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What is acceptable variations in the height of adjoining tiles?

Variation in the height of adjoining tiles is called lippage. This is defined in the ANSI (American National Standards Institute) standard A108.02, Section 4.3.7: "Lippage refers to differences in elevation between edges of adjacent tile modules."

The ANSI standard notes that the perception of lippage is influenced by many factors such as:

A) The allowable thickness variation of the tile modules when judged in accordance with manufacturing standards.

B) The allowable warpage of the tile modules.

C) The spacing or separation of each tile module, which would influence a gradual or abrupt change in elevation.

D) Angle of natural or manufactured light accentuating otherwise acceptable variance in modules.

E) Highly reflective surfaces of tile modules accentuating otherwise acceptable variance in modules.

Additionally, variations in the plane of the substrate will also affect lippage. In many cases, when tile is installed by the thinset method over an uneven substrate, the installed surface will not meet lippage standards.

What is the allowable warpage of a tile?

The ANSI A137.1 standard defined allowed warpage according to the type of tile. For a paver tile (commonly used on floors) the allowable warpage is defined as follows: Warpage. When measured as described in ASTM C 485, the warpage of each tile in the sample shall not exceed 1.0 percent along any edge nor 0.75 percent on either diagonal.

From this formula the allowable warpage can be determined.

Is allowable lippage on an installation calculated from the allowable lippage in the table plus the allowable warpage?

No. This is a common misconception. The amount of allowable tile warpage is not used in the calculation of allowable lippage. Rather, allowable lippage is the total of the inherent (i.e. actual) tile warpage and the allowable lippage from the table. Of course, the actual warpage should not exceed the allowable warpage as calculated above.

This is really rather straightforward if you look at it as follows:

The intent of the standard is to define reasonable lippage from one tile to another. When a tile is warped but within standards, all of the corners cannot be set in the plane of the adjoining tiles. Accordingly,

the tolerance for variation from tile to tile includes the actual warpage of the tile. If there is no warpage, the lippage should not exceed the value in the table.

In the days before isostatic presses, warpage was more common. Today, many mass-produced tiles have exceedingly little warpage.