Installation Guide

22 THIN-SET FACTS
QUESTION
Can ceramic tile, including porcelain tile, be set on DITRA
with unmodified thin-set mortar?
ANSWER
YES. In fact, we recommend it. Here’s why:
Portland cement-based unmodified thin-set mortars are dependent on the presence
of moisture for hydration in order to gain strength. Since DITRA is impervious,
it does not deprive the mortar of its moisture. This allows the cement to properly hydrate,
resulting in a strong, dense bond coat. In fact, after the mortar has reached final set
(usually within 24 hours), unmodified thin-set mortars achieve higher strengths when
cured in continually moist conditions.
QUESTION
Can ceramic tile, including porcelain tile, be set on
DITRA with latex-modified thin-set mortar?
ANSWER
We DON’T recommend it. Here’s why:
Latex-modified mortars must air dry for the polymers to coalesce and form a hard film
in order to gain strength. When sandwiched between two impervious materials such as
DITRA and ceramic tile, including porcelain tile, drying takes place very slowly through
the open joints in the tile covering. [According to the TCNA Handbook for Ceramic,
Glass, and Stone Tile Installation, this drying period can fluctuate from 14 days to over
60 days, depending on the geographic location, the climatic conditions, and whether
the installation is interior or exterior]. Therefore, extended cure times would be required
before grouting if using modified thin-set mortars between DITRA and ceramic tile,
including porcelain tile. If extended cure times were not observed, the results could be
unpredictable. This is even more important to consider in exterior applications that are
exposed to rain as there is the additional concern of latex leaching.
ADDITIONAL NOTES
Over 25 years of field experience and testing by the Tile Council of North America
(TCNA) support the efficacy of using unmodified thin-set mortars to bond ceramic tile,
including porcelain tile, to DITRA in both interior and exterior applications. See relevant
testing data on page 23.
Remember, the type of mortar used to apply DITRA depends on the type of substrate.
The mortar must bond to the substrate and mechanically anchor the fleece on the
underside of the DITRA. For example, bonding DITRA to wood requires latex-modified
thin-set mortar. When bonding DITRA to particularly dry, porous concrete with
unmodified thin-set mortar, the slab should be moistened to saturate the concrete and
help prevent premature drying of the mortar. Excess or standing surface water must be
removed prior to installation. Additionally, all mortars (modified and unmodified) have an
acceptable temperature range that must be observed during application and curing.
In general, pre-mixed thin-set mortars and mastics are not suitable for use in conjunction
with DITRA and DITRA-XL.
Discussion of thin-set mortars and Schluter
®
-DITRA installations
THIN-SET FACTS