Rectangular tiles: Installation considerations

Rectangular tiles are more popular than ever, so there is a need to review those properties and conditions that may affect installation.

Rectangular porcelain tile normally has a slightly convex shape, causing slight variations in the tile's height. According to ANSI (American National Standards Institute) standard A108.02, Section 4.3.7: “Lippage refers to differences in elevation between edges of adjacent tile modules.” ANSI A137.1 defines acceptable warpage in terms of a percentage of the tile’s edge length and also sets a maximum warpage amount in millimeters that cannot be exceeded regardless of tile size.

In general, large rectangles can meet ANSI standards for warpage and still exhibit lippage when the running bond pattern and/or narrow grout joints are used. Since rectangles are most often rectified and a narrow grout joint like 1/8” is typically desired, manufacturers have chosen to address the problem in a practical way, by recommending a 25 percent or 33 percent offset rather than the traditional 50 percent. Remember the tile shape is slightly convex: in a 50 percent offset, the high middle of one tile is adjacent to the low end of the next, creating noticeable lippage. By sliding the middle and end of adjacent tiles closer together, lippage is reduced. If a 50 percent offset and a narrow joint are desired, the specifier and owner should view a mock-up and the resulting lippage before deciding how to proceed with the installation. They may decide to reduce the offset or to increase the joint width.

Subway wall tiles
These tiles often have softer and thinner bodies, exhibiting more warpage than floor tiles. In the case of large wall tiles like 4"x16" or 8"x20", the manufacturer usually recommends the stack bond with 0 percent offset. In this pattern, a credit card joint can be used to obtain a flat surface.

Another important consideration on tile walls is lighting. Wall-wash and cove-type lighting can create shadows and undesirable effects, accentuating even the slightest irregularities in a wall’s surface. Special attention should be given to wall preparation, installation methods and fixture location when this type of lighting is used.