To minimize the undesirable effects of critical lighting:

  • Place lighting such that it will decrease or eliminate the undesirable effects of critical lighting, and install permanent light prior to tile installation to provide the installer the maximum opportunity to reduce undesirable shadows.
  • Reduce the amount of inherent or allowable lippage by ensuring substrates meet required flatness tolerances for ceramic tile, for example by: specifying a mortar bed, pourable underlayment, or other tile substrate that facilitates a flat tile installation; including adequate allowance in the tile specification for substrate preparation; and specifying the trade responsible for required alterations to a substrate that does not meet flatness tolerances for tile.
  • Reduce the amount of inherent or allowable lippage by specifying tile with minimal warpage, such as rectified tile per ANSI A137.1.
  • Minimize the effect of lippage due to warpage by specifying wider grout joints, cushioned or beveled edge tile, and tile sizes and patterns that minimize lippage due to warpage, for example avoid offset or brick-joint patterns or specify a maximum 25% or 33% offset (see ANSI A108.02).





Use of wall-washed and cove-type lighting, where the lights are located either at the wall/ceiling interface or mounted directly on the wall, are popular techniques for producing dramatic room lighting effects. However, when proper backing surfaces, installation materials and methods, and location of light sources are not carefully coordinated these lighting techniques produce shadows and undesirable effects with ceramic tiles, particularly when light strikes the tile surface at a low or nearly flat angle (i.e., nearly parallel to the tile surface). Similar shadows can be created from natural light from windows and doors side-lighting interior walls and floors, and when light shines at a low angle on exterior walls and floors, as commonly occurs in the early morning and evening.

Because such low angle lighting highlights and exaggerates normal and acceptable inconsistencies in tile and tilework, the shadow-producing light is often referred to as "critical light" or "critical lighting." Some of the allowable and acceptable characteristics of tile and tilework that can be highlighted or exaggerated by critical lighting include:

  • Die release lines on the edges of tiles
  • Difference in appearance between a factory edge of a tile versus an edge that has been field-ct by scoring, grinding, or we cutting
  • Allowable warpage in the tile (see ANSI A137.1 for definition and allowances)
  • Allowable lippage (see ANSI 108.02 for definition and allowances)
  • Allowable variation in height between field tile and trim pieces and/or accent tiles, whether such are or are not part of the same tile line

In addition, critical lighting can worsen the appearance of tilework that does not meet the finish flatness and lippage tolerances set forth in ANSI A108.02, which is considered acceptable tile workmanship under certain specified conditions:

  • Tiled floors sloping to drains (see ANSI A108.02)
  • When the project owner does not provide a substrate that meets required flatness tolerances for tile nor contract the tile contractor to correct substrate flatness defects (see ANSI A108.01)