Crack isolation methods and techniques have been practiced for thousands of years to prevent excessive tile cracking. In ancient times, crack isolation involved locating a layer of sand beneath the setting bed so the tile installation could float over any movement below. To deal with expansion, regularly spaced grout joints were filled with tar which was covered with fine sand. The sand bed method is still being used today, although primarily on exterior applications where thick stone or cast concrete tiles are installed without grout.

For the majority of interior and exterior crack isolation work, a sheet or liquid-applied membrane system is used. They are called systems because each brand has components that work together to provide the desired level of protection required for a specific installation. For example, a liquid-applied system may include a base liquid, gel, or paste, and a reinforcing fabric. A sheet system is generally composed of the sheet, an adhesive for laminating the sheet to the setting bed, and a sealant for closing seams. In addition to basic sheet or liquid-applied systems, there are self-adhering reinforced sheets that are used with a companion primer to maximize adhesion. The latest type of crack isolation is built into, and becomes a property of thinset mortar and requires no reinforcing fabric. Crack isolation thinset mortars cost considerably more than regular thinset mortars, but the extra cost is usually offset by labor and material costs for traditional sheet or liquid-applied systems. It is very important to keep in mind that not all crack isolation systems provide the same level of performance, and each brand offers varying degrees or warrantee protection – or none at all.

Regardless of the type, no system can offer a 100% guarantee that tiles installed above the membrane will not crack. Instead, crack isolation systems are designed to reduce the incidence of cracking. Not all systems offer the same level of protection, which is usually gauged by the maximum width of the crack being covered: most systems have a 1/8-inch wide limit. Crack isolation systems cannot be used over structural cracks. Structural cracks can be found in concrete where the slab on one side of the crack is higher than the other side. On wood construction, structural cracking can also be caused by subflooring or underlayment that is too thin, or joists that are too small to carry expected loads. Under some conditions, cracks can occur when two sheets of subflooring meet on a single joist or when the grain of either the subflooring or underlayment does not run perpendicular to the direction of the joists. Some systems claim to bolster flooring systems built over joists spaced too far apart, but for best results, crack isolation systems should only be installed over wood or concrete that is structurally sound.

For maximum strength, plywood underlayment is sometimes specified in place of backer boards, but plywood has a much higher rate of expansion and contraction than either stone or ceramic tiles. As well, it is tougher to adhere tile to wood than to a tile backer board. To overcome these problems, some – but not all crack isolation systems can be applied to plywood underlayment to maximize adhesion and to prevent tile cracking.

As with any other method of installing tiles, a network of movement joints has to be built into any installed crack isolation system. The primary function of any crack isolation system is to overcome substrate movement. Ironically, tiles installed over a crack isolation membrane must be free to move. Any tile installed over a crack isolation membrane without movement joints are subject to both cracking and shearing. With any crack isolation systems, as stated above, there are no 100% guarantees against cracking, but when no movement joints are incorporated into an installation, any warrantee offered by the manufacturer will not be honored. All ceramic and stone tile installations – with or without a membrane system – require a network of movement joints.

There are two approaches to crack isolation: full-coverage and bandage. With full coverage, as you might expect, the entire floor is covered. With the bandage method only the cracked area is covered. The full-coverage method offers the best protection and costs more. The bandage method costs less, but has several problems: limited protection, uneven profile, diminished appearance, and higher maintenance requirements. The bandage method is used primarily over commercial or residential concrete slabs. Covering only a small area, the bandage method creates a bump on the surface of the floor, unless the area to be covered is lowered an amount equal to the thickness of the installed bandage. This can be done, at extra expense, by abrading, bush-hammering or shot blasting. Since slab cracks may not always travel in a straight line, individual bandages may have to overlap to provide good coverage: overlapping sections will add even more height to the repair.

The length of the required bandage is governed by the length of the crack; the width is determined by the manufacturer and is usually based on the size of the tile being installed. As well, the bandage method requires that the treated area be surrounded by a movement joint to separate the tiles directly over the substrate crack – this can cause appearance problems if an exact match between the grout and the movement joint filler is not possible. The movement joint filler, because it is subject to floor or wheeled traffic and regular cleaning, will also need more frequent replacement than joints located at the perimeter of a floor. When the bandage method is specified or requested, installers should protect themselves by sketching a map that shows the location of each existing crack that is to be covered. After sketching the map, have the property owner sign and date the map: if new cracks appear in areas that were not covered by a bandage, it will be the owner’s responsibility – not the installer’s.

Finally, some membrane systems can provide waterproofing protection as well as crack isolation, and at least one (Nobleseal SIS), offers crack isolation, waterproofing, and sound reduction protection.