Chart 1

Deflection. What is it? Need it be checked prior to installation? If so, how is the check performed? Is it the responsibility of the flooring dealer?

In the simplest of terms, floor deflection refers to a floor's change from level caused by live loads (people) or dead loads (refrigerators, stoves, cooking islands, etc.). And why is deflection information important? Mainly because of the increasing use of thin and larger-format ceramic and marble tiles, and the shift from mud-bed installations with a cleavage membrane underneath to thin-bed installations. Deflection problems manifest themselves as cracked tile and grout, and/or tile disbonding.

According to Dan Louis, CEO of Laser Products, more than 70 percent of new construction floors fail to fall within the maximum allowable deflection necessary to support ceramic tile. This puts you at risk.

What is the curious "L/360 formula" used to determine maximum allowable deflection? Simply put, one would measure the floor's span in feet, convert that figure to inches and divide it by 360. For example, if you have a 15-foot span that would be equivalent to 180 inches. Dividing that by 360 would reveal a maximum allowable deflection of 1/2 inch.

How would one measure deflection before installing tile? A simple method is to stretch a tight string across the plane, and then measure the distance from string to substrate. The next step would be to bring in your tile and equipment, and your heaviest helpers. Situate the people and materials in the center of the span and measure the deflection again. Of course, an even more effective means of measuring deflection would be to use a laser level.

Vapor Retarders. I came across a recently published article that reported the American Concrete Institute's (ACI) recent retraction of a longstanding recommendation for the construction and placement of vapor retarders beneath the slab. The ACI has come to the conclusion that use of vapor retarders actually exacerbates the problem they were designed to alleviate.

The ACI's reasoning is that this type of assembly creates the potential for a rainwater reservoir to accumulate in the blotter layer of gravel prior to concrete placement. If true, I am not sure what effect this would have, because vapor retarders are still specified by local authorities.

In April 2001, the ACI published an update regarding vapor retarder locations. In the update, the organization uses the language, "if a vapor barrier is required due to local conditions." ACI goes on to say that rainwater problems associated with the retarder have been recognized and it advises caution with regard to the granular fill layer.

ACI also says that when the use of a vapor retarder is required, the decision on whether to locate the material (retarder) in direct contact with the slab or beneath a layer of granular fill should be made on a case-by-case basis. ACI also provides a flow chart for guidance (see chart 1).

Chart 2

Of course, some black and green marbles can curl from exposure to water in the setting materials. And excess moisture may cause buckling of ceramic tile, discoloration of grout and eventual disbonding. Testing of new top-side moisture control methods continues in the effort to help the tile industry.

Movement Joints. According to most concrete textbooks, about half of the total amount of shrinkage and creep that will occur in concrete takes place within 60 days of its casting. The remaining half occurs over a 20-year time span. So, if a floor covering is placed within the first several months after the slab is cast, slab shrinkage and tile problems are definite possibilities.

Expansion movement joints are used in response to these possibilities in an attempt to forestall problems of slab movement. Other joints are also utilized to protect ceramic floor installations.

In its Handbook for Ceramic Tile Installation, the Tile Council of America (TCA) now uses a heading entitled "Movement Joints --Vertical and Horizontal." TCA defines a construction joint (formerly known as a "cold joint") as a joint that occurs where two successive placements of concrete meet.

A contraction joint is a formed, sawed or tooled groove in concrete that is used to create a weak point to "regulate" the location of cracking that results from dimensional changes of the structure.

An expansion joint is a separation provided between adjoining parts of a structure to allow movement where expansion is likely to exceed contraction (see chart 2). It is this joint that requires proper location by the architect and proper preparation by the ceramic trades. These joints are supposed to be, according to TCA recommendation, 24 to 36 feet in each direction on interior installations. For exterior installations, the dimensions are reduced to 8 to 12 feet in each direction.

Many tile contractors will set tile right over the expansion joint, which is asking for trouble. Certain membrane manufacturers now say that with the use of their products, tile can be installed directly over the expansion joint. However, such systems still are not recognized by the TCA.

Finally, isolation joints are used to make a separation between horizontal and vertical parts such as walls, columns, etc.

When the tile installation is properly separated from the expansion joint, the tile then can expand and contract (due to exposure to sunlight etc.) and the joint itself can move without cracking the tile or the grout. Proper treatment of the joint is outlined in the Tile Council of America Handbook.