Frequently Asked Questions
On this page we have compiled answers to some of the questions our Customer Service and Technical teams commonly get asked. Make use of your browsers search function (Ctrl + F) to quickly locate specific terms. If your question is not answered here, our contact information is located in the column to the right, do not hesitate to call or write us for more information.
Section by Product:
The Mortar Questions
*As the most common of all our frequently asked questions, the issue of mortar has been given a high-priority section of its own. For more product-specific queries, please find the relevant section further down the page.
- Q. What type of thin-set mortar should I use when installing ceramic and stone tile over Schluter-KERDI?
A. Unmodified thin-set mortar is recommended to set ceramic and stone tile over the KERDI membrane.
Portland cement-based unmodified thin-set mortars are dependent on the presence of moisture for hydration in order to gain strength. Since Schluter-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.
Exceptions: Certain moisture-sensitive stones, e.g., green marble, or resin-backed tiles may not be appropriate for use in wet areas or may require special setting materials. Please consult the stone supplier and Schluter-Systems for more information. For an acid-resistant covering, use an epoxy adhesive to set and grout the tile. However, do not use epoxy to install the Schluter-KERDI membrane.
Modified thin-set mortar is not recommended to set ceramic and stone tile over the KERDI membrane.
Modified thin-set 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 Schluter-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.
- Q. How do I know if the thin-set mortar is modified or unmodified?
A. 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 appropriate 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 appropriate product standards for modified thin-set mortars include ANSI A118.4 and ANSI A118.11.
- Q. What type of thin-set mortar should I use when installing Schluter-DITRA?
A. The type of thin-set mortar used to install Schluter-DITRA depends on the substrate material. For example, to set DITRA over plywood or OSB, a modified thin-set mortar meeting the requirements of ANSI A118.11 is used. To set DITRA over concrete or gypsum, Schluter-Systems recommends an unmodified thin-set mortar meeting the requirements of ANSI A118.1. Please consult the Schluter-DITRA Installation Handbook before beginning your tile project to confirm the proper materials selection.
- Q. What type of thin-set mortar should I use when setting tile over Schluter-DITRA?
A. Schluter-Systems requires the use of an unmodified thin-set mortar meeting the requirements of ANSI A118.1 to install ceramic (including porcelain) and stone tile over Schluter-DITRA*. We don’t recommend the use of modified thin-set mortars to set tile over DITRA because these mortars must air dry to cure properly. When sandwiched between two impervious layers such as DITRA and porcelain tile, drying takes place very slowly through the open grout joints. [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.
*Exception: Certain moisture-sensitive stones, e.g., green marble, or resin-backed tiles may require special setting materials. Please consult your stone supplier and Schluter-Systems for more information. For an acid-resistant covering, use an epoxy adhesive to set and grout the tile.
- Q. Can I use a premixed thin-set mortar to install Schluter-DITRA or KERDI and tile?
A. No. Schluter-Systems does not recommend the use of premixed thin-set mortars (mastics/adhesives) in any Schluter-DITRA or KERDI installation.
- Q. Why should I use the Schluter-Shower System?
A. The Schluter-Shower System is an integrated family of products that together form a fully bonded, watertight assembly for tiled showers. Unlike shower pan liners in conventional assemblies, bonded waterproof membranes, such as Schluter-KERDI, bond directly to the mortar bed and protect it from becoming saturated with regular use. In addition, KERDI protects wall cavities from vapor penetration. The Schluter-KERDI-DRAIN provides a simple and secure connection to the KERDI membrane at the top of the assembly. Other components include the Schluter-KERDI-FIX sealant, used to treat protrusions through the membrane, and the Schluter-KERDI-SHOWER- ST/-SC/-SR/-SB prefabricated EPS substrates, which can reduce weight and installation time by replacing the mortar bed. The integrated Schluter-Shower System eliminates leaks, reduces the potential for efflorescence and mold growth in the system, and dramatically reduces total installation time to ensure success and make shower installation easier than ever.
- Q. My shower isn’t the same size as the Schluter-KERDI-SHOWER-ST tray. Can I modify the tray to fit my shower?
A. Yes. The Schluter-KERDI-SHOWER-ST tray can be cut to size to fit various applications. Ideally, the tray should be cut by equal amounts on all four sides to ensure a consistent height of the first course of wall tile. However, slight differences can typically be made up with thin-set mortar when setting the floor tile. The shower base can also be extended past the perimeter of the shower tray using dry pack mortar.
Note: The Schluter-KERDI-SHOWER-SC curb can be cut to size to fit various applications. In addition, multiple curbs (or cut sections of curb) can be placed consecutively to form longer runs, including angled designs.
- Q. What is a water test? How do I perform one on the Schluter-Shower System?
A. A water test is a quality control check performed on any shower installation before setting tile. It basically consists of stopping the waste line with a test plug and filling the base of the shower stall with water to check for leaks. If you are building a barrier-free shower, then you will need to provide a temporary dam (e.g., a 2x4 and silicone sealant) at the threshold. You can quickly check the test plug by filling the pipe and drain with water up to the height of the Schluter-KERDI-DRAIN integrated bonding flange (or a little below). If the plug is watertight, then you can continue filling the assembly. The water level is marked and the assembly is left for a specified amount of time (24 hours is typical). If there are no leaks, the shower passes.
Notes: We recommend waiting a minimum of 24 hours after setting Schluter-KERDI to begin water testing. This will allow the thin-set mortar to reach final set and ensure watertight performance at the seams under normal curing conditions (i.e., room temperature). If temperatures are much lower (e.g., around 50 or 60 degrees), then it is best to wait longer.
- Q. What are the Schluter-KERDI-SHOWER-ST/-SC/-SR/-SB tray, curb, ramp, and bench made of? Can they really support the tile assembly?
A. The Schluter-KERDI-SHOWER-ST/-SC/-SR/-SB tray, curb, ramp, and are made of expanded polystyrene (EPS) foam with a high compressive strength (57 psi). They provide a convenient, lightweight, and stable substrate for the Schluter-KERDI waterproofing membrane and tile. After the tile is installed and grouted, the assembly will support all the loads typical of tiled showers. This can include glass doors or glass block walls.
- Q. Can I install Schluter-KERDI over drywall in my tiled shower?
A. Yes. Once you install Schluter-KERDI over the drywall panels, they will be completely protected from the water and vapor in your tiled shower.
- Q. I’m installing Schluter-KERDI over drywall in my shower. Do I need to tape the joints between panels before installing the KERDI?
A. No. It is not necessary to tape the joints between drywall panels before installing Schluter-KERDI. If you must tape the joints per fire code requirements, make sure to apply a latex primer over the taped joints before installing KERDI. Joint compound can be water soluble and should be protected from the moisture in the thin-set mortar used to install KERDI.
- Q. Can I install glass tile over Schluter-KERDI?
A. Glass tiles are unique products and may require special consideration. Please call Schluter-Systems at (800) 472-4588 to discuss your particular application before installing glass tile over Schluter-KERDI.
- Q. What size trowel should I use to install Schluter-KERDI?
A. A 1/4" x 3/16" V-notched trowel or the Schluter-KERDI trowel, which features a 1/8" x 1/8" square-notched design, is recommended. Mix your thin-set mortar to a fairly fluid consistency (but still able to hold a notch) and solidly embed the Schluter-KERDI into the fresh mortar with the flat side of the trowel (a drywall knife is handy for this purpose as well). It is good practice to periodically lift a corner of the KERDI after embedding it in the thin-set mortar to check the fleece for full coverage.
- Q. Once I have installed the Schluter-KERDI membrane, how long must I wait before setting the tiles?
A. In general, tiles may be installed over the KERDI membrane immediately.
Exception: A water test may be required for some applications (e.g., showers) according to regional building codes. Prior to setting tile, wait 24 hours minimum before water testing to allow for final set of thin-set mortar and ensure waterproof performance at seams and connections.
- Q. I accidentally poked a hole through the Schluter-KERDI while installing the membrane. Do I need to start over or can I simply repair the hole?
A. Holes can typically be repaired by installing a “patch” of Schluter-KERDI or Schluter-KERDI-BAND with unmodified thin-set mortar, making sure to provide a 2" overlap in all directions. Small penetrations may also be repaired using the Schluter-KERDI-FIX sealant.
- Q. I need to remove and reinstall a tile on Schluter-KERDI. How can I do this safely?
A. If you are in the process of installing the tile and the thin-set mortar is still fresh, simply run your margin trowel under the back of the tile to break the suction created by the thin-set mortar and gently remove the tile.
If you have already installed the tile and the thin-set mortar has set, you will need to take a different approach. Remove the grout from around the tile to be replaced and break the tile into smaller pieces with a mallet before removing. Carefully removing the tile in this manner will minimize the risks of damaging the KERDI membrane, but such damage may be unavoidable in some cases. Cover and/or replace the damaged membrane with a new section of Schluter-KERDI installed with unmodified thin-set mortar, making sure to create a 2" (50 mm) overlap on all sides.
- Q. Why should I use Schluter-DITRA in my tile installation?
A. Ceramic and stone tiles are durable, easy to maintain, and hygienic, representing the ideal surface coverings. However, today’s lightweight construction methods can make the installation of hard surface coverings particularly challenging. In order to protect the integrity of the tile assembly, an underlayment that performs multiple functions is required. Schluter-DITRA is specifically designed to allow the installation of ceramic and stone tile over any even and load-bearing substrate. The integration of DITRA’s uncoupling, waterproofing, load-distribution, and vapor management functions makes consistent results viable.
Tile has been successfully installed for thousands of years by incorporating an uncoupling layer, or forgiving shear interface, within the tile assembly. Schluter-DITRA provides uncoupling through its open rib structure, which allows for in-plane movement that effectively neutralizes the differential movement stresses between the substrate and the tile, thus eliminating the major cause of cracking and delaminating of the tiled surface.
Schluter-DITRA provides reliable waterproofing in interior and exterior applications. Its polyethylene composition protects the substrate from moisture penetration, which is particularly important in today’s building environment where most substrates are moisture sensitive.
The distinguishing feature of Schluter-DITRA is the existence of free space created by the configured channels on the underside of the matting. The free space provides an escape route for excess moisture and vapor that could otherwise cause damage to the tile layer above. Thus, DITRA effectively manages moisture beneath the tile covering.
When placed on a solid foundation, columns or pillars can support tremendous loads. The same physical principle applies to Schluter-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 DITRA is virtually incompressible within the tile assembly, the advantages of uncoupling are achieved without sacrificing point load distribution capabilities. The ability of DITRA installations to support and distribute heavy loads while preserving the integrity of the tiled surface has been verified through extensive laboratory and field testing, including applications exposed to vehicular traffic.
- Q. I’m planning to install my tile using Schluter-DITRA. Where do I begin?
A. The first thing to do is plan out the installation according to the Schluter-DITRA Installation Handbook. The Handbook is available on our website for download or you can call Customer Support to request a print copy. The comprehensive Handbook features all the warranted Schluter-DITRA details for ceramic and stone tile installation in both interior and exterior applications over various substrates, such as plywood, OSB, concrete, poured gypsum underlayments, and heated floors. Each application features a detailed illustration and provides direction on substrate preparation, appropriate setting and grouting materials, movement joints, etc.
- Q. What size trowel should I use to install Schluter-DITRA?
A. A 1/4" x 3/16" V-notched trowel, 5/16" x 5/16" V-notched trowel, or the Schluter-DITRA trowel, which features a 4.5 mm x 4.5 mm square-notched design is recommended. Mix your thin-set mortar to a fairly fluid consistency (but still able to hold a notch) and solidly embed the Schluter-DITRA into the fresh mortar with a float or screed trowel. It is good practice to periodically lift a corner of the DITRA after embedding it in the thin-set mortar to check the fleece for full coverage.
- Q. Do I need to provide movement joints when I use Schluter-DITRA in my tile installation?
A. Yes. Schluter-DITRA does not eliminate the need for movement joints. Movement joints are essential in any tile installation to prevent the development of stress in the tile covering due to changes in temperature, moisture, and loading. These include perimeter joints at field edges and restraining surfaces, and surface joints within the tile field at regular intervals. Please refer to the Schluter-DITRA Installation Handbook for further discussion and guidelines for movement joint placement.
Note: Schluter-Systems’ prefabricated movement joint profiles protect tile edges and prevent sound bridges and surface water penetration, resulting in a permanent, maintenance-free installation. The family of Schluter-DILEX prefabricated movement profiles includes a variety of shapes, sizes, and materials to suit different applications.
- Q. Can I install glass tile over Schluter-DITRA?
A. Schluter-Systems does not recommend the use of glass tile over Schluter-DITRA.
- Q. I tried to snap chalk lines on Schluter-DITRA before setting my tile, but the chalk rubs off the plastic easily while I am working. What can I do?
A. One option is to apply hair spray over the chalk lines. You don’t have to worry about the hair spray acting as a bond breaker because the thin-set mortar applied over Schluter-DITRA is locked into the cut-back cavities on top of the membrane. Another option is to use an ink line instead of a chalk line.
- Q. I need to remove and reinstall a tile on Schluter-DITRA. How can I do this safely?
A. If you are in the process of setting the tile and the thin-set mortar is still fresh, simply run your margin trowel under the back of the tile to break the suction created by the thin-set mortar and gently remove the tile. If you have already installed the tile and the thin-set mortar has set, you will need to take a different approach. Cut through the tile assembly down to the substrate around the tile to be replaced. Remove tile, DITRA, and thin-set mortar from the substrate, taking care not to disrupt adjacent tile work. Reinstall a new section of DITRA and tile using the appropriate setting materials per the Schluter-DITRA Installation Handbook.
- Q. Once my tile is installed, it will be higher than the adjacent carpet. What profile would you recommend in this application?
A. The Schluter-RENO-TK profile provides a sloped transition between the tile and carpet and features a channel below the sloped surface to hide and protect the edge of the carpet.
- Q. The tile I’ve chosen for my floor doesn’t have a matching cove base available. I’d rather not select another tile. Do you have any solutions?
A. Yes. Schluter-Systems offers various profiles that allow the user to produce a finished tile base with a cove. The first step is to select a cove-shaped profile that will provide the transition between the floor and the base. For example, Schluter-Systems offers the Schluter-DILEX-EHK in stainless steel, Schluter- DILEX-AHK in anodized aluminum, and Schluter-DILEX-HKW in PVC. Selection between these profiles will depend on the expected stresses (e.g., residential application vs. commercial kitchen) and desired appearance. The next step is to select a profile to finish the top of the tile base that may consist of your floor tiles in a different format (either cut or from the manufacturer). Suitable options include the Schluter-RONDEC for a rounded appearance or the Schluter-QUADEC for a square look. The final result will be a beautiful, durable, and maintenance-free tile base. Note: All of the profiles listed above feature matching inside and outside corners.
- Q. How do I know which profile size to use?
A. Select the profile height according to the tile thickness. If a larger profile is used, it may protrude above the tile layer and may create a trip hazard if it is installed in a traffic area. Typically, profiles should be flush or slightly lower than the tile after installation.