Simply stated, a warped tile is a tile that is not flat. In the laboratory, we measure warpage following the test method ASTM C485, Test Method for Measuring Warpage of Ceramic Tile, which defines warpage more precisely as the curvature of a tile measured as the deviation of the tile surface from a true plane along the edges or the diagonals. This important measurement is one factor that can make a difference between a beautiful installation and one riddled with lippage issues.
The ASTM C485 method is used to measure warpage along each edge and in the center of a tile. Warpage is reported as a positive or negative percentage which indicates whether the tile is convex (positive %) or concave (negative %). Warpage is calculated by dividing the actual measured amount that the tile deviates from flatness by the length of the edge or diagonal, then multiplying by 100 to get a percentage.
In the American National Standard for tile manufacturing, ANSI A137.1, there are limits on the amount of edge and diagonal warpage allowed depending on the type of tile. The specifications include a positive and negative maximum percentage allowed, as well as a cap on the absolute value measured. Without the cap, if the maximum was based solely on a percentage, then the absolute value allowable would get larger with larger tiles. The cap limits this from happening. According to the standard, the lesser of the percentage or the cap sets the maximum allowable warpage.
For example, the maximum allowable edge warpage for a rectified pressed floor tile is ±0.40% or ±0.05", whichever is less. A tile with a facial dimension of 8" would be allowed to deviate from flatness by up to 0.03" along the edge. This is calculated by multiplying 0.40% (0.004) by the edge length. However, if we consider a 24" x 24" tile, while 0.4% of 24" is 0.096", the cap limits the maximum allowable warpage to 0.05".
Now that you are an expert on warpage, check back soon because next time we will be finishing up this blog series with "What is Wedging?"