As we begin this discussion on crack isolation -- which is sometimes referred to as a “Crack I” product -- we first need to define what these products are and what they are not. Also, we need to understand that the sometimes popular term, “anti-fracture” membrane, is actually not accurate, nor is it recognized in the ANSI Specifications.

Crack isolation membranes categories 
Sheet Applied Membranes
Liquid Applied Membranes
Trowel Applied Membranes

The Tile Council of North America (TCNA) in its “Membrane Selection Guide” provides the following definition. “Crack Isolation membranes (ANSI A118.12) for thin-bed ceramic, glass and stone installations act to isolate the tile from minor in-plane substrate cracking. Membranes covered by this definition are bonded to a variety of manufacturer-approved substrates covered by ANSI specifications. In some cases, the trowel-applied products can be used as the adhesive for the tile. Other products within the scope of this category are allowed to cure or are applied as sheet goods and are then used as the substrate for the tile. Membranes may be sensitive to naturally occurring moisture and alkalinity when used over cement and gypsum-based substrates. Consult manufacturer for acceptable limits of moisture and alkalinity.”

Now that we have defined what a crack isolation membrane is, we need to figure out what it is not. Several areas of the U.S. have developed their own “brand” -- or substitute membrane -- to keep the tile from cracking. The reason these folks use these homemade remedies is based strictly on the price. They find it cheaper to substitute the “maybe this will work” product as opposed to an industry tested and proven product from legitimate manufacturers that offer detailed installation instructions, limitations and warranties. 

The use of duct tape over the crack is about as useful as a Band-Aid on an earthquake fault line.  The installation of roofing felt, floorcovering scribing felt and even upside down sheet vinyl glued to the concrete are equally unreliable. The problem with the use of these materials is that they do not function properly as Crack I products, and this is only the beginning of the problem. The floorcovering adhesives used to bond these materials can contribute to mold and mildew growth when the adhesive emulsifies due to moisture vapor transmission through the concrete, and most times they do not provide an adequate bond to the substrate. The felt or sheet vinyl loosens from the concrete and becomes unbonded which leads to hollow sounding tile which is not permitted under ANSI Specifications. Beyond that, these unwise “product placers,” (you can’t really call them installers or professionals) have no manufacturer warranty or support. This is a lonely place to reside especially for the unsuspecting consumer.

The testing of a “standard performance” membrane under ANSI A118.12 requires that the membrane not allow the tile to crack when a gap is mechanically opened under the tile and membrane which measures at least 1/16 inch but before a gap of 1/8 inch.  A “high performance” membrane must be able to handle an opening under the tile of at least 1/8 inch before the tile fails.  Contact the manufacturer and/or the architect to determine which membrane is suitable for your job.

Excerpts from the ANSI Specifications, under the A118.12 Scope heading state several crucial pieces of information, including, “Cracking is limited to horizontal planar movement of the substrate.” This means that the crack can only move laterally, in an in-and-out fashion, much like an accordion or a shifting pattern -- as if you were to rub your hands together in a back and forth motion. Crack I products are not designed to withstand any type of structural or up-and-down movement. If a crack of this description is encountered, consult the architect, engineer or design professional for guidance.

The Scope continues, “It should be noted that while crack isolation membranes are intended to minimize the potential for crack propagation from the substrate through the finished tile or stone installation, they may not always be 100% effective in preventing all defects in the finished tile.” When using these products, be certain to follow the manufacturer’s recommendations to the letter, and if in doubt, consult them for their recommendations. 

The Scope goes on to state, “It is particularly important when dealing with a cracked substrate that expansion joints are properly located and filled with a suitable sealant or prefabricated expansion joint. Movement joints in the substrate shall be carried through the tile installation.”   Expansion joints as defined in the TCNA Handbook are, “(1) A separation provided between adjoining parts of a structure to allow movement where expansion is likely to exceed contraction; (2) a separation between pavement slabs on grade, filled with a compressible filler material; (3) an isolation joint intended to allow independent movement between adjoining parts.” The cardinal rule here is that expansion joints must be honored and always carried through the tile work. No manufacturer will recommend or warrant their products over a true expansion joint as detailed above.

Other joints found in concrete floors, contraction or control joints which are formed, sawed or tooled into the concrete to provide a weakened plane, may be able to be covered to preserve the tile pattern. However, the manufacturer’s recommendations must be followed explicitly in regards to the placement of soft joints adjacent to the groove in the floor.

The Handbook provides two methods for the proper installation of crack isolation membranes under F125 as shown in the accompanying drawings. The practice of crack chasing or following the crack across the floor with the membrane is shown in detail F125 Partial. You see that the installer is given two directions.  (1) “Consult manufacturer for membrane width.” Most manufacturers recommend that their membrane be three times the width of the tile. For example, if the tile is 12 x 12 inches, the membrane would be 36 inches wide.  (2)”One or two soft joints may be required (consult the manufacturer).” Here the choice is left to the membrane provider as to how many soft or sealant joints must follow the crack to allow for movement.

Crack isolation membranes generally fall under one of three categories; sheet, liquid and trowel applied.

Sheet Applied Membranes

Sheet applied membranes usually come in a roll of varying widths and lengths and have the capacity to elongate while staying attached to the substrate, thereby mitigating the cracks. Most products can be installed over concrete surfaces that are clean, dry and free of coatings, sealers or curing compounds with either a latex-Portland cement thin-bed mortar or the manufacturer’s approved adhesive. Sheet membranes may also be installed over properly prepared plywood, primed gypsum underlayment and radiantly heated floors.

Liquid Applied Membranes

Liquid applied membranes are fluid products that are generally applied with a roller, notched trowel or brush to a properly prepared and approved surface. Most manufacturers require that the liquid be applied to a specified thickness which is monitored by using a wet film thickness gauge. Depending on the material requirements, a second coat may be necessary. The majority of these products change color when dry, indicating it is ready to receive tile. 

Trowel Applied Membranes

Trowel applied thin-bed mortars which provide crack protection are a one-step product in that they bond to the substrate and to the tile while allowing for substrate movement without telegraphing through the tile. These products, while much more expensive than a conventional thin-bed mortar, eliminate the necessity of applying a separate crack isolation membrane, thus saving time on the job.

As always, employ qualified installers who have the industry-recognized credentials that demonstrate the skills and knowledge necessary to perform the work involved in your project correctly the first time.