Many tile customers harbor fears of cracks appearing in their newly installed tile floors. If you happen to sell crack isolation membranes, then you probably assure them that the tile will crack unless they avail themselves to the latest in crack suppression technology. However, if you’re a tile contractor, your answer to the possibility of cracks appearing maybe yes, and maybe no. The contractor’s view, while elusive, is more accurate.

Most cracks can be prevented using quality installation materials and techniques. But occasionally, you’ll run into a situation where there are existing cracks in the concrete substrate, control joints that do not happen to fall on a grout joint during the layout, or control joints that need relocation due to the pattern or size of tile. Some membranes in this category also offer additional benefits, such as waterproofing or sound reduction abilities. When the added benefits of crack isolation membranes are properly presented to the customer, many but the most budget-conscious will opt for the additional protection, even at an increased cost, much like the popular electronics warranties we so readily purchase.

Crack isolation membranes are designed to be used over minor cracks with NO vertical displacement. If one side is higher than the other, then the substrate needs more remediation than a crack isolation membrane can offer.

Modern day crack isolation products can be beneficial but their abilities are often overstated and their limitations ignored. They don’t make ceramic tile that stretches, so a membrane with a ½” of movement isn’t really going to do much for the customer in reality. I somehow think they would object to having numerous cracks in their grout joints if there was that much movement.

There is an industry standard for this category of products -- The American National Standard Specification for Crack Isolation Membranes for Thin-set Ceramic Tile and Dimensional Stone Installation, ANSI A118.12. Although this is not an ideal standard in my opinion, it does establish a basic performance expectation criterion by eliminating many products commonly sold and used as crack isolation membranes that provide no benefit to the tile installation. The testing to pass this standard is very specific and confined to a specific group of tile products.

For instance, under the System Crack Resistance Test, 4”x8”x1/2” quarry tile is used to demonstrate a membrane’s crack bridging ability. Quarry tile is pretty tough stuff - tougher than many porcelain tiles of lesser thickness. These membranes are often used under not only porcelain tile but many stone flooring products, which have substantially less movement tolerance than tile and certainly require a much greater level of structural support than quarry tile. To acknowledge the limitations of crack isolation products, the introduction of the ANSI A118.12 standard also states:

“This standard was developed to provide specifiers and installers with the minimum criteria necessary for a material to function as a deterrent to crack propagation from the substrate through the finished thin-set tile or stone installation. Additional tests, which are not a requirement of this specification, may be run when requested for a particular project, using the exact materials for that project.”

In other words, before you make a commitment to using a specific crack isolation product in an application of any appreciable size or value, you may want to ask your favorite manufacturer if the membrane you are considering is rated for both the tile product you are using and the in-service application or duty level. Without exception, manufacturers are happy to help you make the proper selection before the installation. However, post installation, the information offered tends to be rather vague and full of “ifs”. Experience has taught me that it’s prudent in preserving your financial well being to ask questions before, not after the installation. It can be both dangerous and costly to make assumptions.

When membranes are used as underlayments, the supporting floor structure must be adequate. A membrane system does not make any structural contribution to the floor. Appropriate joist spacing, subfloor panel thickness, and fasteners are very critical in this application.

There are a few other big do’s and don’ts with any system that often don’t get the consideration they need before addressing crack suppression that should be considered in the installation process.

Many membrane systems are very sensitive to moisture vapor emission (MVE) that naturally occurs in all concrete slab work. Alkalinity is also a natural part of the concrete product. MVE tolerances prior to application of a membrane vary from 3-lbs. per 1,000 square feet, to free of standing water, depending on product. An alkalinity reading above 9 on the pH scale typically proves caustic to most adhesives.

Again, recommendations vary widely dependent on product and bonding material. A product used successfully in 100 above-grade condos could fail miserably when the same product is used in a basement laundry or slab on grade recreation area. Anytime a product is to be used on grade there should be some concern about suitability for use relative to moisture vapor. When used subgrade, such as a basement or in areas of the country where high water tables exist, the concern is much greater.

Often we hear moisture vapor referred to as hydrostatic pressure. While vapor drive can be strong, hydrostatic pressure can only exist when the installation is below the water table. Ways to measure MVE vary. Plastic sheets, gauges and calcium chloride tests only provide a picture of moisture conditions at the very top of the slab, perhaps 1/2” or less. This is not always an accurate measurement of slab moisture as typically there are an additional 3-1/2” or more of concrete that can also harbor moisture. The most accurate means of measuring slab moisture is an RH test. This is a test where a hole is drilled and probe inserted. Tile people are not accustomed to being concerned about moisture vapor as it has little, if any, effect on the mechanics of the bonding process when using thinset mortars. With membranes, it can become an issue you do not want to encounter.

The system crack resistance test requires only that the quarry tile not crack when the surface is opened 1/16” for liquid systems (1/8” for sheets). Cracking of the grout joints does not constitute failure. Thus, this membrane did pass the test requirements.

While there hasn’t been a study on the subject that I am aware of, we also see a correlation between the size of tile and subsurface moisture condition issues. The bigger the tile, the lesser the floor is able to breathe.

Crack isolation membranes also vary widely in breathing abilities. Some are quite permeable (the ability to pass vapor) while others are impervious. Use common sense not only when you’re installing over concrete, but when using membranes in wood structures over unacclimated basements and crawl spaces common in the southern areas of the country. Homes on fast track construction often have the floors installed within a few weeks of the concrete basement pour. Hundreds of gallons of water must evaporate from freshly poured basements. This moisture moves not only through the three-foot doorway at the top of the stairs, but through the rest of the wood structure as well.

Moisture issues, like swollen edges of OSB panels are quite common and can be magnified when covered with a membrane. Trapping moisture in a wood structure is never a good idea. Some crack isolation membranes advertise as being suitable in lieu of an underlayment thus providing crack suppression while saving the additional cost of a traditional underlayment. Many of these products perform quite well; however, they are very specific on what is required of the supporting wood structure. When using a membrane as an underlayment, there is no contributing value in strengthening the floor system. Rightly or wrongly, many backerboards, while not designed as structural components, can offer limited additional rigidity to the floor support system. Thinset, PVC sheeting, Butyl rubber, and acrylic coatings offer no rigidity whatsoever. If you’re considering a membrane in lieu of an additional layer of underlayment, then make sure the floor meets the recommendations for joist spacing, panel fastening, and thickness. When in doubt, do not be an optimist.

Once we select the proper product, we have still more to think about if the crack isolation membrane is to perform its task. The design of membrane systems is to store and/or dissipate the movement associated with minor inplane cracks. If a substrate moves and/or cracks when covered with a membrane, there will be movement in the membrane.

Each liquid system has a specific film thickness requirement. Most employ a polyester scrim for both reinforcement and assurance of proper thickness. Most also require pretreatment of cracks.

For the membrane to accomplish its task of dissipating that energy, there must be movement accommodation joints. All too often we are called to a job complaint about membrane product performance and find no soft joints in the entire floor installation. It’s particularly important when dealing with a cracked substrate that expansion joints are properly located and filled with a suitable sealant, or prefabricated expansion joint. No movement joints, no expansion abilities - it’s that simple.

Nobody likes a stripe of caulk or a profile in the floor. It was a battle for me on almost every job I did. Many, if not most times, I lost the battle. But, if the floor did come loose or crack at a later date, I did get the opportunity to say, “I told you so.”

Having the same place of business and employees for many years we did get that call on a somewhat regular basis. On the other side of the coin, my business realtor reminds me every time I see him, after my repeated warnings, and his election not to have movement joints installed in his large formal entry, that it still has not cracked. If I were to guess, my estimation about 80% of the time when we did not install the appropriate movement joints we managed to get away with it. But, that also means 20% of the time we did not. In dollars and cents that is a lot of money. However when not allowed to install the appropriate movement joints we typically warned of the impending problem in writing by some means. Consequently, there were very few occasions when we had to go back and make tile repairs free of charge. Let the owner make the decision movement joints and document it if you are forced to grout instead of caulk.

Luckily, we have nearly as many products to choose from as application considerations we must make. So answering the question “what is the best crack isolation membrane” comes with many caveats. Some allow for relocation of control joints some don’t. Others may offer waterproofing or sound reduction in addition to their crack bridging abilities. With an ever-increasing range of products, installation materials and techniques are changing as well. Remember the old adage, if it sounds too good to be true, it is. That is something that still hasn’t changed. Chose your crack isolation system wisely.