Installation Guide
QUESTION: Should a latex-modified thin-set mortar or an
unmodified thin-set mortar be used to install Schluter
®
-KERDI?
ANSWER:
The type of bonding mortar used to apply KERDI must be suitable for the substrate
and mechanically anchor the fleece on the KERDI. For all the substrates listed in
this Handbook (e.g., gypsum board, cement backerboard, etc.), an unmodified
thin-set mortar is recommended. When installing KERDI with unmodified thin-set mortar
over particularly dry and porous substrates, it is recommended to wet the substrate first,
in order to help prevent premature drying or skinning of the thin-set mortar.
Unmodified thin-set mortar must be used to construct all KERDI seams to ensure
watertight performance of the system. Learn more below.
QUESTION: Can ceramic tile, including porcelain tile, be set on Schluter
®
-KERDI 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 KERDI 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 Schluter
®
-KERDI 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 KERDI and ceramic tile, including porcelain tile, drying takes place very slowly through the open
joints in the tile covering. [According to the Tile Council of North America's 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 KERDI 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 wet areas such as showers, as there is the additional concern of latex leaching.
QUESTION:
How do I know if the thin-set mortar is modified or unmodified?
ANSWER:
In general, unmodified thin-set mortar is a blend of Portland cement, sand, and water retention agents that is mixed with water by the user.
The applicable product standard for unmodified thin-set mortars is ANSI A118.1. Look for this standard on the packaging (e.g., product
meets ANSI A118.1 when mixed with water). Unmodified thin-set mortar may also be referred to as dry-set mortar by the manufacturer.
Modified thin-set mortars are similar to unmodified thin-set mortars, but include additional polymers such as latex. The additional polymers
are introduced in either liquid or powder form. In the first case, the user mixes an unmodified or dry-set mortar powder with a liquid polymer
additive instead of water. In the second case, the polymers are added by the manufacturer to the dry blend in powder form. The user then
mixes the powder with water. The applicable product standards for modified thin-set mortars include ANSI A118.4 and ANSI A118.11.
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 in conjunction with KERDI in tiled showers.
• Remember, all thin-set mortars have an acceptable temperature range that must be observed during application and curing.
• Pre-mixed thin-set mortars and mastics are not suitable for use in conjunction with the Schluter
®
-Shower System.
THIN-SET FACTS
Discussion on thin-set mortars and Schluter
®
-KERDI installations
THIN-SET FACTS — Discussion on thin-set mortars and Schluter
®
-KERDI installations 37