Handbook for Ceramic, Glass, and Stone Tile
Installation. “Heavy” indicates a performance level acceptable for shopping malls, stores, commercial kitchens, work areas, laboratories,
auto showrooms and service areas, shipping/receiving, and exterior decks, while “light” indicates a performance level acceptable for light
commercial use in office space, reception areas, kitchens, and bathrooms.
Given that the test assemblies consisted of only a single layer of plywood over joists spaced at 19.2" o.c. (a bending and deflecting
substrate), these ratings demonstrate that DITRA performs extremely well under load while at the same time providing flexibility within the
shear plane.
DITRA provides uncoupling (geometric flexibility) through its open rib structure, which allows for in-plane movement that
effectively neutralizes the differential movement stresses between the substrate and the tile.
When placed on a solid foundation, columns or pillars can support tremendous loads. The same physical principle
applies to DITRA installations. Column-like mortar structures are formed in the cutback cavities of the matting. Loads
are transferred from the tile covering through these column-like mortar structures to the substrate. Since mortar has a
very high compressive strength, DITRA becomes virtually incompressible within the tile assembly and, therefore, doesn’t
sacrifice load-distribution capabilities of the system.
Tile has been successfully installed for thousands of years by incorporating an uncoupling layer, or forgiving shear interface, between the
tile assembly and the substrate. This practice has evolved from the sand-strata method (tile set in mortar over a layer of tamped sand) to
the unbonded mortar bed method (tile set in mortar over a cleavage membrane). However, this alone does not ensure a high-performance
tile installation. The tile covering must be well supported so that loads can be distributed through the assembly to the substructure without
damaging the tile covering. Therefore, a viable tile assembly must be designed to incorporate both support/load distribution of the tile layer
and flexibility within the shear plane (e.g., a traditional unbonded mortar bed allows for flexibility at the shear plane through a cleavage
membrane, but still provides a solid base for the tile layer).
Since DITRA utilizes geometric flexibility in the shear plane rather than material flexibility, the advantages of uncoupling are achieved without
sacrificing load-distribution capabilities of the tile assembly. Thus, it is the combination of geometric flexibility in the shear plane and support
in the normal direction that allows DITRA to protect the tile layer from stresses due to loading and changes in temperature and moisture.
Test Report Number Number of Cycles Passed Rating
TCA-046-03 (B) 13 Heavy
TCA-126-03 (A) 9 Light
Assembly Average Maximum Shear Stress (psi)
Tile over concrete 208
Tile and DITRA over concrete 65