Installation Guide

GYPSUM — Floors, Interior - Ceramic or Stone Tile 9
Every substrate presents unique challenges
GYPSUM
Bonding ceramic or stone tiles directly to gypsum concrete substrates is generally considered questionable or not recommended. The
challenges associated with gypsum-based underlayments include the requirement of an extended drying period before installing tile and
continued sensitivity to the reintroduction of moisture throughout the life of the installation. In addition, since the coefficient of thermal
expansion of gypsum concrete is substantially greater than that of ceramic tile, shear stresses caused by temperature fluctuations can
result in delamination or cracking of the tile covering. This is particularly important when gypsum concrete is used as a thermal mass for
radiant heated floors. With the increasing popularity of radiant heated floors, which typically utilize gypsum concrete, tile installers need a
reliable installation system to address these issues.
Schluter
®
-DITRA’s uncoupling function protects the ceramic or stone tile covering by neutralizing the differential
movement stresses between the gypsum concrete substrate and the tile, thus eliminating the major cause of cracking
and delaminating of the tiled surface.
DITRA’s waterproofing function prevents the reintroduction of moisture to gypsum concrete underlayments, which,
if not prevented, could significantly compromise performance of the underlayment and lead to damage of the
tiled surface.
The residual moisture in gypsum concrete is allowed to escape through the air channels on the underside of the DITRA
matting. This is particularly important since gypsum concrete must dry in order to gain strength.
Since DITRA is virtually incompressible within the tile assembly, the advantages of uncoupling are achieved without
sacrificing point load distribution capabilities.
By addressing all of the challenges associated with today’s fast, lightweight construction methods, DITRA provides a durable installation
system for ceramic and stone tile over gypsum substrates.
Floors, Interior - Ceramic or Stone Tile
D-G-TS-17
Ceramic, porcelain or stone tile
Unmodified thin-set mortar
DITRA or DITRA-XL
uncoupling membrane
Unmodified thin-set mortar
Gypsum concrete
Gypsum concrete
Areas of Application
over gypsum concrete underlayment placed
over structurally sound wood or concrete
subfloors
interior dry or wet areas
Limitations
minimum 2" x 2" (50 mm x 50 mm) tile
Requirements
for wood substrates, subfloor/underlayment
configuration according to detail D-W16-T,
D-W19-T, D-W24-T, or D-W24-XL-T;
D-W24-XL-T for use with DITRA-XL only
where radiant heat tubes are laid over the
subfloor, gypsum poured to a height that is
3/4" (19 mm) above the tops of the tubes is
required before installing DITRA and DITRA-XL
residual moisture in gypsum screed, 2.0%
(percentage by volume) or less before
installing DITRA and DITRA-XL
Substrate Preparation
gypsum – follow manufacturer’s directions
Movement Joints
DITRA and DITRA-XL do not eliminate the
need for movement joints, including perimeter
joints, within the tiled surface. Movement
joints must be installed in accordance with
industry standards and norms; see page 17
of this Handbook, TCNA EJ171, and TTMAC
301 MJ
Setting and Grouting Materials
unmodified thin-set mortar – ANSI A118.1
grout – ANSI A118.3, A118.6, A118.7,
A118.8
Installation Specifications
tile – ANSI A108.5
grout – ANSI A108.6, A108.9, A108.10
gypsum – follow manufacturer’s
recommendations
Other Considerations
since DITRA and DITRA-XL must bond to the
gypsum concrete, follow gypsum
manufacturer’s recommendations regarding
primers and/or special surface preparation
before installing DITRA and DITRA-XL
where a waterproof floor is required, all
DITRA and DITRA-XL seams and floor/wall
transitions must be sealed with Schluter
®
-
KERDI-BAND using unmodified thin-set
mortar; see page 12
certain moisture-sensitive stones, e.g., green
marble, or resin-backed tiles may require
special setting materials. Consult stone
supplier and Schluter
®
-Systems for more
information
vapor barrier on crawl space floors according
to regional building codes
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