Installation Guide

NATURAL STONE 27
Q. What distinguishes DITRA-XL from DITRA?
A. Since the uncoupling function of DITRA is founded on its geometric flexibility to provide a forgiving shear plane, increasing the height
of the product produces additional protection against stresses being transferred between the tile and supporting substrate. Testing
reveals that this increase is significant.
Four assemblies incorporating natural stone installed with DITRA-XL over single-layer plywood subfloors were tested according to the
ASTM C627 Standard Test Method for Evaluating Ceramic Floor Tile Installation Systems Using the Robinson Type Floor Tester as
shown below.
Test Setup:
1. 2" x 2" joists
2. APA-rated “Exposure 1” tongue-and-groove plywood subfloor; 23/32" (3/4" nom.) thickness
3. Latex portland cement mortar (ANSI A118.11)
4. Schluter
®
-DITRA-XL
5. Unmodified thin-set mortar (ANSI A118.1)
6. Stone (12" x 12" x 3/8" nom. for each type)
7. Polymer-modified cement grout (ANSI A118.7)
These test assemblies represent demanding conditions, given the bending and deflection of the single-layer plywood subfloor under
loading and keeping in mind the lower flexural strengths of these types of stones. The results indicate that DITRA-XL provides superior
movement accommodation to limit stress transfer and protect the natural stone surface covering against damage.
It should be noted that one limitation of the ASTM C627 test is that test specimens do not include seams or butt joints between subfloor
panel ends as would be present in an actual floor construction. Subfloor butt joints are located in areas of maximum bending flexural stress
(on the joists).
Q. Then why doesn’t Schluter
®
-Systems warranty DITRA-XL for stone applications over single-layer wood subfloors?
A. Schluter
®
-Systems acknowledges that, in many instances, the market is requesting stone applications over single-layer wood subfloors.
This is due in large part because customers want even transitions to adjacent flooring surfaces and don’t want the additional cost
of installing another layer of wood. Schluter
®
-Systems has responded to this request with DITRA-XL. In our opinion, based on laboratory
testing and field experience, DITRA-XL is the most reliable system for installing natural stone over single-layer wood subfloors on the
market today.
Still, the inherent variability in the flexural strength of natural stone in conjunction with the dynamics of single-layer wood subfloors can
produce substantial uncertainty in predicting the in-service performance of the stone covering. As such, Schluter
®
-Systems cannot
warranty stone coverings over single-layer wood subfloors.
When the risks associated with natural stone and a single-layer wood application are unacceptable to the owner, building design
professional, general contractor, and/or installer, then detail D-W-S is recommended, which includes the use of a plywood/OSB
underlayment (double-layer wood system) in addition to the DITRA or DITRA-XL uncoupling membrane. Double-layer wood systems using
DITRA or DITRA-XL for supporting stone are covered by the DITRA & DITRA-XL warranty (see page 35).
Joist Spacing Stone ASTM C627 Test Results Rating TCNA Test Report Number
19.2" o.c. White Carrara 12 cycles Heavy TCNA-121-07 (A)
24" o.c. White Carrara 7 cycles Light TCNA-121-07 (B)
24" o.c. Crema Marfil 5 cycles Residential TCNA-441-07
24" o.c. Travertine 5 cycles Residential TCNA-441-07