Crack isolation and waterproofing systems may be desired when ceramic tiles are specified over problem-prone substrates or waterproofing is desired or required. One of the most important considerations in their use is they are part of an installation system. Waterproofing systems are currently covered under ANSI A108.13, installation of load bearing, bonded, waterproof membranes for thinset ceramic tile and stone. Currently, there is no standard for crack suppression though one is soon to be introduced. In four years of teaching a generic membrane class using 10 different systems, I can assure you of one thing: no two products are alike relative to installation requirements. Though some products may appear to share similar application techniques, all they are is similar.
Trowel applied membrane systems are typically composed of a liquid and powder and possibly a reinforcing fabric. First, the setting bed is prepared as it would normally be for the direct application of ceramic tile. Most require pretreatment of cracks and/or corners. Application usually consists of keying the material into the substrate with the flat side of the trowel followed by combing the material in a singular direction and another pass with the flat side of the trowel to smooth the surface. There are a few key elements to success using these systems. The notches act as a gauging device in this application. There is a specific film thickness required for this type of system to perform. Typical failure occurs from either engaging the substrate with the trowel, leaving a void, or failure to reinforce the corners. These are the simplest of systems to use but probably one of the most demanding on troweling techniques.
Synthetic sheet membranes are made from chlorinated polyethylene, polyvinyl chloride, and other materials, are available in a variety of lengths and widths, and vary in performance from manufacturer to manufacturer. Generally speaking, these membrane systems have polyester or fiberglass netting bonded to each face so they may be attached to subfloors, and tile to the top surface. Bonding on both sides can often be done with latex-portland cement mortar. With some brands, the sheet may be bonded to the subflooring with glue allowing the tile installation to proceed quickly and without worrying about disturbing the bond between the sheet and the subfloor, a major consideration when using thinset mortar to bond the sheet to the floor. However, when using glue to bond the membrane, it is important to know the moisture vapor and alkalinity level of the slab.
Manufacturers have strict guidelines on vapor emission and alkalinity. Using adhesive over a slab that has excessive vapor emission can prove disastrous. Regardless of the bonding material used, the sheet has to be pressed into the surface of the subfloor. With thinset mortar, this is usually done with a 75- or 100-pound sheet vinyl roller. When using organic adhesive (because it is so sticky), the sheet can be smoothed down with the roller or even a flat trowel. Like all other types of membrane, it is essential that the setting bed surface be prepared using the same criteria required for the direct application of tile - if the surface is not smooth and flat enough for tile, it is not smooth and flat enough for a membrane. With this type of membrane, care must be taken to avoid trapping air below the sheet. This can cause air pockets to form that can seriously reduce the compressive strength of any tiles installed directly above the pocket. Particular attention must be given to any overlapping and sealing seams between neighboring sheets when the main objective is to keep the installation waterproof. Seams may be closed with a solvent or sealant, not to be confused with caulk; there IS a difference. Consult with the manufacturer for specific recommendations.
The Tile Council of America handbook has this year added the term uncoupling membrane. The definition is a system that separates the finished surface from the substrate to allow independent movement between the two and prevent the transfer of stress to the tiled surface. There are several manufactures of these systems. While a sheet type membrane, they typically have different performance properties than other systems. Among the advantages, the system is they allow for elimination of the control joint relocation required with other systems. Uncoupling membranes also allow for vapor transmission that can have a negative effect on most other systems. They can be used either for crack suppression or waterproofing with an additional seaming fabric. They also may be used as a substrate in lieu of additional underlayment or over green concrete. These systems are installed using a suitable thinset mortar.
These are the current type of systems available there continues to be widespread use of untested products never intended for ceramic tile membrane systems. Uses of roofing felt, sheet vinyl, scribing paper, or scrim reinforced Kraft paper with unknown values, glued or unglued to concrete slabs have a long history of failure. These products typically lack the performance features and criteria that would allow effective control of concrete fractures and wood movement without transmission of cracks through the finished tile surface. They exhibit low bonding strength against the expansion and contraction of both the tile and the substrate. In the case of concrete, they also typically lack ability to be exposed to or are not designed for prolonged moisture exposure common in a bonded application. A while back, I received a call from a national homebuilder that gave some interesting statistics on the effectiveness of their use. Prior to using the glue-down felt method over concrete, they were experiencing a 40 percent failure rate during the warranty period. Use of roofing felt cut the figure down to a 20 percent failure rate. They found this risk to be acceptable, as the subcontractor was always judged liable and provided free repairs or replacement. There are many fully warranted and industry approved membranes that probably would cut the product failure rate down to 0 percent. Due to minor cost differences, this builder was not interested as he had a ready source to assume liability during the warranty period, the tile contractor. After the warranty period that liability would pass to the consumer.
Diagnosing the substrate and selecting the proper membrane system requires some good familiarity with not only the substrate but also the performance requirements of the installation. All membrane systems require solid support and with few exceptions recommend an L/360 deflection rating. They all require adequate surface preparation otherwise their performance characteristics may be greatly diminished. In the case of crack suppression, the contractor must make an estimation of the nature of the crack: is it active and moving or dormant and stable? Is it caused by a structural failure? Is there a problem with the structural or subfloor materials? Can the crack(s) be expected to continue advancing into the slab? If the installation is over wood construction, what structural members (or lack of) are causing or contributing to potential damage of the tile and installation system? If the substrate deteriorates because of a structural problem, it is highly unlikely that any crack isolation or waterproofing system could protect tiles or substrate installed over such a surface. On the other hand, if the problem is rooted in the substrate materials, crack isolation systems may help to reduce potential damage. If there are no cracks in the structure surrounding the crack in the subfloor, if the surface height of the substrate on either side of the crack is the same, and if the crack width does not exceed manufacturers' requirements, thinbed crack isolation systems can be effective at reducing tile damage and in the case of waterproofing, protect the substrate. Too many flaws might be an indication that the entire substrate is in poor condition and that additional cracks may appear as time goes on rendering attempts at crack suppression and waterproofing ineffective. Proper assessment of the substrate is critical to the success of the installation and requires good knowledge of substrate materials.
Another important consideration in all crack suppression and waterproofing membranes is provision for movement accommodation. In the application of crack isolation systems, most allow only relocation of control joints, not elimination as commonly done. Manufacturers provide very specific instructions on how these joint 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. This can be of even larger concern if the application also requires waterproofing and should play an important part in the selection process. 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.