Schluter DITRA – Underlayment for Tile Installation

In this tile installation report, we provide comprehensive information on this popular underlayment material, so you can decide if Schluter-DITRA underlayment is right for your tile project. For more on buying, installing, pricing and caring for tile and stone floors click here.

WHAT IS SCHLUTER-DITRA MATTING?

Schluter-DITRA matting is used by professional tile installers to prevent the cracking of tile and grout caused by movement substrate, the foundational material beneath the tile. DITRA is also noted for its ease of use when compared with materials like cement backer board.

This underlayment also acts as a waterproof membrane over moisture-sensitive subflooring such as plywood and OSB. The result of Schluter-DITRA’s performance is a tile floor with outstanding durability and resistance to cracks and moisture.

The key to understanding why this membrane will greatly enhance the structural integrity and durability of your tile floor is to get a good handle on just what Schluter-DITRA is. By the way, contractors sometimes refer to this product simply as DITRA, and we will too.

According to Schluter, their DITRA matting is

“a polyethylene membrane with a grid structure of square cavities, each cut back in a dovetail configuration, and an anchoring fleece laminated to its underside. “

Now, let’s break down this definition into its parts:

• A polyethylene membrane: Polyethylene is the most common plastic available. The Schluter-DITRA matting is a flexible, waterproof form of it.
• With a grid structure of square cavities: These cavities are filled with thin-set mortar that produces a mechanical bond between the mortar and the tile.
• Each [cavity] cut back in a dovetail configuration: Picture a box in which the floor of the box has a length and width greater than the top, requiring that the sides be slanted outward. If you filled the box with mortar and let it cure, you couldn’t simply pull out the block of mortar because its base would be larger than the top. Each DITRA square cavity is shaped like such a box, so the cured thin-set mortar becomes anchored within the cavity, though it doesn’t “stick” to the plastic matting. Because it doesn’t adhere to the Ditra surface, the matting can move with expansion and contraction of the subfloor without transferring that movement to the tile and the grout and causing cracks.
• And an anchoring fleece laminated to its underside: The fleece backing adheres to a subfloor such as concrete or wood using thin-set mortar.

HOW DOES DITRA WORK TO PREVENT CRACKING?

Schluter-DITRA membrane reduces cracking by absorbing the stresses created when the substrate beneath it moves due to shifting, expanding or contracting.

When the substrate moves – and it will in every flooring installation—the DITRA material flexes with it to prevent the movement from transferring through and causing the tile and grout to crack. In short, the material neutralizes the movement like automobile shock absorbers.

With the flexible layer of DITRA installed, the tile isn’t adhered to the substrate. It is mortared to the DITRA underlayment, which in turn, is mortared to the subfloor. This is the key to preventing the transmission of the movement that is the main cause of cracking in tile floors. Interestingly, tile installers have been using sand for thousands of years as a non-rigid layer of material between the substrate and the tile to prevent cracking. Schluter-DITRA simply takes the functionality of a sand layer to the next level of performance by also providing a waterproof vapor barrier.

Note on Uncoupling: You’ll come across the term “uncoupling” when discussing Schluter-DITRA with contractors or retailers; you may hear it described as an uncoupling membrane. The term refers to the fact that the DITRA material prevents the tile mortar from sticking to the subfloor. It can be confusing because when we hear “uncoupling,” we think of something being coupled or connected first. In reality, the DITRA membrane prevents the coupling in the first place.

WHERE CAN DITRA BE INSTALLED?

Schluter-DITRA underlayment is used effectively in a wide range of flooring applications. Here’s an overview of the locations where you can take advantage of the performance DITRA offers.

Wood Substrate: OSB, plywood and framing members should be covered with a material such as DITRA underlayment because wood is very susceptible to changes and to moisture. Wood expands and contracts with moisture and temperatures changes and flexes when bearing loads. These movements will damage tile and grout without the presence of a flexible, motion-absorbing material. Wood will also be damaged by water, something this polyethylene membrane prevents. DITRA can be installed over these wood subfloor systems:

• 16” or 19.2” O.C. (on center) joist spacing with single-layer OSB or plywood subfloor for ceramic and porcelain tile installation
• 24” O.C. joist spacing with double-layer OSB or plywood subfloor for ceramic and porcelain tile installation
• 24” O.C. joist spacing with single-layer OSB or plywood subfloor for ceramic and porcelain tile installation
• Double-layer OSB or plywood for natural stone installation
• Existing vinyl over a suitable subfloor for ceramic and porcelain tile installation
• Structural plank subfloors for tile or stone installation

Concrete Substrate: Concrete is a challenging substrate to cover with hard flooring such as tile and natural stone. The major reason is that concrete can expand nearly twice as much as ceramic tile and grout.

Installing tile directly on concrete WILL result in significant cracking of the flooring within a short period of time when the concrete expands and contracts with changes in temperature.

Here are the specific types of concrete subfloor that should be covered with DITRA prior to the installation of tile or stone flooring:

• Standard concrete subfloor
• Gypsum concrete underlay over wood or concrete subfloor

Floors with Radiant Heat: DITRA is an excellent choice for use in any application where radiant heating is installed in the floor. Heat causes materials to expand, and they shrink again when they cool. That movement will cause tile cracks without a layer of motion-absorbing material such as DITRA. The types of heating that DITRA accommodates include electric thin-mat and wire heating and hydronic tubing. The subfloor may be:

• Wood substrate
• Thin slab lightweight concrete or gypsum
• Structural concrete slab

Floors Requiring Waterproof Installation: Gypsum concrete and wood substrates such as plywood and OSB will absorb moisture and expand. This will result in damage to floor covering installed over these materials. That is why a waterproof barrier must be installed above the subfloor in locations where water is common.

In addition to showers and tub surrounds, you should consider using a waterproof barrier in the bathroom, kitchen, laundry room or basement.

A waterproof barrier can be created using Schluter-DITRA because it is made from waterproof polyethylene. One additional product is required, Schluter KERDI-BAND, a waterproof material used to seal the seams where layers of DITRA abut one another. The KERDI-BAND is applied using unmodified thin-set mortar.

Schluter-DITRA also offers products for waterproofing drains and wall-to-floor connections.

EXTERIOR APPLICATIONS FOR DITRA

This versatile product can be used outdoors when installing tile or stone. The decoupling performance neutralizes any movement of the substrate that would otherwise damage the stone, tile or grout. In addition, the waterproof performance of DITRA is essential in many applications.

Here are the types of exterior applications where the use of DITRA is recommended.

• Concrete substrate
• Wood substrate
• Exterior concrete floors, patios and walkways

THE ALTERNATIVE TO DITRA: CEMENT BOARD UNDERLAYMENT 

Commonly called CBU or cement backer board (or more technically, cementitious backer units), cement board underlayment was originally designed as a substitute for drywall in showers because it is resistant to moisture and mold and won’t deteriorate when wet.

It was soon discovered that CBU also works well as underlayment for tile and natural stone. The board is glued to the substrate and fastened with screws when possible. The seams are taped and covered with thin-set. Tile is installed above the CBU using thin-set mortar.

DITRA VS. CBU: WHICH MATERIEL IS RIGHT FOR YOUR PROJECT?

Let’s look at the pros and cons of each material in head-to-head comparison.

• Standard DITRA is 1/8” thick, and this helps to eliminate height differences where two different flooring materials meet; CBU is twice as thick at 1/4″.
• DITRA is fully waterproof; CBU allows water to pass through it, but is not damaged by the water. DITRA is the right choice when installing flooring over concrete or other material that requires a moisture barrier to prevent damage to flooring.
• DITRA, as a decoupler, helps prevent substrate movement from damaging flooring; When using CBU, the tile is mortared directly to the board, and expansion or shifting of the subfloor is likely to crack or pop the tile or grout.
• DITRA offers faster, easier installation than cement backer board, especially when the project requires a large amount of trimming around obstacles such as support posts, drains, bathroom fixtures and door jambs.
• Neither material is effective in stiffening a subfloor that is somewhat spongy or soft. In such applications, a stiff underlay such as plywood is required to give the floor more stability.
• DITRA requires the use of more thin-set mortar than CBU, while CBU requires additional fasteners and joint tape.
• Schluter recommends the use of its specially-designed trowel for the application of mortar used to install DITRA, but it is not essential.
• The underside of the DITRA membrane contains free spaces between the cavities. These spaces allow excess moisture and vapor to escape, reducing the potential for it to damage the layer of tiles above. CBU structure does not offer this protection.

HOW TO INSTALL SCHLUTER DITRA (STEP BY STEP)

Here’s a step-by-step DITRA installation guide.

Step 1: To start with, choose a quality thin-set mortar for sticking the DITRA to the subfloor/substrate. Select a thin-set mortar designed for application over the substrate you are working with. Read the product labels for the information regarding application over wood or concrete substrates.

Step 2: Mix the thin-set fairly loose but firm enough to hold a notch. This means that when you use a notched trowel on it, the ridges created must stand up rather than settling. Allow the mortar to sit for 5-15 minutes before using it. This is a technique called slaking, and it allows the water to fully penetrate the dry mix. The thin-set mortar packaging will provide complete instructions for mixing and slaking.

Note: Add water to your mixing pail before adding the thin-set mortar. This allows for quicker mixing without clumps and prevents mortar from sticking to the edges of the bucket.

Step 3: Purchase a suitable trowel for the thin-set mortar between the substrate and the DITRA membrane. The DITRA trowel from Schluter is the best choice. It has an 11/64” x 11/64” square-notched design. Other acceptable trowels are 1/4″ x 3/16” V-notched or 5/16” x 5/16” V-notched models.

Step 4: Select DITRA or DITRA-XL underlayment depending on the job’s requirements. See “A Word About DITRA Thickness” below for more information on your options.

Step 5: Measure the space, and cut the first piece of DITRA to fit. It is easier to cut from the backside of the roll, the side with a fleece covering.

Step 6: Apply the thin-set to the substrate with your notched trowel at a 45-degree angle. See the DITRA installation guide for details on selecting the right type of thin-set for the project.

Step 7: Roll out the pre-cut DITRA over the mortar, fleece-side down, and press down the DITRA using a float or screed trowel to remove all air pockets.

Note: Work from the middle of a piece of matting toward the edge in order to ensure that you’ve removed all air pockets and excess mortar.

Step 8: Raise a corner of the DITRA to check to make sure you’re getting 100% contact between the thin-set and the fleece backing of the DITRA. Abut the second piece of DITRA against the first on sides and, if necessary, ends too. There should be NO overlap.

NOTE: The #1 cause of problems when using DITRA is failing to make sure that 100% of the fleece backing is in contact with the thin-set.

Step 9: Trim the DITRA around drains, support posts, door jambs and other obstacles while the mortar is still wet or pre-trim the piece when fitting it, before mortar is applied to the subfloor.

Step 10: Once the DITRA is down, you can begin setting tile immediately. Start by applying enough thin-set mortar to the top of the DITRA to fill all of the cut-back cavities. DITRA strongly suggests you use unmodified thin-set. See “A Word About Unmodified Thin-Set” below for an explanation.

Step 11: Comb additional thin-set mortar over the DITRA with a notched trowel and solidly embed the tile into the mortar. NOTE: Applying a thin-coat of mortar to the back of the tile with the non-notched edge, a technique known as back-buttering, is recommended when installing tiles 12”x12” or larger.

ADDITIONAL STEPS FOR WATERPROOF APPLICATION 

Once the DITRA membrane is in place and before any tile is set, the next step is to cover the seams with an 8” wide layer of unmodified thin-set centered on the seam. It must fill all of the cavities.

Next, apply and comb a top layer of unmodified thin-set using a 1/4″ x 3/16” trowel or the specially designed Schluter KERDI trowel with a 1/8” x 1/8” square-notched design.

Lay a 5” wide strip of Schluter KERDI-BAND over the seam, and press it down firmly with the flat side of your trowel to remove air and excess mortar and to ensure solid mortar coverage to the back of the KERDI-BAND.

To ensure a waterproof seam where the wall meets the floor, use a 10” wide strip of KERDI-BAND centered in the corner using the same method outlined for waterproofing seams.

Here’s a tools and materials list for installing DITRA matting:

• DITRA or DITRA-XL depending on the requirements of the job
• DITRA-Set mortar or similar thin-set mortar
• DITRA trowel or other suitable option (see Step 2 above) for applying mortar to the substrate
• A 1/4″ x 3/8” trowel for applying mortar to the top of the DITRA for tile installation
• Utility knife for cutting the DITRA underlayment
• Chalk line for laying out the DITRA (optional)
• Flat trowel
• Grout float
• 5-gallon mixing pail
• Mixing paddle
• Electric drill – A hammer drill with a handle works best

A WORD ABOUT UNMODIFIED THIN-SETA

It is essential to use unmodified thin-set over DITRA. Here’s why.

Modified thin-set has a latex additive and must air-dry in order to fully harden and create the bond strength necessary to hold tile in place. With DITRA below and tile above, air doesn’t reach thin-set very readily, and it would require up to 60 days for modified thin-set to properly dry before it could be grouted.

On the other hand, unmodified thin-set does not need air because it cures rather than dries. This makes it the right choice for use with DITRA.

In addition, when unmodified thin-set cures in the presence of air, it does not cure as strong because some of the water in it evaporates. DITRA matting and tile help the unmodified thin-set retain more of its moisture as it cures, and this creates a stronger bond than modified thin-set can achieve.

HOW THICK IS SCHLUTER DITRA?

DITRA membrane is available in two thicknesses, 1/8” and 5/16”. Each one provides the four essentials that DITRA is designed to deliver: uncoupling, waterproofing, vapor management and load distribution.

Which thickness you choose depends on the needs of the project you’re tackling.

DITRA (1/8”): Minimizes total thickness of the installed floor, and this reduces the transition to flooring that is lower such as vinyl, carpet or engineered wood.

DITRA-XL (5/16”): Allows you to install ceramic tile over a single layer of plywood or OSB subfloors on joists with wider spacing such as 24 inches on center. In addition, DITRA-XL and standard 5/16” tile creates a floor height that is level with standard 3/4″ hardwood flooring.

Ask your retail flooring specialist which thickness is right for you, if you have a question about it.

WHERE TO BUY  SCHLUTER-DITRA

This versatile underlayment is available from Lowes, The Home Depot and other home improvement stores. Most retail flooring stores carry standard DITRA and DITRA-XL. It’s available on eBay too.

SCHLUTER-DITRA PRICE

Schluter DITRA cost is $1.70 to $2.20 per square foot. The DITRA-XL cost is $2.60 per square foot.

By comparison, CBU backer board cost is $0.80 to $1.20 per square foot. Many professional tile setters believe that the higher cost of DITRA is offset by the fact that it takes less time to install than CBU does due to its ease of use. Less installation time produces lower estimates from contractors and less hassle for DIY installers.

Check out our guide to the many other types of underlayment.

About the Author: Rob Parsell

Rob joined the Home Flooring Pros team in 2014 and is a freelance writer, specializing in flooring, remodeling and HVAC systems (read more).

“I’m the son of an interior designer and picked up an eye for design as a result. I started hanging wallpaper and painting at 14 and learned enough on the job to be the general contractor on two homes we built for our family and did much of the finish plumbing, electrical, painting, and trim work myself.”