A rectified tile is defined as a tile that has had all edges mechanically finished to achieve a more precise facial dimension. Rectified tile is ever increasing in popularity. Most popular are the larger size formats often used in conjunction with smaller modular sizes to create patterns.

A 1/16” drill bit was placed under the center of this tile representing the allowable variation using the normal substrate recommendation of no more than ¼” variation in 10’, and no more than a 1/16” variation in 12”. It becomes very apparent that a much flatter than normal floor is required to receive large tile such as this 12” x 24”.

Rectified tile is also selected for its ability to allow tighter grout joint spacing. From a homeowner or other end user perspective this seems a very desirable product attribute, one they are willing to pay a little extra for. In the sales and selection process what often fails to get any consideration, and therefore no additional compensation, is the increased challenge that the installer faces in achieving those goals. Large tile combined with minimal grout joints creates a host of installation issues and this article both explores and explains some forthcoming changes as well as challenges in achieving an aesthetically pleasing and correctly installed rectified tile product. Some parts of this article are going to be a little rougher reading than most. As you will see, with the proliferation of tile types, sizes and tolerances, the tile professional will be soon be using their calculator for more than totaling up a sale or estimate.

First let’s explore just what is “rectified tile.” Currently, it is whatever the manufacturer wants it to be! I have heard numerous descriptions of what constitutes rectified. There are no current standards to address this product category, however help is on the horizon. Ceramic tile (porcelain tile is a ceramic tile) is a product that shrinks to size as it is made. This is true for all types of ceramic products. 

A rectified tile is made by either sawing or grinding tile that has already been fired to a specific size, thus allowing minimal size variation between pieces. How minimal is currently up to the manufacturer; I have heard variations of .50 %, .039 %, and .025 % used for rectified tile. Normal non-rectified porcelain tile can have a variation of up to 1.5% under current standards regardless of size. There are proposed standards that have been balloted and will likely be soon adopted under ANSI A-137.1, the American National Standards for Ceramic Tile.

Table 1

This revised standard provides new definitions and will set a specific range of variation that a tile must fall within to be considered either calibrated or rectified based on size. The new tolerance range for regular calibrated porcelain tile will be + or - .5% up to a maximum of .08 inches based on size.  For the first time a separate standard will be published for Rectified tile which limits the range of variation to + or - .25 up to a maximum of .03 inches.

Elsewhere in this article is a partial chart (Table 1) showing what the allowable variations will be for a rectified porcelain tile compared to normal calibrated or non-rectified tile. Under this new standard there is also a specific definition for rectified tile that alludes to the edges being mechanically finished as opposed to pressed. Standards are about setting basic performance guidelines so all may compete fairly. The new product standard, when published, will be a great improvement for everyone in the industry. It will provide clarity that is not currently available.

Now let’s move forward to the installation side of the equation and address some challenges faced by the installer. While customers are seemingly willing to part with an extra $.50 to 1.00 for tightly sized tile product, little if any financial consideration is given to the installation. First, let’s talk about the much sought after small grout joint. One recent ad I saw says “do away with grout joints, use only rectified tile.”

When placing a 12”x24” tile at the center of the adjoining tile, the lowest amount of tile; warpage, at the short edge, is placed next to the highest amount of warpage in the center of the adjoining tile. Even in tile with minimal warpage, some variation will occur. By the way, that is a 4’x4’ tile awaiting testing behind the 12”x24” tile, all 100+ pounds of it.

Another is not quite so brash and says 1/16” grout joint recommended. These are all things consumers want to hear and the marketing departments are more than willing to tell them. However, such statements make technical services departments and installers shudder because they are very difficult if not impossible goals to achieve. Let us look at the first statement, 1/16” grout joint recommended. If the manufacturer were to use the allowable variation under the soon to be published standard for a rectified 12”x12” tile, the maximum allowable variation would be .06 inches.

A grout joint of 1/16” equals .0625. in essence at .0025 inches, there could well be no grout joint! Some manufacturers may be able to produce at levels 50-75% tighter than this, but at .0025 you still see the problem. The standards committee, being composed of both manufacturers and labor looked at this and decided the time had come for specific grout joint recommendations.

Until recently it was understood but not written, that a grout joint width 3 times the actual average variation of the tile was a prudent recommendation. With new standards addressing and adding tighter tolerances to tile products coupled with the desire for ever tighter grout joints, the labor side of the standards committee has suggested that a recommended joint width of 3 times actual variation be adopted in the installation standards, ANSI A-108 (The American National Standards for Ceramic Tile Installation) along with the newly revised tile product standard. This proposal will be introduced in spring of 2008 and passage is expected. Seeing a use for those calculators now? Bottom line, whether calibrated (normal) tile or rectified tile, grout joints must allow for the natural variation in tile products.

Tile meeting standards is checked for compliance using established minimum and maximum variations under both American and international standards.


The issue is probably at its worst when using a larger formats in a staggeredpattern,either calibrated or rectified. If the tileismanufactured as a 12”x 24” the lowest amount of warpage will be on the 12” side of the tile, the highest on the 24” side of the tile. When installing this size tile in a running bond pattern the lowest point of warpage is placed right in the middle of the adjoining tile which is at the highest point of warpage. The best installer and the best substrate can not change the fact that all tile has a certain amount of surface variation as part of the firing process. Warpage will also be less in smaller size tile when multiple sizes are used in a modular pattern. The bigger tile gets, the more apparent this naturally occurring part of the manufacturing process becomes.Another consideration that receives little thought but can cause the appearance of poor workmanship is tile warpage. All tile has some amount of warpage. It is not considered a defect unless excessive. The chart elsewhere in this article (Table 1) also shows at what point warpage is considered excessive. In modular patterns this variation can become very apparent. 

Last but not least on the list of challenges with rectified tile installation comes “normal” substrate tolerances. It would be an accurate statement to say anything over 12” in size is not going to have an adequately flat substrate to facilitate tight joints unless tighter tolerances are specifically requested. Normal substrate tolerances seldom seen even when specified call for maximum surface variation on flatness of no more than a 1/4” in 10’, nor more than a 1/16” variation in 12”. In new construction, this is the job of substrate trades and covered in their industry documents and recommendations. In existing or remodeled applications this tolerance recommendation typically falls to the tile or flooring contractor.

Clamping two 12”x24” tiles together, the amount of warpage and the effect on flatness is very visibly demonstrated here. Both of these tiles do meet the old tile standard variation criteria but only one would pass under the proposed product standard revision.

Elsewhere in this article there is a picture of a 12”x24” tile with a 1/16” drill placed under the center representing a 1/16” variation in a 12” area. That small amount of variation caused a 1/8” displacement in one tile! That 1/8” becomes 1/4” in the next tile and 3/8” in the following after only 6’ of area. Large tile with normal grout joints requires a very flat floor in more than likely the 1/8” in 10’ range, not at all typical of the normal construction process. Large rectified tile requires what is known as a “super flat,” which is well beyond the equipment and ability of most in the substrate trades. This leaves the job to the flooring professional that has the knowledge, skill, and products available to him. The time to flatten the floor is prior to the installation. Using a medium bed mortar may allow for minimal build-up providing a minor degree of flattening but that would be an adequate measure in only the smallest of installation areas. All this goes to say that if you are planning on doing big tile it would be prudent to figure some floor prep. If you are using big or rectified tile with small joints you should plan on a lot of floor prep.

The final word: as large rectified tile gains in popularity, particularly in modular patterns, the need grows to educate the end user, sales personnel, and installers about the properties and limitations of both the product and installation. The requirements to provide satisfactory installations of rectified tile with tight grout joints are exacting and not inexpensive.

The substrate flatness required for narrow grout joints is well beyond reach of the typical mason or carpenter. Unless all appropriate parties are properly educated about the product and installation needs of large rectified tile and narrow grout joints, great resistance can be expected in getting the appropriate compensation for the additional substrate work which is inevitable. 

Something given little consideration and a relatively new problem in the tile world is this type of grout darkening due to slab moisture. Using large tile over concrete limits its ability to breathe. In this instance voids were found in the thinset, allowing actual water to collect. The importance of good, even coverage under the tile increases to prevent shading issues.