In order not to muddy the waters right from the start, it is best to go to one of the pre-eminent authorities in the area of tile installation to help us with some basic definitions and to help us set our expectation levels for the performance of tile installations as it relates to cracking. For those of you who want to skip to the end to see how this turns out, I’ll save you a trip – there is no guarantee against the cracking of tile. Hope I didn’t spoil it for you, but just like no one can guarantee against concrete cracking, no one can guarantee against those cracks “telegraphing” up into the tile. But there are several things we can do to minimize this effect.
Let’s first take a look at what ANSI says about the various types of cracks and joints that we will encounter. In the document ANSI A118.12 entitled AMERICAN NATIONAL STANDARD SPECIFICATIONS FOR CRACK ISOLATION MEMBRANES FOR THIN-SET CERAMIC TILE AND DIMENSION STONE INSTALLATION, we find the following terms defined for us:
Isolation joint: A separation between adjoining parts of a concrete structure, usually a vertical plane, at a designated location such as to interfere least with performance of the structure, yet such as to allow relative movement in three directions and avoid formation of cracks elsewhere in the concrete and through which all or part of the bonded reinforcement is interrupted.” (Say that three times fast!)
Construction joint: The surface where two successive placements of concrete meet, across which it may be desirable to achieve bond, and through which reinforcement may be continuous.
Contraction Joint: Formed, sawed or tooled groove in a concrete structure to create a weakened plane and regulate the location of cracking resulting from the dimensional change of different parts of the structure.
Basically, we can divide joints and cracks into two very general categories: moving and non-moving (or active or dormant if you prefer volcano-speak). By identifying which type we are dealing with, we can establish what we can and cannot do when it comes installing tile.
Prior to beginning the tile installation on any given project, the specifier must identify where moving joints are located. These are intentionally placed at specific locations where adjacent materials should move freely. Expansion and isolation joints are the common moving joints seen on concrete slabs. These are usually placed every 24 to 36 feet in each direction on interior concrete slabs and 8 to 12 feet on exterior slabs. In addition, interior tile work that is subjected to direct sunlight (atrium in a mall with sky lights) should also have expansion joints approximately every 12 feet.
Expansion joints must also be placed in tile when the substrate material changes plane or composition, and in any area where tile butts up to restraining surfaces such as curbs, columns, pipes and ceilings. Floor-wall and wall-wall intersections are areas of expansion that are designed in by the specifier and are there to accommodate movement where materials change direction or composition.
At the other end of the spectrum, are the saw cuts (control joints) and cracks that develop due to a one-time event, usually shrinkage as the concrete dries and cures. These cracks are often referred to as “dormant.” Cold joints are typically installed between slab pours done on different work days. These pours should also be identified for the tile installer, because cold joints can result in cracking that may telegraph into the tile. Saw cuts, ones that are cut into the concrete to allow for shrinkage cracking, should also be clearly identified and carried up into the tile installation. For these reasons, movement joints in tile installation are recommended over these joints and should never be less than the width of the original saw cut.
As a rule of thumb, all expansion, control, construction and cold joints should be continued through the tile work, and the continued joints should never be narrower than the original joint. For exterior applications, joints that are 12 feet on center should be a minimum of 3/8” in width, while joints that are 16 feet on center should be at least 1/2” wide.
For interior joints for standard ceramic pavers, the joint is typically the same width as the grout joint being used, but should not be less than 1/4.” When installing mosaics or glazed wall tile, the joint must be at least 1/8” wide, and up to about 1/4”. All of these joints require the use of a flexible material, such as what it often called “backer rods,” to fill in the bottom half of the joint, followed by the use of a sealant such as a silicone. Try as we might to match the color of the cementitious grouts to the silicone or other sealant used, close inspection will show a slight color difference - significant enough to indicate a moving joint somewhere near the row of tile.
With ceramic tile installations being one of the most artful forms of surface finishes we use, it is comforting to know that there are methods of installation that allow us to accommodate such things as cracks and joints. I have always felt it necessary to advise designers and owners that building materials need room to move, and that we should incorporate this movement into our ceramic tile floor and wall installations.
However, when art comes up short of meeting our expectations for the “look” we strive for in any given design, science steps in and new technologies allow us to modify installation methods. In our next installment, we will look into membranes and fabric, and address the issues of bridging and isolation as they apply to cracks and joints in ceramic tile installations.