Crack isolation has been receiving increasing attention from both manufacturers of setting materials and consumers. Early in my career, I heard an industry icon say all tile installations should have a crack isolation membrane. Three decades later, I agree it’s not a bad idea. Barring an exceptional concrete floor placement, it’s safe to say that membranes are always a good idea on slab construction of any type. They may bring many benefits to wood structures as well.

Movement abilities are only one of the many performance tests required to pass the crack isolation standard. They must also pass wet shear bond and surface point loading, among others.

We have all heard the saying that the only guarantee with concrete is it will crack. That is true, but proper specification and finishing techniques can control the cracking to a large extent. Over the years I have learned much about concrete. It’s incorrect to say that it will always crack at inopportune places with no ability to control it. However, the chances of getting that perfect slab are about as slim as getting that perfect tile installation.

Movement is expected with wood structures. No amount of engineering or skill can overcome the fact that wood structures move. There are a lot of good, hard facts that support the use of membranes in most installations. They can be very reasonably-priced insurance policies to provide aid in case of movement in the substrate -- virtually assured whether from wood deflecting or concrete’s lifelong tendency to curl.

Knowing what product to use
Many premium thin-set products on the market today offer exceptional movement characteristics and make it possible to reach some amount of compromise when substrate movement is a consideration. There are other situations where a sheet-applied product may be more suitable. Only the trial and error of experience can determine which choice is better for most tile professionals.

Movement abilities are only one of the many performance tests required to pass the crack isolation standard. They must also pass wet shear bond and surface point loading, among others.

One of the most important considerations -- when any type of membrane is desired for either protection or underlayment properties -- is that it is designed for and performs as part of a tile installation system. There is now a standard for this product category ANSI A118.12 - 2005 or, the American National Standard Specifications for Crack Isolation Membranes for Thin-Set Ceramic Tile and Dimensional Stone.

Until that standard was passed, it was very hard to determine whether a product performed as advertised or in a system configuration other than through manufacturer representations or trial and error. Quite often this occurred at the expense of the contractor. This new standard specification is not a perfect utopian standard by any means. In my opinion it leaves much to be desired and could use some improvement. But the standard does keep products that have no prayer of performing in a tile installation at bay by setting some minimal criteria for performance expectations in a product.

Evaluating the substrate
Occasionally it is left up to the retailer or installation contractor to decide on including a membrane with a tile installation. If the consumer is made aware of the advantages in using a membrane system with their installation the response is often positive.

Diagnosing the substrate and selecting the proper membrane system requires good familiarity with not only the type of substrate but the performance requirements of the installation as well. The types of membrane systems available are as numerous as the manufacturers who make them. They generally share some of the same installation strategies but vary in performance.

Some crack isolation membranes have the option of being applied to the entire floor surface or as a “bandage” for spot applications. An experienced contractor must determine the nature of the crack and if it’s active and moving or dormant and stable. Other questions include:

• Many manufacturers recommend an adhesive application when using their product. When adhesive is used the moisture and pH reading of the slab should be taken prior to installation. Membranes may stop water from going in, but not from coming out, as was the case in this instance.

  Is it caused by a structural failure?
• Is there a problem with the structure substrate materials?
• Can the crack(s) be expected to continue advancing into the slab?
• If the installation is over wood construction, will the structure provide adequate support or will excessive spans and lack of panel thickness overwhelm the abilities of the membrane?

When membranes are used as underlayments, complete support is absolutely critical to performance. If the substrate is questionable because of a structural problem, it is highly unlikely that any crack isolation system could protect tiles installed over such a surface.

Movement accommodation
Another consideration of paramount importance when using any crack isolation membrane is the provision for movement accommodation. Why bother to use a membrane system at all if no movement accommodation provision is made for the membrane to do the job it was designed for: to allow movement? Membranes do not eliminate movement joints; they are designed to work with them. In the application of crack isolation systems, most allow only relocation of control joints, not elimination.

Manufacturers provide very specific instructions on how grout joints over cracks are to be treated. In the case of a true expansion joint -- the joining of two structures such as in a room or building addition -- none allow for expansion joint relocation. If the application also requires waterproofing, this consideration should play an important part in the selection process.

Many contractors are now following Davy Stephens’s preference to use a membrane in each and every installation.

Use of any membrane does not change the requirement of all manufacturers to provide for movement in the tile installation as outlined in the TCA Handbook under EJ 171 or the American National Standards. Providing protection against substrate movement is not possible unless the substrate has some place to move. Following the recommendations of the manufacturer and adherence to industry guidelines will result in a profitable installation for you and achieve the performance your customer expects.

David M. Gobis, a third-generation tile setter, is the technical director for the Ceramic Tile Education Foundation. He has been in the trade for over 35 years and owned a successful contracting business for many years prior to his current position. Mr. Gobis is an author of numerous trade related articles and a frequent speaker at industry events. He is member of the Construction Specification Institute, International Code Council, American Concrete Institute, National Tile Contractors Association Technical Committee, voting member of The American National Standards for Ceramic Tile Installation and Setting Materials (ANSI A108/118), American Society for Testing of Materials (ASTM) C-21 Ceramic Whitewares, and Tile Council of America Installation Handbook committees.