Help prevent cracked porcelain, ceramic or stone tile installations by using a crack isolation membrane that meets ANSI A118.12. Learn how to use liquid or sheet membranes with TCNA Detail F125 to treat cracks in the concrete slab and protect tile from ongoing substrate movement.

Today’s tile and stone assemblies need effective crack isolation!

Why? Structures are built faster, using lighter materials, which leads to increased substrate movement. Add large format tile, and it creates a perfect storm for breakage. All of these factors make crack isolation a critical part of modern tile assemblies.


As you know, the tile contractor is responsible for accepting that the substrate is suitable for tile installation.

Traditionally, a wire-reinforced, 1-1/2” thick mortar bed was used with a cleavage membrane to separate the tile from the substrate.

Now, the best way to prevent cracked tile is to include an isolation membrane. The membrane avoids the mass, weight and height of a mortar bed, while providing better protection to the tile.

Crack isolation membranes are designed to deform with movement, absorbing some of the force, which helps to prevent cracks from transferring to the tile and grout.

For the best results, install a product that meets or exceeds the requirements ANSI A118.12 for high-performance. These membranes isolate tile from horizontal, in-plane substrate movement up to 1/8".

When choosing a membrane, consider end use factors beyond the existing condition of the substrate. Will the tile assembly be exposed to thermal cycling, deflection or heavy traffic? Will the project need waterproofing or impact sound reduction? Many crack isolation membranes offer these additional features.


Liquid membranes provide the simplest, most versatile installation option. They’re faster to apply than sheet membranes, but cure time depends on environmental conditions.

These elastomeric membranes elongate to absorb horizontal movement from the substrate. Always follow the manufacturer’s instructions carefully to achieve the correct film thickness.

The components in sheet-applied membranes vary, but all are installed using primers, adhesives or mortar. Most can be tiled right away.

They do require extra labor for measuring, cutting and piecing together, so plan accordingly.

For tiling over young concrete less than 28 days old, an uncoupling mat membrane provides the best solution.
Air space is part of the design, which allows moisture to escape while absorbing movement stresses to keep the tile and grout intact.


The TCNA Handbook describes use of crack isolation membranes in Detail F125.

Partial coverage addresses existing cracks, while full coverage of the substrate also helps protect against future cracks.

Instead of placing a movement joint directly over an existing crack, F125 partial allows them to be relocated to the next tile joint.

This involves installing a section of membrane over the crack to avoid bonding to the concrete under these tiles. Each section should measure three times the width of the tile along the length of each crack.

On substrates with numerous cracks, or under large format tile, F125 full coverage is recommended, as it provides better protection during future cracking of the slab.

Unless full coverage is specified or approved by the owner, the installer’s responsibility is limited to partial coverage.

For either F125 application, pre-existing shrinkage cracks exceeding 1/8” should be patched prior to membrane installation.

The tile assembly may also require one or two additional soft joints parallel to the relocated cracks, depending on the manufacturer’s recommendation.

Always follow the membrane manufacturer’s directions carefully for the most effective protection.

Remember, crack isolation membranes cannot be used to treat construction, isolation or cold joints.

Any joints that are intended to move must be carried through the tile assembly.

Also, any differential crack may be structural and must be assessed by an engineer, as these cannot be relocated with a membrane.

Ultimately, a crack isolation system controls and directs movement to the soft joints in the tile assembly and this help the tile stay bonded.

Installing movement and perimeter joints per TCNA Detail EJ 171 is critical – even with membrane!

The use of ANSI A118.12 crack isolation membranes is an important investment. Membranes protect the tile assembly and significantly increase its life expectancy.

For more information on crack isolation, consult the references at the end of this video or contact the National Tile Contractors Association.